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1.0 SCOPE ......................................................................................................................................................5 2.0 REFERENCES...........................................................................................................................................5 3.0 DEFINITIONS ............................................................................................................................................8 4.0 BACKGROUND AND SELECTION CRITERIA.......................................................................................10 4.1 BACKGROUND...............................................................................................................................10 4.2 SELECTION CRITERIA...................................................................................................................11 5.0 SUMMARY OF BIOLOGICAL TREATMENT SYSTEMS ........................................................................11 6.0 ACTIVATED SLUDGE SYSTEMS...........................................................................................................11 6.1 DESCRIPTION ................................................................................................................................11 Process Microbiology..........................................................................................................................11 Reaction Kinetics and Fundamental Expressions...............................................................................12 Substrate (BOD) Removal ..................................................................................................................13 Temperature Effect / Importance ........................................................................................................14 Oxygen Requirements ........................................................................................................................14 Process Variations..............................................................................................................................16 Clarification.........................................................................................................................................17 6.2 DESIGN CONSIDERATIONS..........................................................................................................18 Regulatory Effluent Requirements......................................................................................................18 Feed Characteristics: Equalization and Pretreatment Requirements .................................................18 Selection of Reactor and Clarifier Types ............................................................................................20 Oxygen Requirements and Aeration Equipment.................................................................................21 Need for Pilot Plant Data ....................................................................................................................21 6.3 DESIGN PROCEDURE ...................................................................................................................21 Quick, Rough Sizing Basis .................................................................................................................21 Standard Procedures - Design Conditions..........................................................................................21 6.4 SAMPLE DESIGN PROBLEM .........................................................................................................27 Design Conditions ..............................................................................................................................27 6.5 OPERATING STRATEGIES AND ENHANCEMENTS ....................................................................31 Upstream Monitoring ..........................................................................................................................31 Equalization, Spill Diversion, and Pretreatment..................................................................................31 Solids Retention Time ........................................................................................................................31 Routine Operations Monitoring ...........................................................................................................31 Operations Troubleshooting ...............................................................................................................33 7.0 SEQUENCING BATCH REACTOR.........................................................................................................33

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8.0 AERATED LAGOONS ............................................................................................................................34 8.1 BACKGROUND...............................................................................................................................34 8.2 DESCRIPTION................................................................................................................................34 8.3 DESIGN CONSIDERATION............................................................................................................34 Aeration Zone.....................................................................................................................................35 Settling Zone ......................................................................................................................................35 8.4 DESIGN PROCEDURE...................................................................................................................36 Substrate Removal.............................................................................................................................36 Temperature Effects...........................................................................................................................36 Oxygen Requirements........................................................................................................................36 Power Requirements for Oxygen Transfer and Solids Suspension....................................................36 Sludge Accumulation..........................................................................................................................36 8.5 SAMPLE DESIGN PROBLEM.........................................................................................................37 Temperature Effects...........................................................................................................................38 Substrate Removal.............................................................................................................................38 Oxygen and Power Requirements......................................................................................................38 Sludge Accumulation..........................................................................................................................38 8.6 OPERATING STRATEGIES AND ENHANCEMENTS ....................................................................39 Oxygen Transfer.................................................................................................................................39 Short Circuiting...................................................................................................................................39 Algae and Suspended Solids Control.................................................................................................39 Sludge Buildup and Removal .............................................................................................................39 Biomass Return Options ....................................................................................................................39 9.0 AEROBIC ATTACHED GROWTH ..........................................................................................................40 9.1 DESCRIPTION................................................................................................................................40 Process Microbiology .........................................................................................................................40 Applications for Attached Growth Systems ........................................................................................41 Trickling Filters and Packed-Bed Biotowers .......................................................................................41 Rotating Biological Contactor Reactors (RBC) ...................................................................................42 Aerated Biological Filters and Fluidized Beds ....................................................................................42 9.2 DESIGN CONSIDERATIONS .........................................................................................................42 Additional Design Considerations for Trickling Filters ........................................................................43 Additional Design Considerations for RBCs .......................................................................................43 Additional Design Considerations for Aerated Biological Filters and Fluidized Beds .........................43 Equipment Design for Trickling Filters................................................................................................44 Equipment Considerations for RBC....................................................................................................46 9.3 DESIGN PROCEDURE...................................................................................................................47 9.4 SAMPLE DESIGN PROBLEM.........................................................................................................50 9.5 OPERATING STRATEGIES AND ENHANCEMENTS ....................................................................52 10.0 ANAEROBIC SYSTEMS .......................................................................................................................52 11.0 ANOXIC SYSTEMS...............................................................................................................................53

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12.0 NITROGEN MANAGEMENT .................................................................................................................53 12.1 DESCRIPTION ..............................................................................................................................53 Nitrogen Forms...................................................................................................................................53 Process Microbiology..........................................................................................................................54 Process / Reactor Variations For Biological Denitrification .................................................................54 Alternative Reactor Designs ...............................................................................................................54 Alternative Biological Processes.........................................................................................................55 12.2 DESIGN CONSIDERATIONS........................................................................................................55 Effluent Nitrogen Limits.......................................................................................................................55 pH .......................................................................................................................................................55 Temperature .......................................................................................................................................56 Dissolved Oxygen (D.O.) ....................................................................................................................56 Mixed Liquor Recycle Rate and Recycle Ratio...................................................................................56 Power Input to Anoxic Zone................................................................................................................57 Organic Substrate to Nitrogen Ratio...................................................................................................57 Solids Residence Time (SRT) ............................................................................................................58 Hydraulic Retention Time (HRT).........................................................................................................58 12.3 DESIGN PROCEDURE .................................................................................................................58 12.4 SAMPLE DESIGN PROBLEM - PERFORMANCE REQUIREMENTS, EXTERNAL SUBSTRATE REQUIRED AND ROUGH SIZING................................................................................................59 12.5 OPERATING STRATEGIES AND ENHANCEMENTS ..................................................................60 13.0 CHEMICAL ADDITION SYSTEMS........................................................................................................60 13.1 NUTRIENTS ..................................................................................................................................60 13.2 FLOCCULANTS ............................................................................................................................60 13.3 pH CONTROL ...............................................................................................................................60 14.0 NOMENCLATURE.................................................................................................................................61 TABLES Table 1-1.A Table 1-1.B Table 6.1-1 Table 6.2-1 Table 6.2-2 Table 9.1-1 Table 9.1-2 Table 9.2-1 Table 12.1-1 Table 12.2-1 Table 12.2-2 Table 12.2-3 Table 12.3-1 Table 12.3-2 Table 12.3-3

Characteristics of ExxonMobil Activated Sludge Units ...............................................64 Characteristics of ExxonMobil Lagoon Systems.........................................................68 Recommended Values of Ym and b at 20°C ..............................................................69 Criteria for Pretreatment of Activated-Sludge Feed....................................................70 Relative Biodegrability of Certain Organic Compounds..............................................71 Characteristics of ExxonMobil Attached Growth Systems..........................................72 Sample Duty Specification Sheet for Attached Growth System .................................73 Comparison of Vendor Aerated Biological Filters .......................................................75 Wastewater Nitrogen Measurement Parameters........................................................76 Alkalinity Produced During Nitrate - Nitrogen Reduction ............................................76 Mixing Requirements in Suspended Growth Systems................................................77 Theoretical Substrate Requirements For Nitrate - Nitrogen Reduction ......................78 Performance of ExxonMobil Nitrification / Denitrification (N/DN) Systems .................79 Sample Equipment List for Nitrification / Denitrification ..............................................80 Sample Duty Specification Sheet for Nitrification / Denitrification...............................81

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FIGURES Figure 4.2-1 Figure 4.2-2 Figure 6.1-1

Oxygen Demand / Dissolved Organic Reduction Decision Tree ................................82 Typical Wastewater Treatment Flow Plan..................................................................83 General Schematic Of An Activated Sludge Process (Completely Mixed) Biological Treatment (BIOX) System..........................................................................................84 Figure 6.1-2 Typical Plot Of The Relationship Between The Specific Growth Rate Constant And The Limiting Substrate Concentration.................................................................84 Figure 6.1-3 BOD Removal And Sludge Growth Relationships......................................................85 Figure 6.1-4 Conventional Plug Flow Activated Sludge Process....................................................86 Figure 6.1-5 Typical Configuration Of A Step-Feed Aeration Activated Sludge Process................86 Figure 6.1-6 High Purity Oxygen Activated Sludge Process ..........................................................87 Figure 6.1-7 Completely Mixed Activated Sludge With A Selector .................................................87 Figure 6.1-8 Two BIOX Configurations For Total Biological Nitrogen Removal .............................88 Figure 6.3-1 Determination Of Ym From Pilot Unit Batch Yield Test...............................................89 Figure 6.5-1 Succession Of Protozoa And Sludge Development ...................................................90 Figure 8.2-1 Aerated Lagoon Example Layout Of An Aerated Lagoon System .............................90 Figure 9.1-1 Films And Layers On Attached Growth Media ...........................................................91 Figure 9.1-2 Schematic Of Trickling Filter Systems........................................................................92 Figure 9.1-3 Typical Trickling Filter ................................................................................................93 Figure 9.1-4 Schematic Representation Of Rotating Biological Contactor System ........................93 Figure 9.1-5 Conventional And Submerged RBC Units..................................................................94 Figure 9.1-6 Schematic Representation Of An Aerated Biological Filter ........................................95 Figure 9.2-1 Surface Area Correction Curves For RBCs For Temperatures Below 55°F...............96 Figure 9.2-2 Fluidized Bed Flow Diagram (US Filter-Envirex) ........................................................97 Figure 9.2-3 Tricking Filter Media...................................................................................................98 Figure 12.1-1 Simplified Nitrogen Cycle Within A Wastewater Treatment System...........................99 Figure 12.1-2 Total Biological Nitrogen Removal Configurations ...................................................100 Revision Memo 12/01

Section 11.0 updated, Section 12.0 added with associated tables and figures. Old Sections 6 and 7 covering “Aeration Systems for Biological Treatment of Wastewater” and “Clarification Systems for Biological Treatment of Wastewater” have been relocated to new DP Sections XIX-A6 and XIX-A7, respectively.

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1.0 SCOPE ➧

This section presents design considerations, recommended process design procedures, and certain mechanical design details for facilities to biodegrade wastewater containing organic and some inorganic contaminants to meet certain quality goals. Biological treatment facilities include: suspended growth, attached growth, or a combination of both. There are several varieties of suspended growth systems, the most common being activated sludge and extended aeration. Attached growth includes trickling filter, rotating biological contactors, and many others. This Design Practice includes directions for sizing the aeration basin / tank, calculating the aeration requirements, selecting aeration equipment and sizing the clarification systems. Currently, only the standard activated sludge, aerated lagoons, and aerobic attached growth systems (trickling filters, rotating biological contactors, etc.) are discussed in detail. Design information on other biological treatment technologies like sequencing batch reactors, anoxic systems, and others are not covered. Chemical addition systems and sludge management are briefly discussed. Biological treatment systems can be employed to meet complex effluent treatment requirements including aquatic organism toxicity reduction, selected heavy metals removal and nutrient management. For complex applications or where multiple water quality goals are required, it is recommended that ExxonMobil Engineering specialists be contacted. For reference purposes only, a table including the characteristics of ExxonMobil activated sludge units and aerated lagoon systems is provided in Table 1-1A and 1-1B, respectively. The design procedures discussed in this section are set forth to give guidelines in developing biological treatment for wastewater. In all cases, consideration must be given to evaluating key parameters such as the potential to discharge a stream with low dissolved oxygen, oil sheen, toxicity, and/or high residual chemical oxygen demand to ensure compliance with appropriate environmental standards. Alternate treatment / configurations exist which can provide the necessary wastewater treatment.

2.0 REFERENCES 2.1 DESIGN PRACTICES Section XIX-A1 Section XIX-A2 Section XIX-A3 Section XIX-A4 Section XIX-A8 Section XIX-A9 ➧

2.2 GLOBAL PRACTICE GP 3-2-1

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Primary Oil / Water Separators Flotation Units Media Filtration Chemical Flocculation / Specific Ion Removal and Clarification of Wastewater Activated Carbon Treatment Water / Wastewater Chemical Feed Systems

Sewer Systems

2.3 EMRE WATER AND WASTEWATER DESIGN GUIDE (TMEE 080) DG 11-1-1 DG 11-2-1 DG 11-6-1 DG 11-6-3 DG 11-7-1 DG 11-8-1 DG 11-9-1

Granular-Media Filters Fixed Bed Ion Exchange Water Treating Units Chemical Feeders For Boilers & Deaerators Chemical Feeders For Wastewater Treating Wastewater Dissolved Air Flotation System Gravity Belt Filter Press System Aeration Systems

2.4 OTHER REFERENCES 1. 2. 3. 4. 5.

Metcalf & Eddy Inc., Wastewater Engineering Treatment, Disposal, and Reuse, 2nd Edition, McGraw-Hill Inc., New York (1979) Guidelines For Reducing Waste Treatment Cost, ER&E Report No. EE.48E.85 Grady Jr., C. P. L., Daigger, G. T., Lim, H. C., Biological Wastewater Treatment, Theory, and Applications, Marcel Dekker, Inc., New York (1999) Eckenfelder Jr., W. W. and Grau, P., Activated Sludge Process Design and Control: Theory and Practice, Water Quality Management Library Volume 1, Technomic Publishing Co, Inc., Lancaster, PA, (1992) Metcalf & Eddy Inc., Wastewater Engineering Treatment, Disposal, and Reuse, 3rd Edition, McGraw-Hill Inc., New York (1991) ExxonMobil Research and Engineering Company – Fairfax, VA

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2.0 REFERENCES (Cont) 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

Water Environment Federation, Clarifier Design - Manual of Practice FD-8, Lancaster Press, Lancaster, PA (1985) Eckenfelder Jr., W. W., Industrial Water Pollution Control, 2nd Edition, McGraw-Hill Inc., New York (1989) Hayes, B. E. and Cancellare, M. C., Industrial Water Pollution Control Technology Course - Eckenfelder's Method Compared to the Design Practices, 91 ECS2 191 (December 1991) Altemoeller, P. H. and Goodrich Jr., R. R., Guidelines for Sizing Gravity Thickeners - Wastewater Treatment Sludge Applications, ER&E Report No. EE.24E.89 (October 1989) Parker, D. S., The Case for Circular Clarifiers, WATER / Engineering & Management, April 1991 U.S. Environmental Protection Agency, Nitrogen Control Manual, EPA/625/R-93/010, USEPA, Cincinnati, Ohio (September 1993) ENSR, Breakthrough Nite/Denite Process Purifies Wastewater and Saves Millions in Treatment Costs, ENSR Newsletter (1991) 800-722-2400 Great Lakes-Upper Mississippi River Board of State Public Health and Environmental Managers, Recommended Standards for Wastewater Facilities, Health Education Services, Albany, NY (1990) Gerardi, M. H, An Operator's Guide to Protozoa and their Role in the Activated Sludge Process, PUBLIC WORKS, July, 1986 Wilkinson, J. B. and Palis, J. C., Trip Report: Filamentous Bulking in Augusta's Biox, 84ECD 213 (March 1984) U.S. Environmental Protection Agency, The Causes and Control of Activated Sludge Bulking and Foaming - Summary Report, EPA/625/8-87/012, USEPA, Cincinnati, Ohio (July 1987) Givens, S. W. and Grady, C. P. L. et al, Biological Process Design and Pilot Testing for a Carbon Oxidation, Nitrification, and Denitrification System, Environmental Progress Vol. 10, No. 2 (May 1991). Water Pollution Control Research Series, No. 12020, 2/70, Petrochemical Effluents Treatment Practices Detailed, U.S. Department of the Interior, Federal Water Pollution Control Administration (February 1970). Water Environment Federation, Manual of Practice No. 8 - Design of Municipal Wastewater Plants, WEF and ASCE, Book Press, Brattleboro, VT (1992) Eckenfelder Jr., W. W., et al, Activated Sludge Treatment of Industrial Wastewater, Technomic Publishing Co, Inc., Lancaster, PA, (1995) Mange, O., and Gros, H., Technical Advances in Biofilm Reactors, IAWPRC Congress, Nice, (1989) Fiessinger, F., Water Treatment Technologies for the Challenges of the Nineties, in Water Treatment - Proceedings of the 1st International Conference, Elsevier Science Publishers Ltd, England (1991) Sutton, P. M. and Mishra, P. N., Biological Fluidized Beds for Water and Wastewater Treatment, Water Environment & Technology, August 1991 Tsubone, T., Ogaki, Y., Yoshiy, Y. and Takahashi, M., Effects of Biomass Entrapment and Carrier Properties on the Performance of an Air-Fluidized-Bed Biofilm Reactor, Water Environment Research, Volume 64, Number 7 (Nov/Dec 1992) Lessel, T. H., First Practical Experiences with Submerged Rope-Type Biofilm Reactors for Upgrading and Nitrification, Water Science Technology, Vol 23 (1991) Robertaccio, F. L, Polyelectrolyte Guide, ER&E Report No. EE.20E.84, 1984. Water Environment Federation, Guidance Manual for Polymer Selection in Wastewater Treatment Plants Esler, John, Optimizing Clarifier Performance, CPE Services, Inc., New York State Department of Environmental Conservation, Albany, New York. Water Environment Federation, Aeration - Manual of Practice FD-13, Alexandria, Virginia (1988). Clesceri, L., et al, Standard Methods for the Examination of Wastewater, 17th Edition, American Publish Health Association, Washington, DC, 1989. Stover, E. N., Kincannon, D. F., Rotating Biological Contactor Scale-Up and Design, WE&M - Reference Handbook (1980) Thibault, G. T., Wastewater Treatment by Aerobic Biological Oxidation - Alternatives to Activated Sludge, ER&E Report No. EE.62E.76 Lindquist, L. A., Performance of a Rotating Biological Contactor During Cyanide Shock Loading, ER&E Report No. EE.86E.80 Fort, L. R., Enhanced Water Treatment System (EWETS-PjBM/PSM), 92ECS2 104 Eckenfelder Jr., W. W., Patoczka, J., Watkin, A. T., Wastewater Treatment, Chem. Eng. (September 2, 1985)

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2.0 REFERENCES (Cont) 36. U.S. Environmental Protection Agency, Upgrading Trickling Filters, EPA/430/9-78-004, USEPA, Washington D.C. (July 1978) 37. Neu, K. E., Upgrading of Rotating Biological Contactor (RBC) Systems to Achieve Higher Effluent Quality, Including Biological Nutrient Enrichment and Reduction Techniques, Wat. Sci. Tech, Vol. 29, No. 12, pp. 197 - 206 (1994) 38. Hao, O. J., et al, Biological Fixed-Film Systems, Research Journal WPCF, Vol. 63, Number 4, pp 388 - 394 (June 1991) 39. Ibrahim, A. A., et al, Biological Fixed-Film Systems, Water Environmental Research, Vol. 66, Number 4, pp. 336 - 342 (June 1994) 40. Galil, N., M. and Rebhun, A Comparative Study of RBC and Activated Sludge in Biotreatment of Wastewater From an Integrated Oil Refinery, 44th Purdue Industrial Waste Conference Proceedings, pp. 711 - 717 (1990) 41. Kigel, M. Y., Shultis, J. F., Wastewater Treatment Technologies Accomplished in a Pseudofluidized Bed Reactor, Wat. Sci. Tech, Vol. 26, No. 9 - 11, pp. 2501 - 2504 (1992) 42. Ro, K. S., Neethling, J. B., Biofilm Density for Biological Fluidized Beds, Research Journal WPCF, Vol. 63, Number 5, pp. 815 - 818 (July/August 1991) 43. Odegaard, H., Rusten, B., Westrum, T., A New Moving Bed Biofilm Reactor - Applications and Results 44. Missouri Basin Engineering Health Council for the U.S. Environmental Protection Agency , Waste Treatment Lagoons State of the Art, Project #17090 EHX (July 1971) 45. Rich, L. G., Low-Maintenance, Mechanically Simple Wastewater Treatment Systems, McGraw-Hill, New York (1980) 46. Rich, L. G., Designing Aerated Lagoons to Improve Effluent Quality, Chemical Engineering, May 30, 1983, pp. 67 - 70 47. U.S. Environmental Protection Agency, Municipal Wastewater Stabilization Ponds - Design Manual, EPA / 625/1-83/015, USEPA, Cincinnati, Ohio (October 1983) 48. U.S. Environmental Protection Agency, Retrofitting POTW's - Handbook, EPA/625/6-89/020, USEPA, Cincinnati, Ohio (July 1989) 49. Goodrich, R. R. and Urban, D. B., Refinery Process Unit Wastewater Load Factors-Final Report, EE.86E.86. 50. Burdick, C. R., Refling, D. R., Stensel, H. D., Advanced Biological Treatment to Achieve Nutrient Removal, Journal WPCF, Vol. 54, No. 7 (July 1982) 51. Christiansen, J. A., Kilgallen, P., Roy, S., Use of a Commercially Prepared Nitrosomonas and Nitrobacter Inoculum to Induce Nitrification in a Biological Treatment System 52. Hem, L. J., Rusten, B., Odegaard, H., Nitrification in a Moving Bed Biofilm Reactor, Wat. Res. Vol. 28, No. 6, pp. 14251433 (1994) 53. Johnson, W. K., Schroepfer, G. J., Nitrogen Removal by Nitrification and Denitrification, Journal WPCF Vol. 36, No. 8 (August 1964) 54. Kaczmarek, S. A., Biological Treatment for Upstream Concentrated Wastewaters, ER&E Report No. EE.46E.84 (June 1984) 55. Kaczmarek, S. A., Nitrification in Refinery Wastewater Treatment, ER&E Report No. EE.62E.85 (October 1985) 56. Picard, M. A., Faup, G. M., Removal of Nitrogen from Industrial Waste Waters by Biological Nitrification - Denitrification, Wat. Pollut. Control (1980) 57. Schmidt, E. L., Belser, L. W., Nitrifying Bacteria, Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties - Agronomy Monograph No. 9, 2nd Edition (1982) 58. Sekoulov, I., Addicks, R., Oles, J., Post-Denitrification with Controlled Feeding of Activated Sludge as H Donator, Wat. Sci. Tech, Vol. 22, No. 7/8, pp. 161 - 170 (1990) 59. Tiedje, J. M., Denitrification, Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties - Agronomy Monograph No. 9, 2nd Edition (1982) 60. Wilson, T. E., Pickard, D. W., Bizzarri, R. E., A Nitrogen Success Story, Wat. Envir. Fed. (September 1994) 61. Water Pollution Control Federation, Nutrient Control - Manual of Practice FD-7, Facilities Design, Alexandria, Virginia (1983) 62. Randall, C.W., et al, Design and Retrofit of Wastewater Treatment Plants for Biological Nutrient Removal, Water Quality Management Library Vol. 5, Technomic Publishing Co., Inc., Lancaster, Pennsylvania (1992) 63. U.S. Environmental Protection Agency, Nitrogen Control - Manual, EPA/625/R-93/010, USEPA, Cincinnati, Ohio (September 1993)

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3.0 DEFINITIONS Active Solids / Biomass - The portion of the solids in a biological system composed of microorganisms that are actively metabolizing the substrate (removing the organic or inorganic contamination). Non-biodegradable solids accumulate in the activated sludge system and reduce the percentage of active solids (biomass / organisms) in the system. Aerated Lagoon - An oxidation pond with aeration devices. Mixing energy supplied to an aerated lagoon is usually insufficient to completely mix the system. Aerobic - A system or process which is active in the presence of dissolved oxygen. In biological waste treatment, aerobic refers to a microbiological system in which microorganisms use dissolved oxygen in the metabolism of the substrate (remove contaminants). Alkalinity - Alkalinity of a water is its acid-neutralizing capacity. Alkalinity is destroyed in the biological process of nitrification. Nitrification consumes 7.15 mg of alkalinity (expressed as CaCO3) per mg of ammonia-nitrogen nitrified. As a rule of thumb, 150 mg/L alkalinity measured as calcium carbonate is needed for nitrification. Anaerobic - A system or process which is active in the absence of dissolved oxygen. In biological waste treatment, anaerobic refers to a microbiological system in which microorganisms metabolize the substrate in the absence of dissolved oxygen. Anoxic - A term frequently used to describe a system or process which is active in the absence of dissolved oxygen but in the presence of nitrate. In these systems, nitrate, not dissolved oxygen, acts as the terminal electron acceptor for the metabolism of the substrate. Autotrophs - Organisms that obtain carbon for the formation of cell tissue from dissolved carbon dioxide. Biochemical Oxygen Demand (BOD, BOD5, BODULT) - A general measure of organic material in wastewater samples that can be biologically degraded. It is the quantity of oxygen consumed during the biological decomposition (oxidation) of material in water. Certain inorganic compounds that exert an immediate oxygen demand (e.g., sulfite) will be detected in the BOD test. BOD is usually measured over a specific time period; a five-day period is commonly used, with the result expressed as BOD5. If the biological decomposition is allowed to proceed to completion, the quantity of oxygen consumed is termed the ultimate BOD, often designated BODULT and is normally measured over 20 days. In this case, some nitrogen compounds can be oxidized, a process called nitrification. BOD is normally expressed mg/L (ppm). BOD5 is typically 60 percent of BODULT. BIOX - Abbreviation for BIological OXidation and commonly used to describe an activated sludge system but can be used in reference to other aerobic biological oxidation processes used to treat wastewater. Bulking - An upset condition in the settling basin of a biological oxidation system during which the bio-sludge doesn't settle, or settles slowly, and leads to floc or suspended solids carryover with the effluent. Bulking is usually associated with filamentous bacterial growths. Carbonaceous Biological Oxygen Demand (CBOD) - A general measure of organic material in wastewater samples that can be biologically degraded; similar to BOD, but a nitrification inhibitor is used to eliminate the interference of nitrifying bacteria. Chemical Oxygen Demand (COD) - A measure of the amount of organic or reduced inorganic compounds in a sample that can be oxidized by a strong oxidizer, usually potassium dichromate and sometimes potassium permanganate. COD of a wastewater is generally greater than the BOD since the wastewater may contain oxidizable material that cannot be biologically degraded. The COD test is simpler and faster than the BOD test. Caution must be used when analyzing the COD in high salt (chloride) wastewater streams since the salts will interfere with the test results. COD is expressed as ppm or mg/L. Denitrification - The process by which specific microorganisms biologically convert nitrate or nitrite into nitrogen gas. The principal biochemical pathway for denitrification involves using the oxygen bound up in the nitrate or nitrite, and not free oxygen, as the terminal electron acceptor in the oxidation of the organic compounds. Dissolved Oxygen (DO) - Dissolved oxygen level, measured in mg/L (ppm), is an important monitoring parameter for biological systems and receiving water bodies. It indicates whether a biological treatment unit can sustain a healthy aerobic microbial population or whether the receiving water body can sustain microbial, aquatic fish, or plant life. A minimum DO value for healthy aerobic biological treatment systems is between 1 to 2 ppm (mg/L). The maximum concentration soluble in water under normal conditions (saturation concentration) is between 8 to 10 ppm (mg/L) and is a function of salinity and temperature. Higher DO levels are possible in high purity oxygen activated sludge systems. Endogenous Respiration - The energy required for cell maintenance. Other factors such as cell death and predation are usually combined with endogenous respiration in a term called endogenous decay. Enzymes - Biological catalysts. Several vendors offer these chemicals to enhance biotreatment. Facultative Bacteria - Bacteria that can grow in either an aerobic or an anaerobic environment Filamentous Growth - Thread-shaped microorganisms sometimes found in activated sludge plants. Excessive filamentous growth is to be avoided because they are difficult to settle. Filamentous growth can be controlled by maintaining a proper F/M ratio or SRT. Severe filamentous growth may require drastic control, i.e., chlorination or peroxide. Floc - A flocculent mass formed by the aggregation of a number of fine suspended particles.

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3.0 DEFINITIONS (Cont) F/M Ratio - Food to Mass ratio which corresponds to the amount of substrate (organic contamination) removed per day per total mass of activated bio-sludge in the aeration basin. F/M ratio is normally expressed as pounds of BOD5 or TOC removed per day per pound of MLVSS in the aeration basin. The growth and settling characteristics of the biomass in an activated sludge plant depend upon the F/M ratio. F/M ratio varies inversely with the solid retention time (SRT). Heterotrophs - Organisms that obtain carbon for the formation of cell tissue from dissolved organic substrates. Hydraulic Retention Time (HRT) - The length of time the influent wastewater is retained in the aeration basin (not including the effect of sludge recycle; i.e., aeration volume divided by influent volumetric flowrate). Inert Solids - Those solids within the aeration basin that are not active. Inert solids can include non-degradable solids which enter with the feed, activated carbon, cell debris from dead organisms, and extra-cellular by-products produced by living organisms. Mean Cell Residence Time (MCRT) - See Solids Retention Time (SRT). Mixed Liquor - The contents of the aeration basin, consisting of the wastewater and cell biomass. Mixed Liquor Suspended Solids (MLSS) - The concentration of total suspended solids in the aerated section of a biological treatment unit or lagoon. MLSS is normally expressed in units of ppm or mg/L. Mixed Liquor Volatile Suspended Solids (MLVSS) - The portion of the MLSS which volatilizes at 1022°F (550°C). Biological solids (microorganisms) are the main contributor to MLVSS. For systems which add powdered activated carbon, a separate acid digestion step can be done to distinguish between MLVSS due to carbon and MLVSS due to biomass. MLVSS is normally expressed in units of ppm or mg/L. Nitrification - Nitrification is a biological process where ammonia is converted / oxidized to nitrate. The process involves one microorganism species (Nitrosomonas) converting ammonia to nitrite and then a different microorganism species (Nitrobacter) converting the nitrite to nitrate. This term should not be confused with “denitrification," which refers to the biological process of converting nitrates to nitrogen gas. Nutrients - Chemical elements such as nitrogen, potassium, phosphorous, sulfur, cobalt, zinc, and copper, which are essential for microbial growth. As a rule-of-thumb, a ratio of BOD5, nitrogen, and phosphorus (BOD5:N:P) = (100:5:1) is needed for biological activity. Organic Loading - The daily rate of BOD5 applied per unit volume of attached growth system media. Organic loading is expressed in units of lb. of BOD5/d/1000ft3 of system volume or kg of BOD5/d/m3 of system volume. Oxidation Pond - A basin in which wastewater undergoes biological oxidation treatment by the action of algae and bacteria but without the aid of aeration devices, mixing devices, or sludge recycle. pH - A measurement of the acidic or basic character of a solution at a given temperature. It is defined as the negative logarithm (to the base 10) of the hydrogen ion concentration (-log[H+]). Pure water is slightly ionized with a pH of 7, and at equilibrium the ion product, Kw , is [H+][OH–] = 1.01 x 10–14 at 25°C. Generally, biological treatment systems operate best at pH ranges between 6.5 to 8.5. Wastewater outside the 5.5 to 9.5 pH range (before being commingled in the mixing zone of the wastewater effluent and the receiving water body) can potentially cause harm to the receiving water aquatic life as well as the biological treatment microorganisms. Recycle - The portion of solids which is taken from the clarifier underflow and routed back to the aeration basin to control the mixed liquor microbe population in the bioreactor, sometimes referred to as RAS, Recycle Activated Sludge. Return Activated Sludge (RAS) - The portion of solids which is taken from the clarifier underflow and routed back to the aeration basin to control the mixed liquor microbe population in the bioreactor. Sludge Blanket - The level of sludge in the settling basin (clarifier), usually expressed in ft (m). Sludge Volume Index (SVI) - A measure of compaction of sludge after gravity settling. SVI is the volume in milliliters occupied by one liter of activated sludge after 30 minutes of settling divided by the MLSS concentration. SVI = [(ml of settled sludge / liter of initial suspension) / (mg/L of MLSS)] x 1000 Normally, the SVI of the sludge varies with the F/M ratio and the SRT in the aeration basin. Values below 100 are generally indicative of good settling and compaction. Care must be taken in use of the SVI in systems with high MLSS levels, e.g., high purity oxygen systems or very high sludge ages. Solids Residence Time (SRT) - The average length of time the solids are held in the system expressed in days, sometimes referred to mean cell residence time (MCRT). SRT = (Total mass of solids in the aeration basin and clarifier) / (mass of solids lost both intentionally and unintentionally per day) Specific Surface - The total surface area of the attached growth system media per unit of bulk system-media volume - both measured without slime growth - expressed in units of ft2/ft3 or m2/m3. ExxonMobil Research and Engineering Company – Fairfax, VA

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3.0 DEFINITIONS (Cont) Substrate - Carbon and energy sources needed to promote the growth of microorganisms. The organic waste which is used as food by the microorganisms to produce more organisms, water, carbon dioxide, and intermediate by-products. Total Dissolved Solids (TDS) - A measure of all dissolved material in a solution, including inorganic salts (e.g., NaCl, MgCl, etc.) that typically make up the bulk of the TDS measured in the standard lab test. TDS is used to determine the salt levels of wastewater. Measurement for TDS consists of passing a sample through a standard glass fiber filter, and the filtrate is evaporated to dryness in a weighed dish and dried to a constant weight at 356°F (180°C). The material remaining on the filter paper is the total dissolved solids and is reported in units of ppm or mg/L. Conductivity can be used for a quick substitute measurement for TDS (reported in units of micromhos or microsiemens). As a rule-of-thumb, for wastewater streams at pH 7, the TDS of that stream in ppm (mg/L) can be approximated by multiplying the conductivity in units of micromhos or microsiemens by 0.7. Total Suspended Solids (TSS) - The amount of suspended matter removed by a 0.45 micron filter when a wastewater sample is dried at 217°F (103°C) and is reported in units of ppm or mg/L. Inorganic particles such as clay or grit as well as organic particles (biological solids including algae) contribute to the suspended solids concentration. Theoretical Oxygen Demand (ThOD) - The oxygen required to oxidize all organics or reduced inorganic compounds to CO2, SO4, NO3, etc. In practice, the TOD and COD measure by analytical procedures will approach the calculated ThOD. ThOD is normally expressed as ppm or mg/L. Total Organic Carbon (TOC) - The quantity of organically bound carbon in a sample. TOC is commonly used as a replacement for BOD since the test for TOC is significantly faster than the 5-day test for BOD, and the BOD test can sometimes give erroneous results. TOC is normally expressed as ppm or mg/L. Total Oxygen Demand (TOD) - The amount of oxygen required to oxidize all oxidizable substances in a sample, including the biodegradable organic matter. It is measured using a special analytical instrument. TOD is normally expressed as ppm or mg/L. Total Kjeldahl Nitrogen (TKN) - A measure of total organic and ammonia nitrogen (does not include nitrite, nitrate). Volatile Suspended Solids (VSS) - Volatile suspended solids is the portion of the TSS (or MLSS) which volatilizes at 1022°F (550°C). Biological solids (microorganisms) are the main contributor to VSS. VSS is normally expressed in units of ppm or mg/L. Waste Bio-Sludge - The portion of excess biomass that is intentionally discharged from the activated sludge system. Normally bio-sludge is intentionally wasted by purging a portion of the return activated sludge from the clarifier underflow or recycle line.

4.0 BACKGROUND AND SELECTION CRITERIA 4.1 BACKGROUND Biological treatment systems are extensively used for the treatment of commercial, industrial, laboratory and municipal wastewaters. The main reason they are preferred by water pollution control professionals in government, academia and industry is that they generally are the most cost effective end-of-pipe systems to remove simple and complex organic and some inorganic contaminants. They are especially effective in removing several types of contaminants simultaneously, resulting in an effluent that can meet several types of water quality criteria, including oxygen demand, low concentrations of toxic compounds and low levels of aquatic toxicity. However, biological treatment may not be the most suitable technology for low volume, limited compound wastewater. Biological treatment works by employing naturally occurring, living and reproducing microorganisms (bacteria, protozoa, fungi and algae) that use undesirable contaminants in wastewater as food, energy sources, and nutrients. The main difference between industrial wastewater biological systems and sanitary / sewage systems is that pathogenic or disease forming microorganisms are usually not present in significant numbers in industrial systems and the microbe populations are quite different. Hence, this is the reason why sanitary wastewater from human sources at the facility are usually treated separately from industrial wastewater and there is generally a much lower exposure risk to industrial treatment works operators by organisms that can affect human health. Biological systems, whether in the form of controlled bioreactors (activated sludge type) or simple, naturally aerated lagoon / ponds, require two key capabilities in design, the biological reaction zone and biomass / suspended solids settling zone. In many cases, settling of the biomass is the key and most difficult part of the process, since it is needed to meet effluent quality requirements for suspended solids and oxygen demand and different microbe populations exhibit different settling characteristics.

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4.0 BACKGROUND AND SELECTION CRITERIA (Cont) 4.2 SELECTION CRITERIA A description of the different types of biological treatment systems is provided in each of the areas of this practice that covers the technology. The first step in selecting a system is to evaluate the oxygen demand or dissolved organic reduction requirements for the particular application. Figure 4.2-1 provides a decision tree to assess general requirements. The next step is to see how the biological system would fit into the overall treatment plant wastewater system. Figure 4.2-2 provides an example; more details on wastewater treatment process selection can be found in Section XIX-A of the Design Practices. The next step is to evaluate the effluent requirements of the particular application and integrate the expected performance of the biotreatment with the rest of the treatment system. For the petroleum refinery and petrochemical plant applications, aerobic lagoon or activated sludge systems are the most prevalent systems in ExxonMobil plants. New types of bioreactor systems are being suggested and applied for selected applications in municipal and industrial wastewater treatment. Due to a general lack of data on the performance of these systems on petroleum refinery wastewaters, pilot testing would typically be in order for cases where the benefits of these systems over activated sludge are desired. Anaerobic (without oxygen addition) biological treatment is known to have been applied to only one ExxonMobil location on a large scale, for a wastewater from a crude oil production treatment facility. This type of system was applied because of the concentrated nature of the produced water; a rule-of-thumb concentration for consideration of anaerobic treatment is over 3,000 ppm of oxygen demand. In this particular location, conventional aerobic activated sludge treatment followed the anaerobic treatment unit to ensure effluent quality requirements were met. Fixed media or film bioreactors are being applied cost-effectively in selected applications for petroleum based wastewater. These are currently being evaluated for use on hydrocarbon contaminated groundwater and other low organic contaminated wastewater from the petroleum industry.

5.0 SUMMARY OF BIOLOGICAL TREATMENT SYSTEMS The following provides a summary of the various types of Biological Treatment Systems and the sections within this DP covering each type:

•

Aerobic Suspended Growth Systems a. Activated Sludge (Section 6.0) b. Sequency Batch Reaction (Section 7.0) c. Aerated Lagoons (Section 8.0)

• •

Aerobic Attached Growth

(Section 9.0)

Anaerobic Systems

(Section 10.0)

•

Anoxic Systems

(Section 11.0)

6.0 ACTIVATED SLUDGE SYSTEMS 6.1 DESCRIPTION Figure 6.1-1 presents a general schematic of the typical activated sludge process. Pretreated wastewater containing soluble organic and inorganic compounds is introduced into a reactor where an aerobic culture of bacteria and other microorganisms is maintained in suspension. The aerobic environment is maintained by the use of diffused or mechanical aeration, which also serves to keep the contents well mixed. The microorganisms utilize the dissolved substances to obtain energy and, in the presence of oxygen and nutrients, convert them to carbon dioxide, water, and more microorganisms. After a specified period of time, the mixture of microorganisms flow into a clarifier where the microorganisms are separated from the treated wastewater. The majority of the microbial cells (biomass) is returned to the reactor to maintain the desired microbial concentration, while the remainder is purged from the system. (Reference 1, 2) Typically, most activated sludge systems are designed and operated as a continuous flow process. Facilities with small flows may use a sequencing batch reactor (fill-and-draw) approach. Reactors are often designed as completely mixed tanks, although plug flow designs are also common. While there are many variations in process design and configuration, fundamentally they are all similar as described above. (Reference 1, 5) Process Microbiology To design and operate an activated sludge system efficiently, it is necessary to understand the importance of the microorganisms in the system. The most important and abundant group of microorganisms is the bacteria. They are the organisms responsible for the decomposition of the organic matter in the influent. Additionally, they are responsible for forming flocs which permit the bio-sludge to be separated from the treated water. The community also contains protozoa which are not

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) an important means of organic waste removal. However, they do predate on bacteria and remove excess, non-flocculated bacteria from the wastewater, helping to clarify the effluent. A third category are nuisance organisms, those that when present in sufficient numbers interfere with the proper operation of the process. Most problems arise with respect to sludge settling and are the result of filamentous bacteria and fungi. These organisms can reduce the specific gravity of the flocs so that the sludge is very difficult to separate by gravity clarification. Consequently, an effective design is one that allows rapid decomposition of the waste, fosters good microbial flocculation, and selects against nuisance organisms. (Reference 3) There are two primary biochemical reactions that occur in biological wastewater treatment, and both involve the utilization of energy. A portion of the soluble organic matter in the wastewater is used by the organisms as food to obtain energy and the remainder of the organic matter is used for the synthesis into new microbe cells. In simplest form, synthesis can be expressed as: (Reference 1, 3, 5) Organics + O2 + Nutrients → More (New) Microorganisms + CO2 + H2O + Energy + Non-degradable Soluble Residue

Eq. (6.1-1)

The second reaction involves the energy required for cell maintenance. This is the energy needed to keep cells functioning even in the absence of growth. As the amount of organic matter is decreased, less energy will be available for new growth. When the point is reached at which the rate of energy supply (food or organic matter) just balances the rate at which energy must be used for maintenance, no net growth will occur because all energy will be used to maintain the status quo. If the rate of energy supply is reduced still further, the difference between the supply and the maintenance requirement will be met by the degradation of energy sources available within the cell, i.e., by endogenous respiration (the microbes will use each other for a food source). This will cause a decline in the mass of the culture. Simply stated, this reaction can be expressed as: (Reference 1, 3, 5) Microbe Cells (C5H7NO2) + 5 O2 → 5 CO2 + 2 H2O + NH3 + Non-degradable Cellular Residue + Non-degradable Soluble Residue

Eq. (6.1-2)

Contaminants other than organic compounds can also be biologically removed in the activated sludge process. For example, in the presence of two specific types of bacteria, ammonia can be removed from the wastewater through conversion to nitrate. This process is called nitrification, and can be expressed as follows: Nitrosomonas

NH +4

+ 1.5 O2

New Cells + NO 2− + H2O + 2 H+ + Energy

Eq. (6.1-3)

New Cells + NO 3− + Energy

Eq. (6.1-4)

Nitrobacter NO 2− + 0.5 O2

Carrying this one step further, under anoxic (without free oxygen) conditions, nitrate can be biologically removed from the wastewater by conversion to nitrogen gas in a process called denitrification: Microorganisms Organics + NO 3−

New Cells + N2 + CO2 + H2O + OH– + Energy

Eq. (6.1-5)

The microorganisms responsible for carrying out denitrification are similar to the ones responsible for the removal of organics as described in Eq. 6.1-1. However, the principal biochemical pathway for denitrification involves using the oxygen bound up in the nitrate, and not free oxygen, as the terminal electron acceptor in the oxidation of the organic compounds. Reaction Kinetics and Fundamental Expressions

Biological oxidation of organic matter is a process governed by a multitude of reactions catalyzed by microbially produced enzymes. The rates of substrate removal and cell growth depend on the composition and concentration of both the organic material and the metabolizing microbial population, and on the temperature. While the heterogeneous nature of industrial wastewater and the activated sludge microbial community make modeling these kinetics somewhat difficult, some proven approaches have been developed. The activated sludge process has successfully been modeled under the assumption that the organic substrate, S, is limiting to growth. S is usually expressed in terms of BOD5, but COD, TOD and sometimes TOC can also be used. Experimentally, it has been found that the effect of a limiting substrate or nutrient can often be adequately defined using the following expression proposed by Monod. (Reference 3, 4, 5)

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) u =

(um ) (S) (K s + S)

where: u um S Ks

= = = =

Eq. (6.1-6) Specific microbial growth rate, (1/d) Maximum specific microbial growth rate, (1/d) Concentration of growth limiting substrate in solution (e.g., BOD or TOC), mg/L Substrate concentration at one-half the maximum growth rate, mg/L

The effect of substrate concentration on the specific growth rate is shown in Figure 6.1-2 (Reference 1, 3) Substrate (BOD) Removal

The rate of substrate removal is controlled by the growth rate of the microorganisms and is related to this growth by the microorganism yield coefficient. The relationship between the rate of substrate removal and the microbial growth rate can be expressed as follows: u q = Eq. (6.1-7) Y where: q u Y

= = =

Specific rate of substrate removal, (1/d) Specific microbial growth rate, (1/d) Microbial yield coefficient, mass microorganisms / mass substrate removed

The microbial yield coefficient, Y, is a function of the bio-sludge residence time (SRT) and can be described by the following equation: Ym Eq. (6.1-8) Y = 1 + (b) (SRT ) where: Y = Ym =

b = SRT =

Microbial yield coefficient, mass microorganisms / mass substrate removed Maximum yield coefficient, mass microorganisms / mass substrate removed, measured when the microorganisms are in the logarithmic growth phase Endogenous decay coefficient, (1 / day) Sludge retention time, d

For a continuous stirred tank reactor (CSTR) or completely mixed bioreactor at steady state, the solids leaving the system will be equal to the solids produced. Therefore, the growth rate and solids retention time of the microorganisms in the system are related by: 1 = u−b Eq. (6.1-9) SRT where: SRT = u = b =

Sludge retention time, d Specific microbial growth rate, (1/d) Endogenous decay coefficient, (1/d)

Combining Eqs. 6.1-7 and 6.1-9: 1 + b q = SRT Y

Eq. (6.1-10)

The specific rate of substrate removal can also be described, based on a mass balance on the rate of substrate removal: q =

(So − S) ( Xv ) (HRT )

where: q = So = S = Xv = HRT =

Specific rate of substrate removal, (1/d) Feed substrate concentration, mg/L Effluent substrate concentration, mg/L Mixed liquor volatile suspended solids, mg/L Hydraulic residence time, d

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Eq. (6.1-11)

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Combining Eqs. 6.1-10 and 6.1-11 gives an expression for the concentration of active heterotrophic biomass in the reactor: Xv =

(So − S ) Y æ 1 ö + b÷ HRT ç SRT è ø

Eq. (6.1-12)

Note: In Eq. 6.1-12, the term “b” is usually assumed to be zero. This is because a small change in this somewhat unreliable coefficient can result in significant changes to the calculated number. Experience has shown that dropping this term from this equation provides a good approximation of the actual MLVSS. Lastly, the excess biological sludge production can be estimated by the following expression: (Reference 5)

Px = Q Y(So - S)[8.34 {(lb/Mgal)(mg/L)}]

(Customary)

Eq. (6.1-13)

Px = Q Y(So - S)(10-3 kg/g)

(Metric)

Eq. (6.1-13)M

where: Px Q

= =

Y

=

So S

= =

Net excess sludge production, lb/d (kg/d) Wastewater flow rate, Mgal/d (m3/d) mass microorganisms Microbial yield coefficient, mass substrate removed Feed substrate concentration, mg/L Effluent substrate concentration, mg/L

It should be mentioned that various other expressions have been used to describe the rates of specific growth and substrate utilization for the activated sludge process. (Reference 1, 3, 4, 5) What is fundamental in the use of any rate expression is its application in a mass-balance analysis. It does not matter if the rate expression selected has no relationship to those used commonly in the literature, so long as it adequately describes the observed phenomenon. (Reference 5) In addition, where the removal of a specific organic constituent or nutrient (e.g., ammonia) is required, the rates of removal of such substances need to be considered. For example, where ammonia discharge is limited, the kinetics and expressions governing the rate of ammonia removal can be expected to govern the design of the activated sludge process. The model and equations described above for heterotrophic organic removal are also applicable to a system designed for ammonia removal, however, the kinetic coefficients may be different between heterotrophs and nitrifiers. (Reference 11) Temperature Effect / Importance

The effects of temperature must also be considered in the design of biological treatment systems. A general rule of thumb is that the removal rate of organic matter is doubled every 18°F (10°C) temperature rise in the 50 to 105°F range (10 to 40°C). Temperature not only influences the metabolic activities of the microbial population but also has a profound effect on such factors as gas-transfer rates and the settling characteristics of the biological solids. The effect of temperature on the activated sludge process is usually expressed as follows: (Reference 5) RT = (R20)[(1.08)(T–20)] where: RT (R20) 1.08 T

= = = =

Eq. (6.1-14)

Reaction rate at T °C, also qT (specific rate of substrate removal at temp = T °C) Reaction rate at 20°C, also q20 (specific rate of substrate removal at 20°C) Temperature activity coefficient for activated sludge processes Temperature, °C

Oxygen Requirements

The theoretical oxygen requirements can be determined from the BOD5 of the wastewater and the amount of biomass wasted from the system per day. If all the BOD5 were converted to end products, the total oxygen demand would be computed by converting BOD5 to the ultimate carbonaceous BODULT using an appropriate conversion factor. However, during the organic removal, process oxygen is utilized in the process of providing energy for both the synthesis of new cell material (growth) and for basic cell maintenance (respiration). Since at steady state the new cells are wasted from the system, the BODULT of the wasted cells must be subtracted from the total. The remaining amount represents the amount of oxygen that must be supplied to the system. From Eq. 6.1-2, it is shown that the BODULT of one mole of cells (approximate molecular weight = 113) is equal to 1.42 times the concentration of cells. Oxygen is also required for other oxygen demanding compounds not captured in the BOD test, such as nitrogen compounds.

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Without Pilot Data Therefore, the theoretical oxygen requirements for the removal of the carbonaceous organic matter in the wastewater can be computed using Eq. 6.1-15 (Reference 5). Some contingency to the oxygen demand calculation is usually applied to ensure sufficient oxygen is available. If the design flowrate and influent substrate concentration is used in Eq. 6.1-15, a 25 percent contingency is recommended. If the average flowrate and influent substrate is used in Eq. 6.1-15, a 50 percent contingency is recommended.

lb O 2 /d =

Q (S o − S) (8.34) − 1.42 (Px ) f

(Customary)

Eq. (6.1-15)

kg O 2 /d =

Q (S o − S) (10 -3 kg/g) − 1.42 (Px ) f

(Metric)

Eq. (6.1-15)M

where: f

=

Q = So = S = 8.34 = Px =

Conversion factor for converting BOD5 to BODULT (0.45 - 0.70) (this factor is wastewater dependent) Design wastewater flowrate, Mgal/d (m3/d) Influent substrate concentration BOD5, mg/L Effluent substrate concentration BOD5, mg/L Conversion factor [lb/Mgal-(mg/L)] Net excess sludge production, lb/d (kg/d) {calculated from Eq. 6.1-13.}

Y is calculated from Eq. 6.1-8, using Table 6.1-1 to for values of Ym and b. When nitrification has to be considered, the total oxygen requirements can be computed as the oxygen for removal of carbonaceous matter plus the oxygen required for the conversion of ammonia to nitrate as follows: (Reference 5) lb O2 / d =

Q (So − S) (8.34) − 1.42 (Px ) + 4.57Q (Nio − Ni ) (8.34) f

Eq. (6.1-16)

kg O2 / d =

Q (So − S) (103 g / kg)−1 − 1.42 (Px ) + 4.57Q (Nio − Ni ) (103 g / kg)−1 f

Eq. (6.1-16)M

where: Nio = Ni = 4.57 = f =

Q = So = S = 8.34 = Px =

Influent TKN, mg/L Effluent TKN, mg/L Conversion factor for amount of oxygen required for complete oxidation of TKN Conversion factor for converting BOD5 to BODULT (0.45 - 0.70) (this factor is wastewater dependent) Design wastewater flowrate, Mgal/d (m3/d) Influent substrate concentration BOD5, mg/L Effluent substrate concentration BOD5, mg/L Conversion factor [lb/Mgal-(mg/L)] Net excess sludge production, lb/d (kg/d) - calculated from Eq. 6.1-13 using Y which is calculated from Eq. 6.1-8 using values of Ym and b from Table 6.1-1.

With Pilot Data There are two approaches for estimating oxygen demand if pilot plant data are available. The approach used depends on the amount of the data available. For the first method, the oxygen requirements can be calculated using Eq. 6.1-17 if this data is available. Contingency depends on the characteristics of the flowrates and influent substrate concentration plugged into the equation. Add 25 percent contingency when maximum values are used and 50 percent contingency when averages are used. When a large amount of data is collected either with a pilot unit or an existing full scale unit that covers both peak flows and substrate influent concentrations, the recommended approach to calculating oxygen demand is by calculating the daily oxygen demand by Eq. 6.1-15 or 6.1-16 (pairing flow with contaminant level, organics, nitrogen, sulfite, etc.) and using a probability plot. The 95th to 99th percentile of the oxygen demand is normally chosen for design specifications, but the ultimate decision is left up to the designer.

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Rr =

a′ Q (So − S) + b′ ( X v ) V V

where: Rr V a′

Eq. (6.1-17)

= = =

Oxygen utilization rate, (mg/L-d) Aeration tank volume, Mgal (m3) Oxygen utilization coefficient for synthesis, lb (kg) O2/lb (kg) organics removed b′ = Oxygen utilization coefficient for endogenous respiration, lb (kg) O2/lb (kg) VSS-d Q, So, S and Xv as described previously

The total oxygen requirement term Rr is usually determined from the following equation: Rr = KLa (Cs - C) where: KLa = Cs = C =

Eq. (6.1-18)

Overall oxygen transfer coefficient, (1/d) Saturation oxygen concentration, mg/L Actual oxygen concentration, mg/L

Process Variations

The activated sludge process is very flexible and can be adapted to address many types of biological wastewater treatment problems. Diagrams of BOD removal and bio-sludge growth relationships for the process are given in Figure 6.1-3. (Reference 5) The most common configurations are the completely mixed single stage and conventional plug flow designs, and most of the process variations are modifications of these two basic approaches. Each variation tends to offer its own unique advantages. A few of the more common variations of the activated sludge process are discussed below. Completely Mixed Activated Sludge (Figure 6.1-1) - By definition, the contents of a completely mixed reactor are thoroughly uniform. Pretreated influent wastewater is rapidly distributed throughout the basin and the operating characteristics of the system (e.g., MLSS, substrate concentration, oxygen uptake rate, etc.) are relatively constant. As the concentration of substrate in the reactor is the same as in the effluent, only a very low level of food is generally available at any time to the large mass of microorganisms. This characteristic is cited as the major reason why, when compared to plug flow systems, completely mixed systems are better capable of handling surges in organic loading and toxic shocks without adversely affecting effluent quality. This increase in process stability accounts for the reason why so many activated sludge systems have been designed as completely mixed reactors. (Reference 2) Extended Aeration / Nitrifying Activated Sludge - Extended aeration systems are usually complete-mix designs where the microorganisms are kept in the endogenous respiration phase of the growth curve. The attributes of this system are a long aeration time (> 18 hr), a long sludge retention time (> 20 d), and a low food-to-microorganism ratio (< 0.1 (1/d)). This requires the design have a large reactor / hydraulic residence time, and sufficient clarification capacity to handle a relatively high MLSS (3500 + mg/L) While extended aeration requires more air than other activated sludge processes, it results in very low production of waste bio-sludge. These process conditions are also required if ammonia removal (nitrification) is desired. This is because the two microbial species responsible for nitrification have relatively low specific growth rates. Other advantages are a very low effluent BOD (90 to 95% reduction), and resiliency to upsets due to the buffering effect of the large biomass volume. For most cases, this is the preferred fundamental process configuration. Extended aeration processes can be sensitive to sudden increases in flow due to resultant high MLSS loading on the final clarifier. Other potential problems with this system, especially at very high SRTs / low F/Ms, are the selection of filamentous bacteria, and the occurrence of pin-floc (dead cell bodies) in the final effluent. (Reference 2, 3, 5, 7) Conventional Plug Flow Activated Sludge (Figure 6.1-4) - The conventional plug flow configuration has a high organic loading at the influent end of the basin. Loading is reduced over the length of the basin as the organic material in the wastewater is assimilated. Rapid microbial growth and substrate utilization occurs at the influent end of the reactor, while at the downstream end oxygen consumption primarily results from endogenous respiration. Air application is generally uniform throughout the tank. The advantage of a plug flow design is that the high organic loading at the inlet of the process selects against filamentous bacteria growth and often improves sludge settling beyond that realized from a complete-mix reactor. Another advantage is the configuration is flexible and as discussed below, lends itself to modification. (Reference 2)

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Step-Feed Aeration Activated Sludge (Figure 6.1-5) - In a step-feed aeration configuration, influent wastewater is split and introduced into the aeration basin at different points. This provides a more even distribution of oxygen demand, a more equal distribution of the F/M ratio, and helps lessen the effects of shocks. This configuration is restricted to plug-flow reactors, or to multiple completely mixed reactors in series. An attractive attribute of this variation is its flexibility. The ability to control the feed addition points (and often the distribution of aeration in the basin) allows various portions of the reactor to be adjusted to control nuisance organisms, or to provide for denitrification. (Reference 2, 3, 5) High Purity Oxygen Activated Sludge (Figure 6.1-6) - In this variation, high purity oxygen (HPO) is used instead of air. Oxygen is diffused into a series of covered completely-mixed aeration tanks and is recirculated, while a portion of the gas is wasted to reduce the concentration of carbon dioxide. Data and reports on the benefits of oxygen systems have been under review over the years. (Reference 2) The most significant benefit appears to be conferred for high strength wastewater where normal diffused air aeration is insufficient to transfer the required oxygen. The amount of oxygen provided by a high purity oxygen system is about four times greater than the amount that can be added by conventional aeration systems. (Reference 5) Also, because of these high transfer rates, the necessary reactor size can usually be significantly reduced. In general, these savings in volume must be sufficient to offset the additional oxygen cost associated with this system. Additionally, the wellaerated mixed liquor selects against nuisance organisms thereby tending to produce a compact sludge. Solids levels usually range from 4000 to 9000 mg/L depending on the BOD of the wastewater. ExxonMobil has one of these systems at a chemical plant. This system, however, has some issues that must be considered in design. As a result of the buildup of carbon dioxide, pH control tends to be more difficult than in air systems. There is also a need for careful selection of materials used for construction (due to the corrosive atmosphere), and the potential exists for presence of combustible conditions within the reactor vapor space. Additional sensors and control loops on the off-gas are standard design components in HPO systems. (Reference 2, 3, 5, 7) Completely Mixed Activated Sludge with a Selector (Figure 6.1-7) - If the control of nuisance organism is expected to be of significant concern, then consideration should be given to installing an upstream contact or selector tank ahead of a completely mixed aeration basin. In this manner the environmental conditions of the tank (most importantly the dissolved oxygen content) can be controlled to select against filamentous microorganisms. Readily degradable wastewater such as food processing wastes will tend toward filamentous bulking in a completely-mixed system. Complex refinery / petrochemical wastewaters do not typically support filamentous growth and completely-mixed systems without selectors work very effectively. (Reference 2, 7) A disadvantage of using a selector reactor configuration is the potential for odors to occur. Total Biological Nitrogen Removal Activated Sludge (Figure 6.1-8) - Total biological nitrogen removal involves three distinct processes: (1) the conversion of organic nitrogen compounds to ammonia, (2) the nitrification of ammonia to nitrate, and (3) the denitrification of nitrate to nitrogen gas. The first two steps are aerobic processes, the third occurs in the absence of free oxygen (without added aeration). In the first configuration, a small denitrification reactor is installed upstream of the extended aeration / nitrifying activated sludge tank. The reactor is not aerated, and the nitrified effluent from the activated sludge tank is recycled back into this reactor to provide the nitrate source. As denitrification requires organic carbon as an energy source for the microorganisms, this process actually reduces both the organic and oxygen demand loads on the downstream activated sludge. This has the benefit of reducing energy costs and potentially providing a more stable activated sludge operation. A second common configuration involves installing the anoxic denitrification reactor downstream of the activated sludge tank. This configuration negates the need for effluent recycle back to the front end. However, it has the disadvantage of requiring the addition of a carbon source (methanol is often used). Clarification

The clarification of activated sludge is discussed in detail in Design Practice Section XIX-A7, Clarification Systems for Biological Treatment, and therefore will only be briefly mentioned here. A very important aspect of the activated sludge process is the ability to separate and return the biomass to the main reactor. Efficient separation is important for meeting effluent requirements for TSS, BOD, and COD, and for maintaining an adequate concentration of biomass in the bioreactor part of the entire BIOX system. Clarification also acts to reduce the water in the sludge, as the first part of the biomass sludge dewatering system to reduce the volume of the waste sludge. As discussed under Process Microbiology, the type and abundance of the microorganisms and the point on the microbial growth curve at which the system is operating greatly influence the sludge settling characteristics. As the flocculated sludge of a properly operating activated sludge process will have a specific gravity greater than 1.0, most clarifiers are designed as gravity settlers. For difficult to settle sludges (such as during an upset) gravity settling can be assisted by polymer addition. If it is determined either through pilot testing or operating experience that the sludge will not settle effectively by gravity, then either clariflocculators, flotation clarifiers, filters, and other biomass separating equipment can be considered. (Reference 6, 10) ExxonMobil Research and Engineering Company – Fairfax, VA

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) 6.2 DESIGN CONSIDERATIONS In the design of the activated sludge process, considerations must be given to: (1) compliance with regulatory effluent requirements, (2) feed characteristics and pretreatment requirements, (3) selection of the reactor and clarifier type, (4) oxygen requirements and aeration equipment, and (5) need for pilot plant data. Each of these considerations are discussed below. Regulatory Effluent Requirements

The determination of performance objectives is fundamental to selecting an appropriate design basis. Most importantly, the selected design must be capable of complying with anticipated discharge standards in a very reliable way. Hence, consideration must be given to the design of systems that can meet the effluent targets under maximum expected load conditions and perhaps 99+ percent of the time. Consideration should be given to both existing / near term requirements, as well as potential future requirements. If more restrictive requirements are anticipated in the future, or if new processing units are expected to come on-line, consideration should be given to designing the plant to meet these requirements and/or the ease of modification or retrofit. Consideration should also be given to the existing or future need to reduce water usage and/or wastewater discharges. The need to produce an effluent capable of being reused within the process operation can affect the selection of wastewater to be fed to the BIOX reactor as well as the treatment required, and hence the design. In addition to a liquid discharge, wastewater treatment via activated sludge inevitably produces both air and solid waste emissions. If air emissions are a current or future concern, consideration should be given to upstream reduction of volatile contaminants, selection of aeration equipment with lower stripping potential, and possibly covered basins. The ease and cost of disposal of the waste byproduct bio-sludge must also be considered. Restrictive regulations and high waste disposal costs may justify investment in a design which minimizes the production of waste sludge. The design may also be influenced by a need or desire to protect groundwater quality. If leakage to the groundwater is prohibited or not desired, consideration should be given to above ground tankage for the activated sludge reactor. If lined impoundments can be used in lieu of tankage, consideration must be given to selection of the aeration equipment to ensure the integrity of the liner is maintained. Feed Characteristics: Equalization and Pretreatment Requirements

The activated sludge process is capable of treating a wide variety of wastewaters. However, in order to ensure an effective and efficient operation, the characteristics of the feed must be understood and factored into the design. Prior to treatment in an activated sludge system, most wastewaters require some form of pretreatment. One of the most important pretreatment considerations is equalization. Generally speaking, an activated sludge process performs best when fluctuations in the feed to the reactor are dampened and smoothed. It has been found that the microorganisms are quite capable of acclimating to a rather wide range of wastewater characteristics when given time to adapt. This holds true for conditions such as organic loading, temperature, pH, salt content, etc. However, rapid changes in feed characteristics and reactor conditions, even if within the tolerance range of the microorganisms, should be avoided as they can destabilize the system. A minimum of 8 hours equalization at maximum loading is recommended. Table 6.2-1 gives criteria on the equalization and pretreatment requirements of activated sludge feed. A few of the more important feed characteristics are discussed below. 1. Oil Content Free or emulsified oil in the feed can cause several problems. While dissolved oils can be biodegraded by the activated sludge microorganisms, the breakdown of larger oil molecules is limited by their low solubility and relatively small available surface area. As a result, these oils tend to build up in the system and impart a buoyancy to the floc, causing the sludge to settle poorly or to float. The oil can also coat the microbes, hindering diffusion of oxygen. If the oil content of the influent wastewater is greater than 50 mg/L on the average or 100 mg/L at the maximum, upstream pretreatment to remove free and emulsified oils is likely required. Oil content can be measured using the Standard Methods Analysis on a filtered sample. Pretreatment can be induced air / gas flotation, dissolved air flotation, chemical flocculation, media filtration, or similar suspended oil removal technology. For selection of appropriate pretreatment technologies, see DP Section XIX-A, Guidelines for Selecting Wastewater Treatment Systems.

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) 2.

Ammonia, Phosphorus, and Micronutrients

Nitrogen is an important microbial nutrient and must be present in the system. At a minimum, five parts of N are usually required for every 100 parts of BOD5. Refinery wastewater generally contains excess nitrogen, and pretreatment of selected streams (usually steam stripping of sour waters) is often used to minimize the ammonia content. Traditionally, pretreatment has been used as a means of controlling gross discharges of ammonia into and from conventional nonnitrifying activated sludge systems. Normally, maximum ammonia levels of about 200 mg/L in the activated sludge influent can be tolerated without upsetting the process of organic carbon removal. Free (not dissociated) ammonia will have an effect on the organic carbon removal and therefore the maximum tolerable level of ammonia really depends on the pH of the wastewater. As many regulatory agencies now severely restrict the amount of effluent ammonia and/or total nitrogen, many activated sludge systems must now be designed for ammonia removal (nitrification) and possibly total nitrogen removal (denitrification). In this regard, pretreatment may be necessary to keep ammonia levels low enough to prevent toxicity to the nitrifying bacteria. For example, non-ionized or free ammonia (NH3) may begin to inhibit nitrifying bacteria at a total ammonia concentration (at pH 7.0 and 20°C) of 20 to 50 mg/L in the aeration basin. With proper acclimation and buffering, however, much higher concentrations of ammonia can often be treated in a nitrifying system without microbial inhibition. (Reference 11) Pretreatment is also advantageous to reduce the volume of the nitrifying activated sludge reactor. While high concentrations of ammonia (200+ mg/L) can be converted to nitrate in suspended growth nitrifying systems, this usually requires hydraulic residence times of two or more days necessitating large reactors. (Reference 11, 12). The additional reactor costs need to be compared to the cost for the pretreatment. Phosphorus is another important nutrient which must be present in the wastewater feed. Although phosphates may enter refinery and petrochemical plant wastewater from boiler and cooling tower blowdown and from spent phosphoric acid catalyst from polymerization units, most refining wastewaters are deficient in phosphorus and additions are required. One part P is usually required for every 100 parts BOD5, but enough should be added to provide a residual phosphate concentration of 0.1 to 0.3 mg/L in the effluent. Phosphoric acid is generally used for this addition. Adequate pumping capacity and control are a part of the design process. Micronutrients are also required for a healthy and efficient biological population. These are typically available in sufficient quantities in refinery or chemical plant wastewater. Micronutrient requirements are listed below (Reference 20):

3.

MICRONUTRIENT

AMOUNT, mg/mg BOD

Ca

62E-04

Co

13E-05

Cu

15E-05

Fe

12E-03

K

45E-04

Mg

30E-04

Mn

10E-05

Mo

43E-05

Na

50E-06

Se

14E-10

Zn

16E-05

Hydrogen Sulfide

Streams containing hydrogen sulfide should be pretreated (such as the steam stripping of sour waters) so that the concentration in the reactor feed is < 50 mg/L. Levels above this can be inhibitory to non-acclimated activated sludge microorganisms. In addition, hydrogen sulfide exerts an immediate oxygen demand which can dramatically lower the dissolved oxygen content in the reactor. Even if the dissolved content in the bulk liquid is in the 1 to 2 mg/L range, the conditions within the sludge floc may in fact be anaerobic which can select for nuisance filamentous organisms. Sources of hydrogen sulfide within the refinery include sour water condensates, spent sulfidic caustic, and desalter wash water and tank bottom waters in warm and salty water environments. Sources of H2S within chemical plants include steam cracking sulfidic treatment, merox units, and other sulfidic treating processes. (Reference 7)

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) 4.

Toxic and Inhibitory Compounds

5.

Some materials are toxic or inhibitory to the microorganisms. Others can be destabilizing when introduced in shock quantities. These compounds can include cyanides, heavy metals, sulfides, biocides, alcohols, solvents, detergents, and organoamines such as MEA and morpholine. In the event that such compounds are likely to be present in the feed, sufficient equalization and pretreatment must be provided to ensure a stable activated sludge operation. Additionally, certain compounds (e.g., MTBE) may not be amenable to biodegradation, or biodegradable only after an extended period of acclimation. Table 6.2-2 lists the relative biodegradability of certain organic compounds, while Table 6.2-1 includes limiting concentrations of various constituents. MEA

6.

The most troublesome of these inhibitory compounds is MEA. This is due to its widespread use in refineries and its impact to the activated sludge. Although the exact biochemical breakdown pathways are unclear, MEA is likely degraded by an acclimated activated sludge to alcohol and ammonia. The alcohol is destabilizing to the biomass and usually results in floating sludge in the aeration basin and clarifier and a deterioration in system performance. In nitrifying systems, the ammonia is converted to nitrate. This consumes alkalinity and close attention to pH is often required to keep the pH above 6.5. Often times, the concentrations of nitrifiers in the mixed liquor are not adequate to handle this excess ammonia loading, and ammonia will break through in the effluent. For these reasons, MEA content in the feed should be kept to below 20 mg/L. Wastewaters containing MEA can potentially be pretreated by chemical oxidation with hydrogen peroxide (sometimes coupled with Fenton's Reagent - a ferrous sulfate catalyst), and ionization and adsorption onto specific ion exchange resins. Adsorption onto activated carbon is not recommended as MEA is only minimally adsorbable. High levels of MEA can often be cost effectively reduced by source control involving optimization of the equipment handling MEA to reduce losses. pH

7.

Facilities should be provided to keep the pH in the reactor between 6.5 and 8.5. Proper pH is critical to the survival and performance of the microorganisms, and excursions outside of this range can be detrimental. Nitrifying organisms in particular are pH sensitive and can quickly be inhibited at pHs below 6.5. At pH's above about 10, all biological activity will cease until the pH is reduced back down. Temperature The temperature in the aeration basin affects both the rate of contaminant removal, and the sludge settling characteristics. Proper operation of most activated sludge systems require an operating temperature between 50 to 105°F (10 to 40°C). As the rate of substrate removal generally doubles for each 18°F (10°C), higher temperatures will result in more complete removal of the substrate. However, temperatures above 105°F (40°C) can become inhibitory for mesophilic organisms and even result in cell death. At temperatures below 50°F (10°C), the rate of substrate removal is extremely slow. Nitrification at this temperature may not occur. At this temperature, the rate of sludge settling in the clarifier is also reduced. Temperature control is therefore a very important design consideration, and all design calculations should be performed using the lowest expected operating temperature. Measures to control feed and aeration basin temperature include wastewater segregation (especially the segregation of uncontaminated rainwater), cooling towers, steam heating, the ability to switch between hot and cold cooling tower blowdown, bypass capabilities on desalter brine heat exchangers, and temperature control on sour water stripper bottoms, or using insulated and/or below ground lines and tanks. If below ground lines and tanks are used for heat conservation, consideration must be given to prevent the risk of potential leaks to soil and groundwater by double lining the lines and tanks. Some biological systems (such as composting of waste sludge) utilize bacteria, which survive at temperatures in the 115 to 140°F (46 to 60°C) thermophilic range. However, an activated sludge plant should not be designed to operate at this temperature range without careful analysis by experts and pilot testing.

Selection of Reactor and Clarifier Types

One of the main steps in the design of an activated sludge process is the selection of the type of reactor and clarifier. Operational factors that govern this selection include:

•

Reaction kinetics

• •

Oxygen-transfer requirements

•

Expected sludge settling characteristics

• •

Local climatological / environmental conditions

Nature of the wastewater

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) The advantages and disadvantages of various reactor configurations were given in the Description Section. Given the variations in wastewater loading that can be expected in many refinery and chemical plants, a completely mixed design is usually preferred due to its resilience to slug loadings of organic and toxic materials. Where sludge disposal costs are high, or where nitrification is required, a completely mixed extended aeration / nitrifying design might be selected. However, if the control of nuisance organisms is expected to be of significant concern, then consideration should be given to designing at least the first portion of the aeration tank with a plug flow configuration. An alternative would be to install an upstream contact or selector tank ahead of a completely mixed aeration basin. A separate reactor or stage could also be installed if complete biological nitrogen removal (denitrification) was desired. Basin geometry is not critical as long as it allows effective and economical aeration and mixing. As discussed in the aeration systems subsection, the basin depth is usually selected depending upon the aeration equipment specified. The current trend of specifying diffused aeration systems tends to support the selection of relatively deep tanks (23 ft (7 m) or greater). Where possible, the total reactor volume should be divided into at least two independent parallel basins. This will help to improve the overall service factor by allowing operations to continue when one basin is out of service. As discussed in detail in DP Section XIX-A7, Clarification Systems for Biological Treatment, most activated sludge clarifiers are designed as gravity settlers. However, if either pilot testing or operating experience indicate that the sludge will not settle efficiently by gravity, then clariflocculators or perhaps flotation clarifiers should be considered. Polymer addition for poorly flocculating sludges can also be of benefit. If poor settling is due to entrainment of air bubbles within the floc, a degasifier should be considered. As denitrification (nitrate conversion to nitrogen gas) can also disturb settling, the settled sludge should be quickly returned to the aeration basin to prevent anaerobic conditions from forming in the clarifiers. If the poor sludge settleability is due to the occurrence of nuisance organisms, a selector tank should be considered. Oxygen Requirements and Aeration Equipment

The importance of accurately determining the system oxygen demand cannot be overstated. Thorough characterization of the wastewater must be performed in order to properly estimate the oxygen requirements under normal and maximum load conditions. It is recommended that a 2 mg/L dissolved oxygen level be maintained in the aeration basin. If sufficient oxygen is not provided, the system will not be capable of completely oxidizing the organic wastes and meeting the required effluent parameters. Secondly, the predominant bacteria will be the filamentous nuisance type which will result in severe operating problems. To prevent these problems, a safety factor of 25 to 50 percent is commonly employed when estimating oxygen requirement or the 99th percentile of oxygen demand is chosen for design purposes. For a complete discussion on selecting appropriate aeration equipment, refer to the Aeration Systems subsection. Need for Pilot Plant Data

Whenever feasible, pilot plant tests should be performed on the actual wastewater to be treated. Such testing can provide critical site specific information to help ensure that the most effective and economical configuration and design are selected. This is especially important for atypical wastewater where there is little actual operating experience treating the stream. This is particularly the case for many petrochemical wastewaters. Pilot plant testing can determine reaction kinetics, sludge settleability characteristics including the tendency to select for filamentous organisms, excess sludge production, oxygen requirements, and the effects of changes in F/M and SRT. These parameters are all very important in the process of selecting an appropriate system configuration and performing the actual design. In the absence of this information, less reliable “typical" values must be used for design, making it difficult to optimize the plant design and cost.

6.3 DESIGN PROCEDURE Quick, Rough Sizing Basis

For Quick, Rough Sizing Basis, following the Standard Procedures using the “non-pilot case." Consult ER&E Report No. EE.86E.86 (Reference 49) for load factors when necessary. Table 1-1 may also give information that can be used as a guide. Standard Procedures - Design Conditions

Continuous flow stirred tank reactor with cell recycle Either carbonaceous or carbonaceous plus ammonia removal Note: This Design Procedure is written to incorporate both pilot unit and non-pilot unit design cases.

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Step 1: Determine the Design Flowrate and Develop a Comprehensive Wastewater Characterization

Determining the Design Flowrate One of the most important and complex parts of wastewater treatment facility design is to develop a design flowrate and the contaminant loading rate in terms of mass per time. The flowrate chosen for equipment sizing depends on location factors, equalization tankage, process unit operation and variability, stormwater management, load growth for expansion, preinvestment philosophy for infrastructure for future facilities, contingency, operating philosophy and penalties for not meeting wastewater quality requirements. Since the design flowrate is the key sizing factor and ultimately determines the cost and operability of treatment systems, it is recommended that careful analysis be made and experienced engineering assistance be consulted. For more details on determining the Design Flowrate, refer to Design Practice Section XIX-A1, Primary Oil / Water Separators under the Design Parameters Section. Prepare a comprehensive analysis of the wastewater to be treated. The following parameters should be measured or estimated for both average and maximum values: flow, BOD5, either COD or TOD, oil and grease, ammonia, Total Kjeldahl Nitrogen, pH, temperature (min/max), total suspended solids, total dissolved solids, chlorides, phosphorus, and heavy metals. Where sufficient data exists, probability charts should be prepared for each of these parameters. Typically, facilities tend to be designed to handle loadings at approximately the 95% probability level. In the absence of such data, use the following guidelines: (Reference 13) Aeration basin capacity: Minimum temperature Maximum daily organic mass loading Maximum daily hydraulic loading after equalization (8 hrs minimum recommended) = design flowrate Clarifier surface area: Clarifier surface area should be evaluated at the design flowrate conditions. If peaks are typically of short duration, then the daily flow rates may govern; if peaks are of long duration, peak values should be assumed to govern to prevent the solids from overflowing the tank. Step 2: Determine the Need for Pilot Testing

Pilot testing is particularly recommended for unusual or atypical wastewaters. These would include wastewaters with: – Unusually high (> 500 mg/L) or low (< 100 mg/L) BOD5; – High ammonia or TKN values (> 50 mg/L) when ammonia and/or nitrogen removal is required; – Only one or two constituents; – Unusually high concentrations of heavy metals or other inhibitory compounds (e.g., MEA) – Significant concentrations of unusual or hard-to-degrade compounds (e.g., MTBE) Step 3: Select a Design Sludge Retention Time, SRT

The design SRT must be sufficient to prevent washout of the microorganisms, achieve the desired effluent quality, and provide a stable operation within a reasonable margin of safety. Recommended SRTs: For conventional non-nitrifying activated sludge - 15 days For extended aeration/nitrifying systems - 25 days For pilot plant testing, this will be the SRT at which you will initially run the unit. Note: For non-recycle cases (aerated lagoons), SRT = HRT. Design information for such cases is in Section 8.0 of this Design Practice. Step 4: Select an approximate Hydraulic Retention Time, HRT

The design HRT must be sufficient to provide adequate time for the microorganisms to degrade the contaminants. The HRT must be large enough to sustain the desired SRT, which at steady state within reasonable ranges achieves the desired effluent quality. At this point the design HRT is not known, but for pilot test purposes select a value appropriate for the wastewater as follows: For conventional non-nitrifying activated sludge - 12 hrs For extended aeration/nitrifying systems - 24 hrs For unusual or difficult-to-degrade wastewaters - 24 hrs

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Step 5: Determine the Maximum Yield Coefficient, Ym

The maximum heterotrophic yield Ym is defined as the change (increase) in biomass divided by the change (decrease) in substrate measured during logarithmic cell growth. A. Pilot case Grow up a culture of microorganisms acclimated to the wastewater and conditions specified in Steps 3 and 4. Take an aliquot of influent wastewater and acclimated biomass and place them in an aerated batch reactor so that the concentration of organisms in the reactor is approximately 400 mg/L. Aliquots from the reactor should be taken periodically (approx. every 6 hours) and measured for VSS, BOD5 and either COD or TOD. This should be continued until the rate of COD/TOD removal begins to decline (usually within 2 days). Plot VSS (y axis) versus substrate concentration (x axis). The maximum slope of the plot of VSS versus substrate will give Ym. (Reference 17) See Figure 6.3-1 for an example. B. Non-pilot case Select a Ym from the literature appropriate for the type of wastewater being treated. (Reference 5, 11)

Recommended values (at 20°C) can be found in Table 6.1-1. Step 6: Determine Y, the Actual Observed Yield, and b, the Endogenous Decay Coefficient A. Pilot case After your pilot reactor has reached steady-state at the conditions specified in Steps 3 and 4, determine Y and b. Y, the observed yield, can be computed directly by dividing the mass of VSS wasted daily from the pilot reactor and the mass of substrate consumed daily. That is,

Y = mass of VSS wasted each day/mass of substrate consumed each day Knowing Ym, Y, and SRT, the endogenous decay coefficient b can be computed using Eq. 6.1-8: Y =

Ym 1 + (b) (SRT )

from Eq. (6.1-8)

B. Non-pilot case The approach here is to first select a value for the endogenous decay coefficient b from the literature. In general, the values for b reported are similar for most refinery / petrochemical / domestic wastewaters. Recommend values can be found in Table 6.1-1. Now the observed yield Y can be calculated directly using Eq. 6.1-8 above. Step 7: Determine the Specific Organic Removal Rate q, and the Design HRT A. Pilot case The specific organic removal rate can be determined directly from the pilot plant results using Eq. 6.1-11:

q =

So − S ( Xv ) (HRT )

from Eq. (6.1-11)

If the HRT selected in Step 4 was not adequate to sustain the SRT required to meet the required effluent quality, the necessary HRT can now be calculated by simply rearranging Eq. 6.1-11 and solving for HRT at the required effluent concentration S: HRT = (So - S) / (Xv)(q)

Eq. (6.3-1)

Alternatively, if the desired effluent quality has been obtained, the pilot unit can be run at lower HRTs to determine the minimum HRT necessary to meet the requirements. Note: If the pilot unit was not run at the minimum design temperature, then q (and the corresponding HRT) must be corrected for temperature as shown in Eq. 6.1-14:

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) qtd = qtp[1.08(td–tp)] where: qtd = qtp = 1.08 = td = tp =

Eq. (6.3-2)

q at the design (minimum) temperature q at the pilot temperature Temperature activity coefficient Design (minimum) temperature, °C Pilot plant temperature, °C

Note: An HRT of at least 10 hours (0.42 days) is recommended for conventional activated sludge and at least 18 hours (0.75 days) for extended aeration / nitrifying systems. B. Non-pilot case Calculate the specific organic substrate rate using Eq. 6.1-10: 1 + b SRT q = Y

from Eq. (6.1-10)

As Y was estimated at 20°C, correct q if necessary to the design (minimum) temperature using Eq. 6.3-2 above. Calculate the required hydraulic residence time HRT using Eq. 6.3-2 above. A value of Xv, must be assumed or calculated from Eq. 6.3-4. If calculated, Eq. 6.3-4 requires a value for HRT, which should be approximated in Step 4. Note: An HRT of at least 10 hours (0.42 days) is recommended for conventional activated sludge and at least 18 hours (0.75 days) for extended aeration / nitrifying systems. Step 8: Determine the Aeration Reactor Volume

Now that the design HRT of the aeration reactor has been determined, the reactor volume can be determined as follows: V = Q(HRT) Eq. (6.3-3) where: V Q

= =

Reactor volume, Mgal (m3) Design flow rate, Mgal/d (m3/d)

Step 9: Determine the Mixed Liquor Suspended Solids and the Waste Sludge Production A. Pilot case The MLSS concentration under aeration and the waste sludge production (mass/d) will be determined by operating the pilot unit at the design HRT, SRT, and organic loadings. The ratio of active biomass (MLVSS) to total mass of solids (MLSS) should also be determined. The concentration of solids in the clarified underflow, the concentration of solids in the effluent, and consequently the biomass wasting rate (volume/d) from the sludge recycle line that is necessary to maintain the specified SRT can also be determined (see Eq. 6.3-4). Note that the design MLSS is not necessarily the operating MLSS. MLSS will change in operations due to wastewater and/or operational changes. However, a design MLSS is needed to determine the aeration basin and clarifier design. B. Non-pilot case The MLVSS can be determined rearranging Eq. 6.1-12 with b equal to zero: Xv =

(So − S) Y (SRT ) HRT

where: Xv = So = S = Y = SRT = HRT =

Eq. (6.3-4)

Mixed liquor volatile suspended solids, mg/L Influent substrate concentration BOD5, mg/L Effluent substrate concentration BOD5, mg/L Mass of VSS wasted each day/mass of substrate consumed each day Sludge retention time, d Hydraulic residence time, d

To compute the total mass of solids under aeration MLSS, divide the MLVSS by 0.8 which is a typical ratio of MLVSS to MLSS. Note that the design MLSS is not necessarily the operating MLSS. MLSS will change in operations due to wastewater and/or operational changes. However, a design MLSS is needed to determine the aeration basin and clarifier design.

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) The waste sludge production Px can be calculated directly from Eq. 6.1-13: Px = QY(So - S)[8.34 (lb/Mgal)(mg/L)]

from Eq. (6.1-13)

Px = QY(So - S)(10-3 kg/g)

from Eq. (6.1-13)M

where: Px Q Y So S

= = = = =

Waste sludge production, lb/d (kg/d) Design wastewater flow rate, Mgal/d (m3/d) Mass of VSS wasted each day/mass of substrate consumed each day Influent substrate concentration BOD5, mg/L Effluent substrate concentration BOD5, mg/L

Assuming the wasting will be accomplished from the sludge recycle line, determine the biomass wasting rate Qw using the following equation for SRT: V⋅X (Q w X r + Q X e )

SRT =

where: V X Qw Xr Q Xe

= = = = = =

Eq. (6.3-5)

Volume of the aeration reactor, Mgal (m3) Concentration of solids under aeration (MLSS), mg/L Sludge wasting flow rate, Mgal/d (m3/d) Concentration of MLSS in the recycle stream, mg/L Design wastewater flow rate, Mgal/d (m3/d) Effluent suspended solids concentration, mg/L

In the absence of pilot plant data, assume Xr is equal to 3 times X, making R = 0.5, but a safety factor of 2 is usually employed making R = 1.0 and Xe is equal to 30 mg/L. Step 10:Determine the Theoretical Oxygen Requirements The theoretical oxygen demand can be determined directly from Eqs. 6.1-15 or 6.1-16: lb O2 /d =

kg O2 /d =

Q (So − S) (8.34 ) − 1.42 (Px ) f Q (So − S) (10 −3 kg/g) − 1.42 (Px ) f

from Eq. (6.1-15)

from Eq. (6.1-15)M

For extended aeration / nitrifying systems: lb O2 /d =

Q (So − S) (8.34) − 1.42 (Px ) + 4.57 Q ⋅ (Nio - Ni ) ⋅ (8.34) f

from Eq. (6.1-16)

kg O2 /d =

Q (So − S) (10 −3 kg/g) − 1.42 (Px ) + 4.57 Q ⋅ (Nio - Ni ) ⋅ (10-3 kg/g) f

from Eq. (6.1-16)M

where: f

=

Conversion factor for converting BOD5 to BODULT. In the absence of actual data on this relationship, a value of 0.6 is recommended for f Other terms as defined previously

Eq. 6.1-16 actually overestimates the oxygen requirements. This is because a portion of the nitrogen will not be oxidized to nitrate, but instead will be taken up by the microorganisms.

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Two approaches for estimating oxygen demand can be taken if pilot plant data are available. requirements can be calculated using Eq. 6.1-17: Rr =

a′ Q (So − S) + b′ ( X v ) V V

where: Rr a′

In one case, the oxygen

from Eq. (6.1-17)

= =

Oxygen utilization rate, ((mg/L)(day)) Oxygen utilization coefficient for synthesis, lb (kg) O2/lb (kg) organics removed b′ = Oxygen utilization coefficient for endogenous respiration, lb (kg) O2/lb (kg) VSS-d Q, V, So, S and Xv as described previously

As described in detail in the Aeration Systems subsection, the total oxygen requirement term Rr is usually determined from Eq. 6.1-18: Rr = KLa (Cs – C) where: KLa = Cs = C =

from Eq. (6.1-18)

Overall oxygen transfer coefficient, (1/d) Saturation oxygen concentration, mg/L Actual oxygen concentration, mg/L

To determine a′ and b′ from pilot plant data, plot Rr/Xv (y axis) versus (So – S)/Xv(HRT) (x axis). The slope of this line is a′, and the y intercept is b′. Once the coefficients a′ and b′ have been determined, the total mass of oxygen required per day can be determined as follows: lb O2/d = [a′ Q(So - S) + b′ (Xv)V](8.34 ((lb/Mgal)(mg/L)))

Eq. (6.3-6)

kg O2/d = [a′ Q(So - S) + b′ (Xv)V](10-3 kg/g)

Eq. (6.3-6)M

Another option can be used when a large amount of data is collected either with a pilot unit or an existing full scale unit that covers both peak flows and substrate influent concentrations. When this is the case, the recommended approach to calculating oxygen demand is by calculating the daily oxygen demand by Eq. 6.1-15 or 6.1-16 (pairing flow with contaminant level, organics, nitrogen, sulfite, etc.) and using a probability plot. The 95th to 99th percentile of the oxygen demand is normally chosen for design specifications, but the ultimate decision is left up to the designer. Step 11:Determine the Aeration and Mixing Requirements

The oxygen requirement as calculated in Step 10 is termed the Actual Oxygen Requirement (AOR, lbs O2/d) under operating conditions. There are two separate procedures for determining the aeration and mixing requirements depending on whether surface aerators or diffused aerators are used. Design Practice Section XIX-A6, Aeration Systems for Biological Treatment contains the following: a selection criteria guide to identify which type of aeration system should be used, a description of each type, the design considerations, the design procedures, a sample design problem, and operating strategies and enhancements. Please refer to DP Section XIX-A6 for this Step. Step 12:Determine the Nutrient Requirements

Nutrient requirements can be satisfactorily estimated assuming a 100:5:1 BOD5:N:P ratio. lb Nir/d = 0.05Q(So – S)(8.34 ((lb/Mgal)(mg/L)))

Eq. (6.3-7)

kg Nir/d = 0.05Q(So – S)(10-3 kg/g)

Eq. (6.3-7)M

lb P/d = 0.01Q(So – S)(8.34 lb/Mgal-(mg/L)

Eq. (6.3-8)

kg P/d = 0.01Q(So – S)(10-3 kg/g)

Eq. (6.3-8)M

where: Nir = Nitrogen required, lb/d (kg/d) P = Phosphorous required, lb/d (kg/d) Other terms as previously defined

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Step 13:Determine the Recirculation Ratio and the Clarifier Dimensions The sizing criteria of a clarifier are based on overflow rate and solids loading. Design Practice Section XIX-A7, Clarification Systems for Biological Treatment contains the following: a selection criteria guide to identify which type of clarifier should be used, a description of each type, the design considerations, the design procedures, a sample design problem, and operating strategies and enhancements. Please refer to DP Section XIX-A7 for this Step.

6.4 SAMPLE DESIGN PROBLEM Design Conditions

Design a completely mixed suspended growth activated sludge system capable of meeting the following effluent requirements: Effluent Requirements: BOD5 = 20 mg/L O&G = 5 mg/L TSS = 20 mg/L NH3 = 2 mg/L Sample Problem Step 1: Determine the Design Flowrate and Develop a Comprehensive Wastewater Characterization The following wastewater characterization data were obtained: Design flow = 5.70 Mgal/d (21,600 m3/d) Max. hourly peak flow = 2.2 times max. daily flow =12.54 Mgal/d Min. temperature = 60°F (15°C) Design BOD5 = 300 mg/L Design Ammonia = 50 mg/L Design TKN = 65 mg/L Design O&G = 30 mg/L Design COD = 550 mg/L Design Sulfides = < 10 mg/L Max. dissolved solids = 3,000 mg/L Design Phosphorus = negligible Influent suspended solids to reactor assumed to be negligible Heavy metals are negligible Sample Problem Step 2: Determine the Need for Pilot Testing

Due to the tight effluent requirements, the relatively high ammonia loading, and the need to ensure essentially complete nitrification, pilot testing was selected. The following data were determined from the pilot tests: Pilot Unit Reactor Volume = 200 gal (0.75 m3) SRT selected = 25 days HRT selected = 0.6 d Flow = 331 gal/d (1.25 m3/d) So = 300 mg/L Nio = 65 mg/L (50 mg/L of which was ammonia) T = 68°F ( 20°C) At steady-state: S (BOD5) = 60 mg/L N = 0 mg/L O&G = 1.5 mg/L Xv = 4,500 mg/L X = 5,400 mg/L Xr = 14,600 mg/L TSS effluent = 15 mg/L Mass of biomass wasted (including effluent TSS) to maintain SRT = 0.30 lb/d (0.14 kg/d) Mass of substrate consumed each day = 0.66 lb/d (0.30 kg/d) A clarifier solids-flux analysis was not performed (see DP Section XIX-A7 for a sample design with Solid Flux Data)

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Sample Problem Step 3: Select Sludge Retention Time, SRT

As mentioned above, a SRT of 25 days was selected based on pilot data. Sample Problem Step 4: Select an Approximate Hydraulic Retention Time, HRT

As mentioned above, a HRT of 0.6 days (14.4 hours) was selected based on pilot data. Sample Problem Step 5: Determine the Maximum Yield Coefficient, Ym A. Pilot Case The following data was generated by growing up a culture of microorganisms acclimated to the wastewater and conditions specified in Steps 3 and 4.

Data from batch yield test: TIME (hr)

VSS (mg/L)

BOD5 (mg/L)

0

490

300

6

545

246

12

588

197

18

630

150

24

678

102

30

713

75

36

710

66

40

700

60

48

704

55

From the slope of the plot of VSS vs. BOD (see Figure 6.3-1): Ym = 0.9 Sample Problem Step 6: Determine Y, the Actual Observed Yield, and b, the Endogenous Decay Coefficient A. Pilot Case Y, the observed yield, can be computed directly by dividing the mass of VSS wasted daily from the pilot reactor and the mass of substrate consumed daily. That is: Y = mass of VSS wasted each day/mass of substrate consumed each day Mass of biomass wasted (including effluent TSS) to maintain SRT = 0.30 lb/d (0.14 kg/d) Mass of substrate consumed each day = 0.66 lb/d (0.30 kg/d) Therefore Y = 0.30/0.66 = 0.45 Using Eq. 6.1-8 to solve for b:

Y =

Ym 1 + (b) (SRT )

0.45 =

0 .9 1 + (b) (SRT )

b = 0.04 Sample Problem Step 7: Determine the Specific Organic Removal rate q, and the Design HRT From Eq. 6.1-11:

q =

(So − S) 300 − 60 = ( Xv ) (HRT ) ( 4500 ) (0.6)

q = 0.090

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) To correct to the minimum temperature use Eq. 6.3-2: q (design) = 0.090[1.08(15-20)] q = 0.0613 Use Eq. 6.3-1 to solve for the necessary HRT to achieve 20 mg/L BOD5: HRT =

So − S 300 − 20 = ( Xv ) q ( 4500 ) (0.0613 )

HRT = 1.01 d (24.24 hrs) Sample Problem Step 8: Determine the Aeration Reactor Volume Determine the total aeration volume from Eq. 6.3-3:

V = Q(HRT) = 5.7(1.01) V = 5.8 Mgal (21,800 m3) Sample Problem Step 9: Determine the Mixed Liquor Suspended Solids and the Waste Sludge Production Determine the MLVSS under aeration at the design HRT using Eq. 6.3-4: Xv =

(300 − 20 ) (0.45 ) ( 25) 1.01

Xv = 3100 mg/L Determine the F/M ratio: F/M =

BOD5 ( X v ) (HRT )

F/M =

300 mg / L (3100 mg / L ) (1.01 d)

= 0.1 (1/d) (0.1 to 0.4(1/d) acceptable range) From the pilot test Xv/X = 83%, therefore: X = 3100/0.83 X = 3750 mg/L The excess sludge production can be calculated using Eq. 6.1-13: Px total = QY(So – S)(8.34) Px total = 6000 lb/d (2700 kg/d) The amount of sludge that will need to be wasted from the sludge recycle is calculated by subtracting the mass of solids lost in the effluent from the total excess sludge production: Px = 6000 - 5.7 Mgal/d(15 mg/L)[8.34 ((lb/Mgal)(mg/L))] Px = 5300 lb/d (2400 kg/d)

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Use Eq. 6.3-5 to calculate the wasting rate from the recycle line: Xr = X(Xr/X from pilot test) Xr = 3750 (14,600/5400) = 10,150 mg/L SRT =

25 =

( V ) ( X) (Q w ) Xr + QXe

(5.8) (3750 ) Q w (10150 ) + 5.7 (15 )

Qw = 0.077 Mgal/d (290 m3/d) Sample Problem Step 10: Determine the Theoretical Oxygen Requirements The theoretical oxygen requirements can be determined on a daily basis using Eq. 6.1-16 for the duration of the pilot test assuming the pilot ran long enough to capture both peak flows and loads. Pair data for each day is substituted into Eq. 6.1-16 to generate a large scope of oxygen demands. This data is then plotted on a probability plot and the 99th percentile of oxygen demand is used for design. Paired daily data is shown below.

lb O2/d =

5.7 (295 − 20 ) (8.34 ) – 1.42(5995) + 4.57(5.7)(60 – 0)(8.34) 0 .6

O2 = 26,300 lb/d (12,000 kg/d) After plotting the all the values generated by Eq. 6.1-16 using paired (flow + load + Px) daily data on a probability plot, the 99th percentile happens to be 27,800 lb/d (12,700 kg/d) Sample Problem Step 11: Determination of Aeration and Mixing Requirements Refer to the selection criteria guide in DP Section XIX-A6, Aeration Systems for Biological Treatment to determine what type of aeration system should be used. For this case, a diffused system should be used because at an 18 ft depth the estimated power requirement for a coarse bubble diffused system is 667 bhp while a surface aerator system power requirement is 800 bhp. In general, diffused aerators are preferred over surface aerators except for shallow applications (e.g., lagoons). Follow the sample design problems under DP Section XIX-A6 to determine the aeration and mixing requirements (sample data is the same). A sample design problem using the same information is also shown under the surface aeration system for illustrative purposes. Based on these requirements, determine the number and dimensions of the aeration tanks and specify the aeration and mixing equipment. Sample Problem Step 12: Determine the Nutrient Requirements The nutrient requirements can be determined using Eqs. 6.3-7 and 6.3-8:

Available N = 65(5.7)(8.34) = 3100 lb/d (1400 kg/d) Required N = 0.05(5.7)(300 - 20)(8.34) = 665 lb/d (300 kg/d) so no nitrogen addition is required. Required P = 0.01(5.7)(300 - 20)(8.34) = 135 lb/d (60 kg/d) Sample Problem Step 13: Determine the Recirculation Ratio and Clarifier Dimensions Refer to the selection criteria guide in Design Practice Section XIX-A7, Clarification Systems for Biological Treatment to determine the type of clarification system. For this case, circular gravity clarifiers are selected. Follow the sample design problems under DP Section XIX-A7 to determine the requirements for recirculation ratio and clarifier size (sample data is the same).

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) 6.5 OPERATING STRATEGIES AND ENHANCEMENTS The most important operating strategy for any activated sludge system is maintaining operating stability. accomplished by: – Closely monitoring the quality of the wastewater upstream of the treatment facilities; – Providing adequate spill diversion and equalization capabilities – Providing and maintaining the necessary pretreatment facilities – Maintaining the proper SRT of the activated sludge system via continuous sludge wasting – Performing routine monitoring of the operation – Proper control of clarifier recycle ratio

This is best

Upstream Monitoring

Upstream monitoring is critical as a means of detecting changes in the composition and quantity of the wastewater, thereby providing time to react to the change. On-line pH, TOC or TOD analyzers are recommended on each major trunk line sewer feeding the wastewater treatment plant. Equalization, Spill Diversion, and Pretreatment

Sufficient equalization should be provided to ensure a relatively even loading to the activated sludge basin. In the event of a spill or other significant change in wastewater quality, additional spill diversion facilities are essential. Most activated sludge systems can tolerate periodic loadings above their normal design provided the system is given time to acclimate and adjust. Similarly, adequate pretreatment (oil / water separation, neutralization, etc.) must also be provided and maintained. Solids Retention Time

Solids Retention Time (SRT) is important because it controls the growth rate of the microorganisms, and hence the effluent quality (Figure 6.1-3). Maintaining a proper and stable SRT ensures that the microorganisms are kept at the proper point on their growth curve, which in turn will provide the desired substrate removal and settling characteristics. Experience has shown that most refinery activated sludge systems perform best at SRT's between 25 and 35 days. These systems produce a high quality effluent low in BOD5 and suspended solids, have excellent settling characteristics, are tolerant to upset, and produce relatively low quantities of waste biological sludge. It is important, however, that sufficient clarification capacity is available to tolerate the relatively high solids loadings during periods of high flows. At low SRT's, adequate bioflocculation does not occur and the sludge will float on the surface of the clarifier and/or be washed out of the system in the effluent. This phenomenon is frequently observed after an upset when the system is recovering and the microorganisms are growing rapidly. At very long SRT's, the biomass can actually become starved and start to rapidly decay. This results in the occurrence of dead cells (pin floc) in the effluent, and a system which is often weak and subject to upset. Filamentous nuisance bacteria also can take over a system at very high SRT's. Their large surface area-to-volume ratios give them a competitive advantage over other bacteria when food, nutrients, and/or oxygen are in short supply. These bacteria can cause serious problems with sludge settling, and some species (e.g., Nocardia) are notorious foam producers. One common operating mistake is to control SRT via batch wasting. The solids are allowed to build up in the system and then periodically wasted to a lower level. This form of operation tends to be destabilizing to the biomass and is not recommended. Rather, continuous wasting to a optimum SRT (taking into account the solids lost in the effluent) is the preferred approach. Routine Operations Monitoring

The following evaluations are very important for ensuring a stable operation: 1. Dissolved Oxygen The DO in each aeration basin should be monitored continuously. The DO should be kept above 2.0 mg/L at all times. This level of DO helps ensure that both the bulk fluid as well as the interior of the microbial floc are aerobic. The DO should be watched closely whenever there is an increase in organic loading, and especially if there is an increase in loading of reduced sulfur compounds (such as H2S from spent caustics and H2SO3 from wet gas scrubbing operations). These compounds exert an immediate oxygen demand which can quickly deplete the oxygen, especially within the microbial floc. Dissolved oxygen probes are usually placed a few feet below the water surface. When low DO levels are predicted in specific areas due to aeration layout, the probe is sometimes placed in this area to ensure a 2.0 mg/L level throughout the system.

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) 2.

pH

3.

The pH should be monitored continuously in each aeration basin. The pH should be maintained at 6.5 to 8.5 at all times. The nitrifying bacteria are especially sensitive to pHs below 6.5. Temperature

4.

The temperature should be monitored continuously in each aeration basin. Temperatures above 105°F (40°C) and below 50°F (10°C) should be avoided. The nitrifying bacteria are especially sensitive to temperature extremes, and little or no nitrification should be expected at temperatures below the stated minimum. Microscopic Observation A daily or at least routine microscopic examination (100X) of the mixed liquor or recycle sludge can provide a great deal of information on the overall health and performance of the system. The biomass should be observed and notes recorded on the relative abundance of filamentous bacteria, free swimming flagellates and ciliates, crawling and stalked ciliates, and rotifers. A change in activity or dominance can provide an operator with an opportunity to correct for possible deterioration in the activated sludge efficiency (Reference 14).

5.

A microscopic examination should be performed with the following measures: a. Record the date, time, temperature, and location of the sample; b. Record for correlation purposes the DO, MLSS, F/M, SRT, SVI, and effluent TSS and BOD, TOC, or TOD concentrations; c. Scan a slide until 100 protozoa have been identified and listed into appropriate dominance groups; or scan three slides and count the number of protozoa in each group, then total the number of protozoa in each group. Regardless of the number of slides examined, the three highest totals are the dominant groups. d. Scan the slide for filamentous bacteria. Although bacteria are not easily counted, estimate the percentage of the floc that is filamentous. A correlation between dominant groups, sludge characteristics, and effluent quality can be developed over a relatively short period of time and used as a daily operational measure to maintain a healthy sludge and a high quality effluent. A typical representation of the succession of protozoa and bio-sludge development is given in Figure 6.5-1. Successions of dominant groups and their corresponding sludge and effluent conditions are the important or critical parameters. In general, a mixed liquor indicative of a stable operation and high quality effluent will contain a large variety of stalked and crawling ciliates and rotifers, and a small percentage of the bacteria (< 10%) will be of the filamentous type. A high percentage of flagellates and free-swimming ciliates often can be correlated to an increase in the F/M ratio. The absence of protozoa usually indicates a system that has been highly stressed (by high oxygen demand, organic or toxics shocks, etc.), or too high a wasting rate where the higher life forms are being washed from the system. A high percentage of filamentous organisms can often indicate either low DO, low nutrient levels, and/or an F/M ratio that is outside of the desirable range (either too high or too low). Sludge Settleability The settling characteristics of the mixed liquor should be observed and recorded at least daily. This is best done by adding a liter of mixed liquor to a 1000 ml graduated cylinder and observing its settling characteristics over a 30 minute period. The settled volume after 30 minutes (times 1000) divided by the MLSS concentration is called the Sludge Volume Index, or SVI. This simple test provides a good measure on how well the sludge will settle in the clarifier, and combined with microscopic evaluations can provide a useful tool for early detection of adverse changes to the system. A properly operated standard activated sludge system should have an SVI < 100, with only a minor amount of suspended matter in the clarified water. This is not true for other biological treatment systems which will be discussed in more detail in a future edition of this design practice. If a properly operated standard activated sludge system has an SVI > 150, often combined with very clear clarified water, an excessive amount of filamentous bacteria may be growing. The high SVI is due to the fact that filamentous bacteria do not settle or compact well. The reason the clarified water is so clear is because the dense filaments act as a filter and as they settle remove any solids from the water. Conversely, the occurrence of a high degree of suspended solids in the clarified water is often the result of a high amount of pin floc, indicating too high an SRT. A high degree of suspended solids can also indicate a system that was overloaded with organic compounds or ones that can cause toxicity to the microbes.

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6.0 ACTIVATED SLUDGE SYSTEMS (Cont) Operations Troubleshooting

The following suggestions are provided to help address the more common types of operating problems. 1. Excessive Filamentous Bacteria

2.

3.

4.

Ensure that all feed and operating parameters are within the correct range. If necessary, the return sludge can be treated with a chemical oxidant to selectively destroy the filamentous organisms. Filamentous bacteria are selectively destroyed due to their high surface areas. Recommended dosages are 5 - 20 mg/L as free available chlorine (sodium hypochlorite is typically used), and 100 mg/L for hydrogen peroxide. (Reference 15, 16) Organic and/or Toxic Shocks Locate and stop or divert the source of the destabilizing material. Ensure that all feed and operating parameters are within the correct range. Hydrogen peroxide can be added if needed as a supplemental source of oxygen. In the event of a significant decrease in MLSS, bring in waste activated sludge from any nearby activated sludge plant. Commercially prepared innocula are very effective but are usually only justified for severe upsets when activity must be very quickly restored. High Effluent TSS - If due to too high an SRT or a drop in organic feed to the plant (F/M too low), gradually increase the sludge wasting rate to lower the SRT. A cationic polyelectrolyte can be added to the clarifier inlet to help settle the suspended solids. Jar tests should first be performed to select the proper type and dosage, otherwise this treatment will be ineffective. (Reference 9) Incomplete or Loss of Nitrification - Ensure all feed and operating parameters are within the correct range, including alkalinity. Identify and stop any releases of toxic or inhibitory materials to the aeration basin. Ammonia-adsorbing zeolites can be added to the aeration basin to control the discharge of ammonia while nitrification is reestablished. If after taking the above steps nitrification is still not reestablished, consider inoculating the system with either a nitrifying activated sludge from a nearby plant, or a commercially prepared innocula. Unable to Establish Nitrification - The inability to establish nitrification is usually due to the following reasons: + + +

5.

HRT too low (< 10 hours) SRT too low (< 10 days) Carbon levels in the aeration basin too high, resulting in the nitrifiers being out competed by the heterotrophic bacteria.

+ Temperature too low (lower than 70°F, 21°C) + Alkalinity below 150 mg/L as CaCO3 For existing systems SRT can be increased by adding fixed media, to the aeration basin. These media provide a surface on which organisms can attach and grow. This helps to keep the slower growing nitrifiers from being washed out of the system, allowing them to accumulate and subsequently remove the ammonia. The effectiveness of these treatments are currently being evaluated in the Code 500 R&D program. Polymer addition can also indirectly increase SRT because the polymer increases solids compaction in the clarifier bottoms. Excessive Foam - Foam on the top of the aeration basin is due to surface-active materials in the wastewater. These can be such things as detergents, degreasers, emulsified oils from unit cleaning operations, and surfactants from fire-fighting foams. Microorganisms also produce surfactants as a means to solubilize and degrade oil in the water. Some forms of filamentous foam-producing bacteria can actually become established in huge numbers at the surface of the water. In general, upstream control of the influent wastewater and maintenance of the proper operating parameters will prevent the occurrence of excessive foaming. In the event of excessive non-biological foam, chemical anti-foams can be added to control the foam. Foam produced by nuisance filamentous organisms can best be controlled by establishing the proper operating conditions and eliminating these organisms from the system.

7.0 SEQUENCING BATCH REACTOR A variation of the Activated Sludge process for smaller facilities is the Sequencing Batch Reactor (SBR). The SBR is a fill-and draw activated sludge treatment system. Aeration and sedimentation / clarification are carried out sequentially in the same tank. There is no return activated sludge system because all the biological treatment and settling occur in the same tank. SBR systems have not been used within ExxonMobil because they were developed subsequent to installation of most systems. They operate in a semi-batch mode versus the more familiar continuous operation, and their bio-sludge level in the effluent may not be as low as with activated sludge. SBR's are also generally more appropriate for high concentration wastewaters and lower flowrate applications.

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8.0 AERATED LAGOONS 8.1 BACKGROUND In the 1950's and 1960's, shallow biological treatment ponds or lagoons were commonly installed at industrial and municipal facilities to provide secondary treatment of wastewater and sewage. Wind induced currents on the lagoon water surface provided natural aeration to the water and, in many cases, this amount of oxygenation was sufficient to provide reliable treatment. Over time, quality requirements on the treated effluent became more stringent. Many existing lagoon systems were upgraded, adding mechanical aeration in the inlet of the system while using the remaining portion for settling / stabilization. Many facilities were replaced with more controlled types of biological treatment, such as activated sludge systems, in order to meet the lower discharge limits. In recent years, however, industry efforts in pollution prevention and the installation of better pretreatment (oil water separators and flotation units in the petroleum / petrochemical industry) have helped to reduce contaminant loads to waste water treating systems. These changes in influent quality often make lagoon upgrades more feasible when compared with installing much more sophisticated and expensive process equipment, such as biological reactors, when upstream free oil can be controlled to less than 50 ppm. This section of the design practice will focus on how to estimate the sizing of mechanically aerated lagoons for a modern petroleum refining / petrochemical facility. For grass roots locations, or those where a major expansion of processing equipment is planned, an evaluation must be done to determine wastewater characteristics. These characteristics need to be compared with current as well as near term government quality requirements in order to evaluate whether aerated lagoons can reliably meet effluent contaminant concentrations.

8.2 DESCRIPTION An aerated lagoon or pond is a biological system where wastewater is treated on a continuous, flow-through basis without biosolids (microorganism) recycle. Oxygenation is accomplished by forced aeration (mechanical surface aerators or submerged diffusers) and/or from wind induced surface aeration. Many references discuss lagoons with aeration throughout the entire lagoon system. Except in unusual circumstances, this configuration is not adequate to meet modern effluent quality requirements. The effluent water from a traditional aerated lagoon (without settling) generally contains a significant quantity of suspended solids. If this high suspended solids content cannot be tolerated, because of its impact on BOD or TSS limits, post treatment of the effluent may be applied to lower it. If land area is available, sometimes a series of lagoons can be used to obtain improved treatment and to allow settling of biological solids. Most new or upgraded lagoon systems have been partitioned with a barrier so that a portion of the lagoon is used for aeration and the remaining portion is used for settling. Baffles and/or barriers are also used in lagoon systems to ensure proper flow distribution, preventing short-circuiting or poor volume utilization. Where a settling section is provided, the biomass sludge may be removed from the lagoon periodically. In a properly designed system, with adequate depth and capacity, sludge removal may need to be done every few years. See Figure 8.2-1 for an example layout. Aerated lagoon systems are less efficient than activated sludge processes since there is no microorganism or biosludge recycle. They can, however, be designed to reduce incoming BOD5 by 60 to 90 percent and phenol by 80 to 95 percent. Effluent characteristics include < 10 ppm (mg/L) residual oil and a suspended solids content of about one-half the incoming BOD5 , but not less than 50 to 100 ppm (mg/L), for a completely mixed aerated lagoon (upstream of the settling section). Better effluent quality can be obtained with a settling section. See Table 1 in DP Section XIX-A for additional details on typical removal efficiencies for lagoons and other treatment systems. Table 1-1 of this DP contains operational and performance parameters for ExxonMobil Activated Sludge Units and Lagoon Systems Lagoon systems (with aerated and settling zones) can be inexpensive wastewater treating units that are easy to operate and maintain. Their application is limited, however, due to the large plot area required and limited BOD reduction when compared to other biological treatment systems. Lagoons, especially when aerated, can also be a source of air emissions. Aerated lagoon systems should be used only when space is available, the cost of land is not prohibitive, and there are no regulatory (or other) restrictions on this type of installation. See Decision Tree in DP Section XIX-A (Figure 3): Guidelines for Selecting Wastewater Treatment Systems.

8.3 DESIGN CONSIDERATION As stated earlier, lagoons systems work most effectively in a two basin operation (aeration and settling). Where land area is not available for two basins, or a existing single pond system is to be upgraded, the single lagoon can be partitioned with a barrier. The first zone would be aerated allowing for substrate removal while the second zone would allow for solids settling. The need for a liner for both the aeration and settling zones should be assessed when upgrading or installing a new lagoon system. This decision is a site specific one depending on local environmental guidelines, soil permeability, and location and use of underlying groundwater.

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8.0 AERATED LAGOONS (Cont) Aeration Zone

Feed characterization is an important step in the design of any biological oxidation system. Although lagoon systems are capable of treating a wide range of wastewaters, their efficiency is affected by a number of factors such as free oil, sulfides, cyanides, etc. For maximum efficiency, the influent should be of the same quality required for an activated sludge plant. Limiting concentrations of a number of constituents for activated sludge plants are given in Table 6.2-1. The lagoon may operate at higher loadings than those displayed, but at a reduced efficiency. Additional information on feed requirements for biological treatment systems is given in the Activated Sludge subsection of this Design Practice. Due to lack of sludge recycle (lagoon HRT = SRT), the aeration zone of a lagoon system is less efficient than activated sludge processes, achieving 60 to 90 percent BOD5 reduction and 80 to 95 percent phenol reduction. The temperature in the aeration basin can also affect substrate removal efficiency. Recommended temperatures for biological treatment are between 50° and 105°F (10 to 40°C). In general, the rate of substrate removal is doubled for every 18°F (10°C) temperature rise in this range. More detailed information on the affect of temperature on biological systems is given in the Activated Sludge subsection of this Design Practice (Section 6.0). Aerators serve two purposes in an aerated lagoon system: to meet oxygen requirements for substrate removal and to provide the necessary power requirements for complete mixing and solids suspension. Suspended biosolids concentrations in the aeration zone of a lagoon system are lower than in an activated sludge system (2500 to 6000 TSS for activated sludge vs. 100 - 350 mg/L TSS for an aerated lagoon). Although it is not necessary to maintain total solids suspension in an aerated lagoon, it is important to provide sufficient power for complete mixing. In dealing with dilute wastes, or in lagoons with long retention times, it is possible for the horsepower requirements for mixing to exceed the horsepower required for oxygen transfer. Estimating power requirements for both oxygen transfer and complete mixing will be discussed below in the design procedure section. Figure 4.2-1 of DP Section XIX-A6 presents a decision tree for aeration selection. Due to the relatively shallow basin depth usually encountered in a lagoon system (6 to 12 ft), mechanical surface aerators are often used. A complete discussion on aeration systems, including depth requirements and circles of influence for complete mixing, is given in the Aeration subsection of this Design Practice. For screening purposes, a residence time of one day in the aeration section will usually allow for adequate substrate removal. A depth in this section of 6.6 ft (2 m) can also be used as a first pass estimate in sizing the basin. Other rules of thumb for aeration, mixing, and settling requirements, will be discussed throughout this subsection. These numbers should be used as a fist pass for screening estimates only and not for use in detailed design. Settling Zone

If the suspended solids concentration from the aeration zone exceeds permitted discharge levels, a settling zone must be provided. Usually, settling is accomplished in a large, relatively shallow earthen basin. In designing a settling basin, the following issues should be addressed: (1) adequate retention time must be provided to achieve the desired degree of suspended solids removal (2) sufficient volume must be provided in the basin for sludge storage (3) algal growth must be minimized (4) odors that may develop as a result of the anaerobic decomposition of the accumulated sludge must be controlled. A minimum retention time of one day is usually required to settle the majority of the settleable suspended solids. Adequate volume must be factored into the design to allow for sludge storage so that accumulated solids will not significantly affect retention times. Periodically, the settled sludge may need to be removed in order to maintain minimum retention times and control odors in the basin. Extended retention times in the settling basin can promote algal growth, which can contribute to effluent TSS and BOD levels. Although the growth rate is site specific, depending on available nutrients and sunlight, algae can usually be controlled by limiting the retention time in the settling basin to within 2 - 10 days. Odors arising from anaerobic decomposition can be usually be minimized by maintaining a minimum water depth of 1 meter in the settling basin. In warmer climates, a deeper basin may be needed. For screening purposes, a residence time of five days in the aeration section with a depth of 6.6 ft (2 m) will usually allow for adequate solids settling as well as minimal odors and algal growth. These numbers should be used as a first pass for screening estimates only and not for use in detailed design.

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8.0 AERATED LAGOONS (Cont) 8.4 DESIGN PROCEDURE Substrate Removal

First order kinetics can be assumed for substrate removal in an aerated lagoon. The following equation is used to determine an appropriate retention time in the aeration section of the lagoon system. The organic substrate, S, is usually expressed in terms of BOD5 but COD can also be used when BOD5 is not available. S / So = 1 / (1 + q HRT) where: S So q HRT

= = = =

Eq. (8.4-1)

Effluent substrate concentration (mg/L) Influent substrate concentration (mg/L) Rate of substrate removal (1/d) Hydraulic residence time in the aeration basin (d)

A typical value for q is given in the sample problem below (Section 8.5). In cases where systems are being designed to accommodate facility expansions or as part of a grass roots plant, actual wastewater flowrate and concentration data may not be available. If this is the case, load factors can be estimated using ER&E Report No. EE.86E.86 Refinery Process Unit Wastewater Load Factors - Final Report. Once the required HRT is defined, flowrate can be used to calculate basin volume. Basin depth can then be used to determine the surface area needed for the aeration zone. Temperature Effects

As discussed earlier, system temperatures can have a significant effect on substrate removal and must be considered in the design of an aerated lagoon system. Temperature effects are discussed in more detail in the Activated Sludge subsection. The relationship relating temperature and removal rate is stated as: qtd = (q20)[(1.08)(td-20)] where: qtd q20 td

= = =

from Eq. (6.3-2)

Rate of substrate removal at design (minimum) temperature, td °C (1/d) Rate of substrate removal at 20°C (1/d) Design temperature, °C

Oxygen Requirements

In order to provide ample oxygen for treatment, a basis of 1.2 times the total influent chemical oxygen demand (COD) is often used as a rough rule of thumb. A more detailed explanation on determining oxygen requirements is given in the Aeration subsection of this DP. Power Requirements for Oxygen Transfer and Solids Suspension

After the oxygen requirement for the aeration basin has been determined, the total basin aerator horsepower (for surface aerators) can be calculated using an oxygen transfer rate of 2 lb O2/ bhp-hr (0.9 Kg O2/ bhp-hr). The HP needed for solids suspension is then calculated. As stated earlier in this subsection, less power is required to keep solids in suspension in an aerated lagoon than in an activated sludge system. As a rule of thumb, 60 to 100 HP/million gallons (16 to 26 HP/1000 m3) is sufficient to provide power for complete mixing and solids suspension. The number of aerators should be chosen so that the total HP is spaced evenly throughout the aeration zone, maintaining complete coverage without creating any dead zones. If surface aerators are used, they should be placed such that the complete mixing zones are just about touching but the impingement pattern diameters are not overlapping. Surface aerators are also associated with a minimum depth requirement (increasing depth with increasing HP). Refer to DP Section XIX-A6 (Aeration Systems for Biological Treatment) for more details on surface and diffused aeration. Sludge Accumulation

If sufficient power is supplied to the aeration section of the lagoon system, there should be minimal solids accumulation in this area. There will, however, be solids accumulation in the settling basin. For screening purposes, an estimate of 530,000 lb (240,000 Kg) of sludge will be produced per year per million gallons of wastewater flow can be used. Sludge accumulation is highly variable and depends on several site specific factors such as substrate removal, influent TSS, and effluent quality. It is recommended that the following calculations be used to more accurately determine the rate of sludge accumulation:

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8.0 AERATED LAGOONS (Cont) SS = SSo + Xv / 0.8 where: SS SSo Xv

= = =

Eq. (8.4-2) Suspended solids in the aeration basin effluent (mg/L) Suspended solids in the aeration basin influent (mg/L) Lagoon volatile suspended solids (mg/L) [Xv/0.8 = suspended solids contribution excluding influent suspended solids]

Xv can be calculated using Eq. (6.1-12). For the aerated section of the lagoon, the hydraulic residence time is equal to the biomass sludge retention time (HRT = SRT) making this equation: Xv = ((So - S) Ym ) / (1 + b HRT) where: S So HRT Ym b

= = = = =

Eq. (8.4-3)

Effluent substrate concentration (mg/L) Influent substrate concentration (mg/L) Hydraulic residence time in the aeration basin (d) Maximum microbial yield coefficient, dimensionless Endogenous decay coefficient, d-1

Determining values for Ym and b with and without pilot data are discussed in detail in the Activated Sludge Section of this DP (Section 6.0). The amount of sludge that will settle in the basin over one years time can then be calculated as: Sludge = (SS - SSe) Q (8.34 lb/Mgal ⋅ (mg/L)) 365 day/yr where: Sludge SS SSe Q

= = = =

Eq. (8.4-4)

Sludge accumulation in settling basin per year (lb) Suspended solids in the aeration basin effluent (mg/L) Suspended solids in the settling basin effluent (mg/L) Design wastewater flowrate (Mgal/d)

This is a conservative estimate of accumulation in that it does not take degradation of the settled sludge into account. In reality, some of the settled biomass (30% - 60%) will degrade over time. In the absence of data, a 40% reduction of the biological portion (Xv/0.8) of the settled biomass may be assumed. The modification of equation Eq. (8.4-4) to take sludge biodegradation into account yields: Sludge = (SS - SSe - (Xv / 0.8) ⋅ kd) Q (8.34 lb/Mgal ⋅ (mg/L)) 365 day/yr where: kd

=

Eq. (8.4-5)

Fraction of biosolids degraded (0.4 typical)

By assuming that the settled solids will compact to a 10% solids sludge (90% water, 10% suspended matter) with a specific gravity of 1.06, and by using the basin configuration, the reduction in depth due to sludge accumulation can be calculated. The reduction in depth will be equal to: Sludge / (A (1.06) (0.10) 62.4 lb/ft3) = ft/yr where: A

=

Eq. (8.4-6)

Area of the settling basin, ft2

The solids should be cleaned out of the settling basin at intervals chosen such that the settling basin will have sufficient residence time even at maximum sludge levels, and the basin will have sufficient depth to control odors.

8.5 SAMPLE DESIGN PROBLEM Design an aerated lagoon system capable of meeting the following effluent requirements: COD = 80 mg/L = 20 mg/L BOD5 TSS = 55 mg/L The following wastewater characterization data were obtained: Average flow = 0.38 Mgal/d (1440 m3/d) Design flow = 0.90 Mgal/d (3400 m3/d) Design Influent COD = 300 mg/L Design influent TSS = 80 mg/L Minimum design temperature

=

77°F (25°C)

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8.0 AERATED LAGOONS (Cont) Kinetic coefficients (Ym and b shown here taken from Table 6.1-1) Ym b

= =

0.5 0.1 d-1

q (typical value)

=

1.6 d-1 at 20°C

Lagoon depth

=

6.6 ft (2 m)

Temperature Effects

Use Eq. 6.3-2 to correct for minimum temperature: q25

=

(1.6)[(1.08)(25-20)]

q25

=

2.3 d-1 at 25°C

Substrate Removal

Use Eq. 8.4-1 to solve for retention time in the aeration basin (HRT) based on COD concentrations: = 1/(1 + q HRT) S/So HRT = (300/80 - 1) / q HRT = (3.75 - 1) / 2.3 HRT = 1.2 days Solve for aeration basin volume: V = (HRT) Q = 1.2 days (0.90 Mgal/d) = 1.08 Mgal For an aeration basin with a depth of 6.6 ft, the surface area will be 21,900 ft2. Oxygen and Power Requirements

Influent COD in lb/d COD

=

(So) Q (8.34 lb/Mgal ⋅ (mg/L))

COD

=

300 mg/L ⋅ 0.90 Mgal/d (8.34 lb/Mgal ⋅ (mg/L)) = 2,250 lb/d

Oxygen required for COD reduction O2 = 1.2 ⋅ 2,250 lb/d = 2,700 lb O2/d HP required for oxygen transfer HP = (2,700 lb O2/d) / (24 hr/d ⋅ 2 lb O2/bhp hr) HP = 60 bhp Check HP/Mgal to ensure it is adequate for complete mixing: HP / V = 60 bhp / 1.08 Mgal = 56 bhp/Mgal Rule of thumb is 60 to 100 bhp/Mgal Increasing the power to the aeration basin to 80 bhp meets this requirement (74 bhp/Mgal) This 80 bhp requirement can be satisfied with 16 - 5 HP, 11 - 7.5 HP, 8 - 10 HP surface aerators, etc., depending on the size and depth of the aeration basin. Please see the aeration subsection of this DP for more details. Sludge Accumulation

Calculate the mixed liquor volatile suspended solids: = ((So - S) Ym ) / (1 + b HRT) Xv = ((220 mg/L) 0.5) / (1 + 0.1) Xv = 100 mg/L Xv Calculate the suspended solids in the aeration section effluent: SS = SSo + Xv / 0.8 SS = 80 mg/L + (100 / 0.8) mg/L SS = 205 mg/L

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8.0 AERATED LAGOONS (Cont) Assuming 40% of the biosolids in the settled sludge will degrade, calculate sludge buildup in the settling basin: Sludge =

(SS - SSe - (Xv/0.8) ⋅ kd) Q (8.34 lb/Mgal ⋅ (mg/L)) 365 day/yr

Sludge = (205 - 55 - (100/0.8) ⋅ 0.4) 0.90 Mgal/d (8.34 lb/Mgal ⋅ (mg/L)) 365 day/yr Sludge = 274,000 lb/year Choose a retention time for the design flow rate of 0.9 Mgal/d: 5 days will be chosen in order to provide settling and minimize algal growth. This, along with a basin depth of 6.6 ft (2 m), defines a volume of 4.5 Mgal (602,000 ft3) and a surface area of 91,000 ft2. Calculate reduction in settling basin depth: Sludge / (A (1.06) (0.10) 62.4 lb/ft3) (274,000 lb/yr) / (91,000 ft2 (1.06) (0.10) 62.4 lb/ft3) = 0.45 ft/yr

8.6 OPERATING STRATEGIES AND ENHANCEMENTS Oxygen Transfer

Older lagoon systems often used photosynthesis and surface contact with the atmosphere to provide the necessary oxygen for treatment. As loads to the wastewater treatment system increase due to process expansions, additional aeration may be needed to meet effluent discharge limits. The calculations shown above may be used to determine oxygen requirements for proper treatment and to determine aerator power requirements for oxygen transfer and complete mixing. Caution is advised when adding aeration to an existing pond. The relatively shallow depth in most ponds is usually not recommended for a diffused aeration system. In addition, depending on the HP needed, a surface mechanical aerator (most commonly used in lagoon systems) may require a lagoon depth of 6.6 ft (2 m) or greater to avoid scouring the bottom of the basin. Please refer to Figure 4.2-1 of DP XIX-A6 for more information on choosing an aeration system. Short Circuiting

Poor oxygen transfer may also be caused by short circuiting, a common problem in lagoon systems. When this occurs, the actual volume in the lagoon is not used effectively, due to poor pond configuration and/or temperature stratification, resulting in poor treatment in the “dead" zones. Other effects include ineffective use of aeration equipment, promotion of algal growth in dead zones, and surface scum accumulation. The most effective way of correcting short circuiting is the use of baffles. Various materials have been used for baffle walls including fiberglass, PVC, and synthetic rubber. In addition, floating baffles are available from a number of manufacturers. These baffles are supported by a continuous float at the top and a chain weight at the bottom. Baffles can be held in place by anchors located at the lagoon dikes. Floating baffles are especially useful for situations where upgrades to existing facilities, such as the addition of surface aerators, are to be made in the future. Dye or tracer testing can be used to evaluate the severity of short circuiting. It is recommended that EMRE Environmental Specialists be consulted for proper methods and to provide assistance in planning a test program. Algae and Suspended Solids Control

Extended retention times in the settling basin can promote algal growth, which can contribute to effluent TSS and BOD levels. Although the growth rate is site specific, depending on available nutrients and sunlight, algae can usually be controlled by limiting the retention time in the settling basin to within 2 - 10 days. High TSS levels can also be the result of inadequate settling basin volume. If this is the case, sections can be adjusted using floating baffles and/or by repositioning aerators. Sludge Buildup and Removal

Settled sludge will need to be removed from the lagoon occasionally. A dredge or vacuum truck can be used for this purpose. The dredged material can then be sent through the normal dewatering and disposal process. One disposal option, if feasible, is the mixing of dewatered biomass with oily sludge prior to landfarming. The biomass provides nutrients and aids in soil conditioning. Biomass Return Options

With stricter effluent limits, it may become difficult for sites with lagoons to consistently meet these regulations without building costly activated sludge plants. As discussed here, lagoon systems may be upgraded to include baffles and aerators. Although these steps can improve effluent quality, they may not be sufficient in the future.

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8.0 AERATED LAGOONS (Cont) As part of the Code 500 Environmental R&D program, EMRE has investigated improvements in lagoon efficiency by increasing the concentration of biomass in the aeration zone. This increase in biomass should result in an improvement in biodegradation rates and a decrease in contaminant levels. One conceptual design which would be low cost, low maintenance, and easy to install is the suspended baffle design. In this system, water leaving the aeration zone would flow through a “race track" configuration of suspended baffles. Solids settling below the baffles would then be pumped back to the aeration zone. Recycling biomass may also improve nitrification by increasing system SRT (SRT greater than 20 days is usually required for nitrification), providing additional time for growth of the specialized microorganisms required for ammonia removal. Biomass return for aerated lagoons remains a conceptual technology. It has not been field tested and cannot be recommended for full scale use without further investigation. Sites considering replacement of an aerated lagoon system with a more expensive activated sludge plant should consult EMRE Environmental Specialists for additional information on this upgrade option.

9.0 AEROBIC ATTACHED GROWTH 9.1 DESCRIPTION Reactors in which microbial growth occurs on or within a solid medium are termed attached growth, supported growth, or fixedfilm reactors. Examples of these types of systems include trickling filters, packed-bed bioreactors, rotating biological contactors, aerated biological filters, and fluidized bed reactors. In each of these systems, a solid medium is provided onto which the microorganisms attach and grow. This media may be crushed rock, sand, burnt or expanded clay, activated carbon, Raschid or pall rings, vertical or cross-flow plastic packing, and the like. Systems which combine the attributes of both attached and suspended growth are commonly referred to as combined or coupled attached growth systems. Historically, these systems have been viewed as less effective than activated sludge. Consequently their use has traditionally been for roughing treatment or applications with effluent limits of 30 to 50 ppm (mg/L) BOD and TSS, and for applications with relatively small flows where operator attention is minimal. However, recent advances in the design, performance, understanding, and cost of these systems are increasing the number of potential applications, especially when biological nutrient removal is considered (Reference 1, 2). Attached growth systems have several advantages over activated sludge. One primary advantage is that a higher concentration of microorganisms per unit volume is possible. This results in more rapid oxidation of the dissolved contaminants and potentially smaller reactors. Secondly, because the microorganisms are retained in the reactor, the performance of the system is not dependent on the settling and recycling of cell biomass. As discussed in Part A of this Subsection, the biomass settling characteristics are often a major design and operating constraint in suspended growth activated sludge systems. Additionally, this independence from the microbial growth and sludge wasting rates allows slower growing, “specialist bacteria" to develop. This is particularly advantageous for removing ammonia as well as difficult-to-degrade substances such as methyl tertiary butyl ether (MTBE). Attached growth systems also tend to be more resilient to toxic shocks and upsets. This is because the shocks only destroy the outer slime layer and therefore the system can quickly re-establish itself (Reference 1, 2, 3). Process Microbiology

The process microbiology of an attached growth system is very similar to that of a suspended growth system. Both processes utilize a diverse population of bacteria and protozoa to biochemically oxidize the dissolved organic matter and organic nitrogen / ammonia compounds to carbon dioxide, nitrate, and water. As the pretreated wastewater is applied to the medium, the surface quickly becomes coated with a viscous, slimy layer containing bacteria and other organisms. The bacteria remove organics by adsorption and assimilation of soluble and suspended constituents. Oxygen is supplied from the natural or forced circulation of air through interstices in the filter media, or in submerged designs it is diffused upward through the media and liquid (Reference 2, 3). Depending on the type and design of the reactor, the microbial buildup may create an anaerobic interface within the slime layer. This furthers the growth of facultative and anaerobic organisms which may play a role in the breakdown of the organic constituents. However, the aerobic organisms at the upper microbial surfaces provide the basic mechanism for organic removal and conversion. Figure 9.1-1 depicts the layers and reactions that typically occur within the biological film (Reference 1). Excessive anaerobic layers due to over loading the attached media are not desirable since they can result in undesirable odors.

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9.0 AEROBIC ATTACHED GROWTH (Cont) Applications for Attached Growth Systems

Given the wide variety of potential configurations and designs, applicability of attached growth systems is often difficult to determine except on a case-by-case basis. Generally speaking, however, attached growth systems may be cost effective for the following conditions: (Reference 1, 2, 4, 5, 6)

•

Add-ons to existing activated sludge plants for nitrification, denitrification, or heavy metals removal

•

Contaminants which are difficult to biodegrade, and/or require a specialist or slow growing bacteria for biodegradation, and/or require a long cell residence time (greater than 15 days)

•

Weak and medium strength wastewater (BOD5 less than 400 mg/L) requiring a short cell residence time (less than 5 days) for treatment

•

Weak and medium strength wastewater (BOD5 less than 400 mg/L) having a tendency to cause sludge settling problems (bulking)

•

Low concentrations (less than 100 mg/L) of suspended solids

• •

To eliminate suspended solids in addition to one of the above functions When the land available is not sufficient for an activated sludge or aerated lagoon system

Trickling Filters and Packed-Bed Biotowers

In trickling filters and packed-bed biotowers, wastewater is normally introduced at the top of the reactor through a distribution system and flows or trickles down through the media. Upflow systems are also common in newer packed-bed designs (Reference 4). Simplified schematics of a trickling filter system and a packed-bed biotower are shown in Figure 9.1-2. Figure 9.1-3 shows typical trickling filter configurations. The traditional trickling filter media of rock and gravel has now largely been replaced by synthetic packing media (Reference 1). However, based on cost, low loading applications still predominantly use rock and slag media. The synthetic media is desirable due to increased available surface area for microbial attachment and better flow characteristics. The latter is important in maximizing oxygen transfer efficiency, and minimizing short-circuiting and plugging. A comparative list of synthetic trickling filter / packed bed media can be found in Reference 63. The filters and biotowers for downstream system are constructed with an underdrain system for collecting the treated wastewater and any biological solids that have become detached from the media. The collected treated wastewater and biological solids are passed to a settling tank where the solids are separated from the treated wastewater. In practice, a portion of the treated wastewater is recycled to the filter, often to dilute the strength of the incoming wastewater and to maintain the biological slime layer in a moist condition. Trickling filters are classified by hydraulic- or organic-loading rates. The range of loadings normally encountered and other operational characteristics are shown below (Reference 5): CLASSIFICATION Filter Medium Hydraulic Loading, gpm/ft2 (m3/m2day) BOD5 Loading, lb/103 ft3day (kg/m3day) Depth, ft, (m) Recirculation Ratio Sloughing BOD5 Removal Efficiency, % Nitrification

LOW RATE

INTERMEDIATE RATE

HIGH RATE

SUPER HIGH RATE

Rock, slag

Rock, slag

Rock

Plastic

ROUGHING

TWO-STAGE

Plastic, redwood

Rock, plastic

0.02 - 0.06

0.06 - 0.16

0.16 - 0.64

0.2 - 1.20

0.8 - 3.2

0.16 - 0.64

(1.17 - 3.52)

(3.52 - 9.39)

(9.39 - 37.6)

(11.7 - 70.4)

(46.9 - 187.8)

(9.39 - 37.6)

5 - 25

15 - 30

30 - 60

30 - 100

100 - 500

60 - 120

(0.08 - 0.4)

(0.24 - 0.48)

(0.48 - 0.96)

(0.48 - 1.6)

(1.6 - 8.0)

(0.96 - 1.92)

6-8 (1.8 - 2.4)

6-8 (1.8 - 2.4)

3-6 (0.9 - 1.8)

10 - 40 (3.0 - 12.2)

15 - 40 (4.6 - 12.2)

6-8 (1.8 - 2.4)

0

0-1

1-2

1-2

1-4

0.5 - 2

Intermittent

Intermittent

Continuous

Continuous

Continuous

Continuous

80 - 90

50 - 70

65 - 85

65 - 80

40 - 65

85 - 95

Good

Partial

Little

Little

None

Good

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9.0 AEROBIC ATTACHED GROWTH (Cont) Rotating Biological Contactor Reactors (RBC)

A typical RBC system consists of circular plastic disks mounted on a series of horizontal shafts in a tank. A simplified schematic of an RBC system is shown in Figure 9.1-4 and 9.1-5. The shafts are slowly rotated (1 to 2 rpm) by either a mechanical or compressed air drive. Typically, approximately 40% of the media are immersed in the wastewater. The wastewater passes through the contactor by simple displacement and gravity. The rotation of disks alternately exposes the attached biofilm to the organic material in the wastewater and to atmospheric air. The unit generally consists of several rows, or stages, of disks. Depending on the organic loading conditions, each stage will show varying biofilm composition thickness, and color. During shock loads, e.g., toxics, the first stage of RBCs takes the hardest hit, preserving the latter ones. Because of the shearing forces, the excess biological film tends to slough off and is carried with the wastewater and removed in a clarifier. Organic overloading, insufficient rotational speed, or other conditions may cause excessive or uneven biomass buildup on the media, resulting in structural damage to shafts and media, the inability to maintain rotational speeds, or reduced treatment efficiency. Although the process has many operating advantages (personnel support, low energy usage) its limitations must be understood and accounted for if it is to be used successfully. In the past RBC were primarily designed based on a hydraulic loading expressed in gal/ft2/d or m3/m2/hr. However, the design approach has been changed to primarily use of soluble BOD per unit of surface area of disk media or in case of nitrification, lb NH3/103ft2 or kg of NH3/m2. Aerated Biological Filters and Fluidized Beds

The distinction between these systems and tricking filters / packed-beds is that their hydraulic design is such that the media are submerged in the reactor liquid during normal operation. In a biologically aerated filter, the media are stationary during normal operation, and held in place by gravity. In the fluidized-bed reactor, the media are expanded or fluidized as the incoming flow passes upward through the reactor. A schematic representation of an aerated biological filter is shown in Figure 9.1-6. The wastewater is introduced at the top of the reactor, and air is introduced into the media through an air diffusion system located above the filter underdrain system. One potential advantage of this system is that biotreatment and solids separation occur in the same reactor, eliminating the need for separate secondary clarification. As newly grown biomass and suspended solids build up in the reactor and head loss increases, the unit is taken offline and backwashed. In the conventional biological fluidized-bed reactor, wastewater is introduced at the bottom of the reactor at a hydraulic loading rate or upflow velocity sufficient to expand the bed media, resulting in a fluidized state. To date, the media employed in most full-scale fluidized-bed reactors have either been silica sand or granular activated carbon. The fluidized media particles provide a vast surface area for biological growth, in part leading to the development of a biomass concentration approximately 5 to 10 times greater than that normally maintained in a suspended growth activated sludge reactor. Consequently, the hydraulic residence time required for a fluidized bed reactor can in many situations be on the order of one to two hours versus 10 hours or more in an activated sludge system.

9.2 DESIGN CONSIDERATIONS General 1. pH - The pH of the feed should be controlled in order to maintain the effluent pH of the attached growth system to within 6.5 to 8.5. Control of the pH is especially important for the attached growth systems since they lack the built-in equalization of activated sludge plants and aerated lagoons. 2. Temperature - Attached growth system performance is influenced by the temperatures of the liquid and the slime. The aerobic portion of the slime is assumed to be at the same temperature as the liquid. A decrease in this temperature results both in an increase in the thickness of the aerobic film and in somewhat reduced efficiency. The adverse effect can be reduced by providing protection such as housing, windbreaks, reduction in settling tank detention time, and reduced recirculation to reduce heat loss. When wastewater temperatures less than 55°F (13°C) are expected, the required surface area of the RBC must be increased to compensate for cold temperature effects. For surface area correction curves for temperatures below 55°F (13°C) see Figure 9.2-1. 3. Hydraulic Loading - Since the mass of organisms in the attached growth system is fixed, the only way that the rate of substrate removal can be increased is for the substrate concentration to increase. Thus, as the flow rate or the hydraulic loading is increased, the percent of the applied substrate which can be removed must decrease. 4. Organic Loading - The organic loading is the mass of substrate applied per unit time divided by the volume of the treating system. This parameter is used as design basis for sizing all attached growth system. An increased load of organic material in the feed increases the outlet BOD5, reduces the removal efficiency (percent reduction in BOD5), and increases the total mass of removed organic matter.

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9.0 AEROBIC ATTACHED GROWTH (Cont) 5.

6.

Specific Surface Area of the Media - Substrate removal will increase as the specific surface area is increased because the availability of more surface means that more microorganisms can be obtained within each unit volume of attached growth system. The physical configuration of the media will determine the flow patterns which affects the wetted surface area, mass transfer, etc. Specific surface area may range from 12 to 50 ft2/ft3 (39 to 164 m2/m3) for rock media and 24 to 60 ft2/ft3 (74 to 197 m2/m3) for plastic media. Oxygen Transfer Effect - Oxygen limitation will adversely affect the performance of the aerobic treatment systems. In extreme cases, nuisance organisms may proliferate thereby preventing the growth of organisms capable of removing the substrates. If odors and substandard performance are to be prevented, the concentration of substrate entering the system should be lowered by use of recirculation. For trickling filters, the influent substrate concentrations in excess of 400 ppm (mg/L) as TOD could cause an oxygen limitation. For RBCs, manufacturers' recommended upper limit on influent loading is 4 to 6 lb soluble BOD5/103 ft2 (0.02 to 0.03 kg soluble BOD5/103 m2).

Additional Design Considerations for Trickling Filters

1.

2.

Recirculation - Recirculation increases both the percentage and the total mass of organic removal. Up to a point, recirculation tends to distribute the organic load throughout the entire filter, rather than just over the top portion. While recirculation increases the organic load and reduces the once-through residence time, it also reduces concentrations and repeatedly contacts the material in the recirculated stream with the slime. In addition, dilution of the influent reduces shock loadings and improves BOD removal. In the past, recirculation has been reported to improve the efficiency (performance) of rock filters. However, based on a more recent literature, it appears that the benefits of recirculations are due primarily to improved wetting and flushing of filter media. Recirculation as applied to synthetic media involves a somewhat different concept than is applied to rock filters. Typically synthetic film media require a higher minimum wetting rate (flow per unit area) to induce a biological slime to develop throughout the depth of the medium. Thus, recirculation in synthetic filter media is required to maintain the required degree of wetting for a given medium. A trickling filter should generally be designed for at least 100% recycle. The recirculated stream is normally taken from the unsettled filter effluent. Alternatively, clarified effluent can be recirculated; this arrangement avoids putting excess solids back onto the filter. Air Movement - Convection provides the air that maintains aerobic conditions, so ventilation of the filter bed is essential for good performance. The difference between the temperatures of the air (dry bulb) and the water (inlet) determines the movement of air through the filter. A differential of 4°F (2°C) or more will normally result in sufficient air being supplied. The direction of air flow through the filter depends on the direction of the temperature differential; both directions are acceptable. Fans can be used to move air through the filter if the temperature differential is insufficient, but this is not common practice. Fans will also cool the water, but they increase the possibility of air pollution.

Additional Design Considerations for RBCs

1.

2.

Media Rotational Speed - In the treatment of wastewater with BOD up to 300 ppm (mg/L), performance increased with rotational speed to 60 ft/min. (18 m/min.) with no improvement noted at higher speeds (Reference 7). Increasing rotational speed increases contact, aeration, and mixing, and would therefore improve efficiency for high BOD wastewaters. However, increasing rotational speed rapidly increases power consumption, so that an economic evaluation should be made between increased power and increased area. Volume to Surface Ratio - Optimum volume to surface ratio has been reported to be about 0.12 gal/ft2 (0.005 m3/m2) of media.

Additional Design Considerations for Aerated Biological Filters and Fluidized Beds

Most of the aerated biological filters and fluidized beds are vendor designed. Due to lack of experience of these attached growth systems within ExxonMobil, some examples of vendor designs and their descriptions are listed below. Table 9.2-1 compares several vendors' sizing bases. If an aerated biological filters and/or fluidized beds are being considered in your site, it is highly recommended to perform a pilot test.

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9.0 AEROBIC ATTACHED GROWTH (Cont) 1.

2.

Granular Activated Carbon (GAC) “Fluidized" Beds (Figure 9.2-2) - It utilizes a biofilm on and in activated carbon granules. The bacteria, thus, grow on and in the carbon and consume the organics in the water phase. Some of the organics are claimed to be adsorbed by the carbon providing a food source for the bacteria when the dissolved organics in the water phase have been consumed. As the bacterial film grows on the carbon, the density of the granule reduces and it rises in the suspended or fluidized bed. A stirrer knocks the rising particle and causes the bacterial growth to fall off (slough). The carbon density is thus increased due to the loss of bacterial film and it sinks back into the fluidized bed. The sloughed biomass as suspended solids are then collected and can either be discharged with the water phase if the effluent TSS consent is not exceeded, or can be routed to a thickening / dewatering system like DAF. Each reactor is designed with a 100% recycle ability so that the bed is always kept fluidized in a low or nil fresh feed volume scenario. A granular activated carbon “Fluidized Beds" is manufactured by US Filter-Envirex. US Filter-Envirex also uses a method of introducing dissolved oxygen into the water which is claimed to further enhance the process. Compressed air is passed through a molecular sieve to remove the nitrogen and the rich oxygen stream then pressurizes / saturates a slip stream from the reactor effluent. This water is then mixed with reactor feed and achieves about 26 ppm (mg/L) dissolved oxygen versus the 6 ppm (mg/L) achievable using air alone at atmospheric pressure. Thus, a much richer dissolved oxygen supply is made available to the biomass. Sand “Fluidized" Bed - It is similar to the granular activated carbon “Fluidized" bed, except that sand is used instead of activated carbon media. Other differences in the process are:

•

The oxygen saturation system

•

3.

4.

5.

Biomass removal form the sand is effected by withdrawing sand through a pumped recycle system Dorr-Oliver (UK) Ltd is a vendor for this type of the fluidized bed. Fixed Bed with Expanded Clay - This process utilizes fixed upflow beds using expanded clay as the support medium for the fixed film. The beds is expected to be backwashed on about a once a week frequency. These upflow beds would also provide some ordinary filtration capability as well as biological activity which could be advantageous. Lurgi is one of the vendors for fixed bed with expanded clay and below is a typical design range. Moving Bed Biofilm Reactor (MBBR) - This process uses a non-cloggable biofilm reactor with low head-loss and a high specific biofilm surface. The biofilm carrier elements, manufactured by a vendor called Kaldness, are made of high density polyethylene (bulk density of 170 kg/m3) and shaped like wagon-wheels (nominal diameter of 9.1 mm and length of 7.2 mm). A maximum filling of about 70% by volume is used which corresponds to a specific growth area of biofilm of about 400 m2/m3. Captor® Media-Activated Sludge Process - Captor® process utilized reticulated biological support media to retain large amounts of active biomass in the reactor. The biological reactor would be filled with recirculated polyurethane pads having about 97% void space with internal and external surfaces for biomass attachment and/or entrapment. The pads are 1 in. x 1 in. x 0.5 in. in dimension and have a specific gravity of about 1.0 when filled with water. Vendor claims that nitrification performance is not affected at low reactor temperature like 50°F (10°C).

Equipment Design for Trickling Filters

1.

Inlet Distributor - The distribution of the wastewater over the top of the filter should be continuous and even. This is accomplished by rotating distributors. Although fixed sprinklers can also be used as distributors, they are not recommended. Rotating distributors are revolving, hollow arms with either orifices or spray nozzles along their length: one-, two-, and four-arm distributors are common. The reaction force from the momentum of the liquid as it discharges from the orifices or nozzles causes the arms to rotate. Clearance of 6 to 9 in. (150 to 225 mm) should be allowed between the bottom of the distributor arm and the top of the bed. This clearance permits the wastewater streams from the nozzles to spread out and cover the bed uniformly, and it prevents ice accumulations from interfering with the distributor motion during freezing weather. For uniform distribution over the area of the filter, the flow rate per unit of length should be proportional to the radius 2 to 5 ft (0.6 to 1.5 m). If the liquid flow rate is insufficient to keep the rotor turning at the proper speed then a dosing siphon or an electric motor should be used. It is important to have even flow distribution to the filter media at all rates from the minimum to the maximum expected, so it is necessary to have at the minimum flow rate both sufficient momentum to turn the distributor and a hydraulic design that distributes the feed over the entire top surface.

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9.0 AEROBIC ATTACHED GROWTH (Cont) 2.

Media - The media or packing can be plastic sheets, plastic shapes, crushed rock, or slag. Crushed rock and slag have been used for a long time, but plastic media is light in weight and has a high specific surface, and is preferred. The media (packing) should have the following characteristics:

•

Provide a large surface area for microbial film growth

•

Allow the liquid to flow evenly in a thin sheet over the microbial film

•

Sufficient void space to: + Provide free flow of air (bed ventilation) + Allow organic solids sloughed from the microbial film to be carried away + Provide flow without clogging or flooding

•

Resistant to chemical attack by the wastewater

• •

Resistant to fracture from alternate freezing and thawing

•

Structural integrity at the temperatures of operation

Uniform size

• Structural strength to support thick (> 1/4 in or 6.4 mm) slime films 2a. Plastic Media (Figure 9.2-3) - Normally, it is the preferred media. It can be fabricated from polypropylene, polyethylene, polystyrene, and polyvinyl chloride. These plastics differ in their structural strengths, softening point, brittleness, and resistance to swelling, softening, and dissolving in solvents and other chemicals. It is particularly important to select a plastic that will maintain its shape, strength, surface, and structural integrity in use. Check with the supplier to verify that the plastic media has sufficient strength at the expected operating depth and temperatures when loaded with slime. Polyvinyl chloride media, for example, may become too soft at temperatures around 110°F (43°C) and above. The most common form of plastic tricking-filter media is irregular sheets that are corrugated, pleated, zigzagged, chevron shaped, etc. These are staked and spaced in the filter to form a hollow, honey-comb structure. Plastic media are used at high hydraulic loadings and at organic loadings higher than those applied to rock filters. Plastic media filters are usually loaded at hydraulic rates of up to about 1.6 gpm/ft2 (3.9 m3/m2-hr). The slime sloughs continuously from plastic packing, much of the biological growth is suspended in the liquid being recirculated, and only a small portion actually adheres to the filter media. Modular plastic media is self supporting, however, the dumped plastic media requires enclosure which must support it vertically. Consequently, the enclosures serve primarily to minimize splashing and heat loss, and to improve the esthetics of the reactor. 2b. Rock Media - Rock media filters are not recommended for new designs, because they cannot be built high and, therefore, require a large plot area. Rock media has a small specific surface, and should be graded (screened) because with mixed sizes, the small stones tend to fill the interstices between the large ones, and reduce the amount of space for microbial growth. Rocks should be between 2 and 4 in. (50 to 100 mm), a compromise between large surface area and large void space. Crushed rock should have a uniform shape to provide space for microbial growth. No flat stones should be used because they pack tightly and fill voids. Volcanic traprock and granite make excellent rock media, but limestone makes a poor media. Rock media should be unloaded and placed in the filter carefully to keep dirt and chipped rock, which can plug the underdrain tile, out of the bed. If stone media is used the enclosure must be of reinforced concrete or some other material capable of holding it in place vertically. 3. Filter Bed 3a. Shape - Trickling filters can be circular or rectangular. They are normally circular because rotary distributors are used. 3b. Diameter - The filter area required can be provided in one bed up to about 200 ft (60 m), but it is often desirable to provide more than one unit. Two filters provide flexibility for either parallel or series operation, and permit operation when one filter is shut down for repairs. Removal efficiencies in plastic-media filters are approximately constant over a broad range of diameters, ∅, and depths, D, as long as the product (∅ ⋅ D) remains constant. However, since the plot area required increases with diameter, trickling filters are generally constructed as deep as is feasible. The pressure drop through a filter is so small that there is little danger of flooding the bed at hydraulic loadings up to several gallons per minute per ft2 or m3/m2-hr. 3c. Depth - Filter depth is limited only by the available hydraulic head and the strength of the packing and the underdrains. The depth is usually 20 to 40 ft (6 to 12 m) with plastic media (up to 20 ft (6 m) without additional intermediate structural supports for the media) and 6 to 10 ft (1.8 to 3.0 m) with rock media. A filter should be constructed with the most economical height-to-diameter ratio (considering all of the components) consistent with the required total filter volume, hydraulic loading, structural strength of the packing and underdrains, and plot limitations. ExxonMobil Research and Engineering Company – Fairfax, VA

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9.0 AEROBIC ATTACHED GROWTH (Cont) 3d. Underdrains - The underdrains collect the treated liquid and lead it to a channel crossing the bottom of the filter, convey the air into the filter, and distribute the air. Underdrains are designed to permit simultaneous, counter-current passage of liquid and air. The air normally enters through openings in the channel at both sides of the filter. The air inlet vents must be large enough to pass the quantity of air necessary for the proper aerobic operation of the filter. Prior to the advent of plastic media, almost all underdrain systems were made of vitrified clay because of the weights which they had to support. Plastic media are not as heavy, however, so that the underdrain system supporting many of them are simple metal gratings. The filter foundation is normally a two-part, reinforced concrete slab - the parts being divided by the channel through the diameter. The slab slopes toward the channel, generally at a pitch of 1 to 2%. The underdrain tiles, which are placed directly on the slab, slope at the same pitch. Slots in the top of the underdrains collect the treated water. To allow the liquid to flow into the underdrains without flooding (liquid should not fill all of the void spaces at the bottom of the media), and to allow the 25% of the area of the filter, and liquid in the channels should not occupy more than 50% of the total available cross-sectional flow area. The underdrain blocks in the middle of the filter have the greatest flow rate, so if the flow of both air and liquid is satisfactory in these blocks, it will be satisfactory everywhere. Standard underdrain blocks are provided in enough sizes to meet the requirements for most applications. The liquid flows through channels in the underdrain blocks unit it falls into the central effluent channel. The effluent channel should slope sufficiently to maintain a liquid velocity of at least 1 to 2 ft/s (0.3 to 0.6 m/s) at minimum flow to prevent solids from depositing. The channel should be deep enough to avoid submerging the underdrains and to allow sufficient space above the liquid for flow of the necessary amount of air. The liquid flowing in the effluent channel should not occupy more than 50% of the total available cross-sectional flow area. 4. Effluent Handling - In normal operation, the microbial slime ages, dies, and is washed from the media. The rate of film wash-off or sloughing increases if the supply of oxygen is insufficient, and also varies with the hydraulic load. Normally, unless there is another downstream unit, such as an activated sludge plant, a trickling filter is followed by a solids removal step. Effluent-handling equipment should be designed to handle water containing 300 to 800 ppm (mg/L) of suspended solids. If a sedimentation tank is provided to remove solids from the filter effluent, it can be a clarifier, tank, basin, or pond, and should be designed on the following bases:

• •

An overflow rate of 700 to 1,000 gpd/ft2 (1.19 to 1.70 m3/m2/hr)

•

Sufficient weir length so that the hydraulic flow over the weir does not exceed 10,000 gpd/linear ft of weir (5.2 m3/hr per linear meter of weir)

A retention time of 1 to 2 hours

•

5.

6.

Inclusion of mechanical sludge collectors Shock Loads of Toxic Wastes - Due to the relatively short retention time of the wastewater in the reactor or because only organisms on the surface may be killed, the trickling filters has the ability to survive shock loads of toxic wastes. Thus, as the dead organisms are removed by sloughing, a layer will be exposed which has not been subjected to the toxic material. If the shock load of toxic material is of long duration, however, or of a type which will be adsorbed onto the biofilm, then a trickling filter can be severely affected. Materials of Construction - The walls of the trickling filter can be made of sheet steel, corrugated steel, plastic sheets, or reinforced concrete.

Equipment Considerations for RBC

1. 2.

Shafts - RBC shafts are used to support and rotate the plastic media. The shaft length may range from 5 to 27 ft (1.5 to 8.2 m), and normally the media on the shaft are 12 ft (3.7 m) in diameter. The standard length is 25 ft (7.6 m). Media - The media used for RBCs are manufactured of high-density polyethylene (HDPE) and are provided in different configurations or corrugation patterns. Corrugations increase the available surface area and enhance structural stability. The types of media are classified based on the area of media on the shaft and are commonly termed low- (or standard) density, medium-density, and high-density. Standard-density media, defined as media with a surface area of 100,000 ft2 (9290 m2) per 27 ft (8.2 m) shaft, have larger spaces between media layers and are normally used in the lead stages of an RBC process train. Medium- and high-density media have surface areas of 120,000 to 180,000 ft2 (11,149 to 16,723 m2) per 27 ft (8.2 m) shaft and are used typically in the middle and final stages of an RBC system where thinner biological growths occur. Higher density, thicker media is used when temperature are higher than normal (50 mil thick).

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9.0 AEROBIC ATTACHED GROWTH (Cont) 3.

4.

5.

6.

7.

Drive Systems - Most RBC units are rotated by direct mechanical drive units attached directly to the central shaft. Other drive system utilizes an air drive assembly that consists of deep plastic cups attached to the perimeter of the media, an air header located beneath the media, and an air compressor. The release of air into the cups creates a buoyant force that causes the shaft to turn. Both systems have proven to be mechanically reliable. Variable speed features can be provided to regulate the speed of rotation of the shaft. A relatively new upgrade to RBC is the submerged biological contactors (SBC). SBCs are 75 to 90% submerged and have a dual air driven system. The drive air header is located at the tank bottom on the upward side of the SBC and delivers air to the aircups to provide rotational torque. The process air header is located on the opposite side and delivers air to the media envelope. The air rushed up into the media's radial passages to the center core of the SBC. This action both shears biomass maintaining a relatively thin layer and aerates the internal attached biofilm that would not normally be exposed to atmospheric oxygen because of the greater submergence of the SBC. Tankage - Tankage for RBC systems has been optimized to 0.12 gal/ft2 (0.0049 m3/m2) of media, resulting in a stage volume of 12,000 gal (45.4 m3) for a 100,000 ft2 (9,290 m2) shaft. (Reference 5) Based on this volume, a detention time of about 1.4 hrs is provided for a hydraulic loading of 2 gal/ft2/d (0.08 m3/m2/d). A typical sidewater depth is 5 ft (1.52 m) to accommodate a 40% submergence of the media. An additional freeboard of 6 in. (150 mm) is typically provided. Enclosures - Segmented fiberglass-reinforced plastic covers are usually provided over each shaft. In some cases, units have been housed in a building for protection against cold weather, for improved access, or for aesthetic reasons. RBCs are enclosed to 1) protect the plastic media from deterioration due to ultraviolet light, 2) protect the process from low temperatures, 3) protect the media and equipment from damage, and 4) control the buildup of algae in the process. Covers are recommended to prevent biomass washoff due to rain and wind storms. Safety - H2S concentration of 10 ppm (mg/L) or more in the wastewater stream may be stripped out by the trickling filter with generation of noxious ground level odors. Gas masks or other safety equipment should be located near the trickling filter if there is a possibility that H2S will get into the wastewater. Effluent Handling - If a sedimentation tank is provided to remove solids from the RBC effluent, it can be a clarifier, tank, basin, or pond, and should be designed on an overflow rate of 700 - 1000 GPD/ft2 (1.19 to 1.70 m3/m2/hr).

9.3 DESIGN PROCEDURE Whenever feasible, pilot tests should be performed on the actual wastewater to be treated. Such testing can provide critical site specific information to help ensure that the most effective and economical configuration and design are selected. This is especially important for atypical wastewater where there is little actual operations experience treating the streams. A sample duty specification sheet for attached growth systems can be found in Table 9.1-2. Quick, Rough Sizing Basis / Without Lab / Pilot Test Data - The following Quick, Rough Sizing Basis is for the purpose of attached growth system size screening only. The rules of thumb should be used with caution. In addition, contingency of 10 20% should be added depending upon the application. A. Trickling Filter

Obtain a value for np, a constant characteristic of the packing, from a vendor. If one is not readily available, use np = 0.5 for plastic packing. For reaction-rate constant, K′, use the following: K′ = 0.018 to 0.074 for refineries or K′ = 0.0029 to 0.018 for petrochemical plants For different depth of filter media, the diameter of the trickling filter bed can be obtained as follows: é êæ 4Q ö φ = êç ÷ êè π ø ë

æ In(So /S ö çç ÷÷ è K′ D ø

é êæ 17.64Q ö φ = 0.305 êç ÷ π ø êè ë

1 np

ù ú ú ú û

1/2

Without Recirculation

æ 0.305 ln(So /S) ö çç ÷÷ K′ D è ø

1 np

(Customary)

Eq. (9.3-1a)

(Metric)

Eq. (9.3-1a)M

1/ 2

ù ú ú ú û

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9.0 AEROBIC ATTACHED GROWTH (Cont) ì ï ï ï ï ï ï φ = í ï ï ïπ ï ï ïî

1/2

4Q (1 + Nrr ) é ê (K′ D) ê ê æ (S /S) + N rr ê Inç o êë çè 1 + Nrr

ì ï ï ï ï ï ï φ = 0.305 í ï ï ïπ ï ï ïî

1/np

ù ú ú öú ÷ú ÷ú øû

ü ï ï ï ï ï ï ý ï ï ï ï ï ïþ

With Recirculation

(Customary)

Eq. (9.3-1b)

(Metric)

Eq. (9.3-1b)M

1/2

17.64Q (1 + Nrr ) é ê 3.28 K′ D ê ê æ (S /S) + N rr ê lnç o êë çè 1 + Nrr

where: φ Q So S K′ D Nrr np

= = = = = = = =

1/np

ù ú ú öú ÷ú ÷ú øû

ü ï ï ï ï ï ï ý ï ï ï ï ï ïþ

Diameter of trickling filter bed, ft (m) Total influent flow, gpm (m3/hr) Influent BOD5, ppm (mg/L) Desired effluent BOD5, ppm (mg/L) Reaction-rate constant for a particular packing and temperature, dimensionless Depth of filter media, ft (m) Recirculation ratio, dimensionless Packing characteristics, dimensionless

The recirculation ratio, Nrr, is based on the desired diluted BOD5 concentration in the influent. S − Sd Nrr = o Sd − S where: Nrr So Sd S

B.

= = = =

Eq. (9.3-2)

Recirculation ratio for individual filters, dimensionless Influent BOD5 concentration, ppm (mg/L) Desired BOD5 concentration in the diluted feed, ppm (mg/L) Effluent BOD5 concentration, ppm (mg/L)

Rotating Biological Contactor

The following typical design information for rotating biological contactors are based on wastewater temperatures above 55°F (13°C). Hydraulic Loading, gal/ft2-day (m3/m2-day):

2.0 to 4.0 (0.08 to 0.16)

Organic Loading, LF: lb Soluble BOD5/103 ft2-day (kg/m2-day)

0.75 to 4.0 (0.0037 to 0.0196)

lb Total BOD5/103 ft2-day (kg/m2-day) Maximum Loading on First Stage, LF max.:

2.0 to 4.0 (0.0098 to 0.0196)

lb Soluble BOD5/103 ft2-day (kg/m2-day) lb Total BOD5/103 ft2-day (kg/m2-day) Hydraulic Retention Time, hr: Effluent BOD5, ppm or mg/L:

2.5 to 6 (0.0123 to 0.0294) 6 to 12 (0.0294 to 0.0588) 0.7 to 1.5 15 to 30

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9.0 AEROBIC ATTACHED GROWTH (Cont) 1. 2.

Choose an organic loading factor, LF, from the table above. The required surface area of RBC is A rbc =

12Q x So LF

(Customary)

Eq. (9.3-3)

A rbc =

0.024Q x So LF

(Metric)

Eq. (9.3-3)M

where: Arbc Q So LF

3. 4.

= = = =

Surface area, ft2 (m2) Total influent flow, gpm (m3/hr) Influent soluble or total BOD5, ppm (mg/L) Organic loading factor, lb BOD5/103 ft2day (kg/m2-day)

Choose a design organic load peaking factor, LPF. As a first guess, use LPF = 2 x LF. Determine the loading per unit area by Max Loading 12Q x So x LPF = Unit Area A rbc

(Customary)

Eq. (9.3-4)

Max Loading 4.88 Q x So x LPF = Unit Area A rbc

(Metric)

Eq. (9.3-4)M

where: Max. Loading per Unit Area Q So LPF Arbc

5.

6.

= = = = =

lb BOD5/103 ft2day (kg/m2day) Total influent flow, gpm (m3/hr) Influent soluble or total BOD5, ppm (mg/L) Organic load peaking factor, lb BOD5/103 ft2day (kg/m2-day) Surface area, ft2 (m2) calculated from step 2

Check if the computed maximum organic loading rate per unit area is acceptable by checking against the maximum loading on the first stage, LF max., on the table above. If the max. loading per unit area is not acceptable, modify the design organic load peaking factor, LPF. Determine area based on average flow using an overflow rate by A avg flow =

1440Q qover

(Customary)

Eq. (9.3-5)

A avg flow =

Q qover

(Metric)

Eq. (9.3-5)M

where: Aavg flow = Q = = qover

Surface area based on average flow using overflow rate, ft2 (m2) Total influent flow, gpm (m3/hr) Overflow rate, gal/ft2·day (m3/m2-hr)

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9.0 AEROBIC ATTACHED GROWTH (Cont) 7.

Determine area based on peak flow using an overflow rate by A peak

flow

=

1440 Q x LPF qover

(Customary)

Eq. (9.3-6)

A peak

flow

=

204Q x LPF qover

(Metric)

Eq. (9.3-6)M

where: Apeak flow Q LPF qover

8.

= = = =

Surface area based on peak flow using overflow rate, ft2 (m2) Total influent flow, gpm (m3/hr) Organic load peaking factor, lb BOD5/103ft2d (kg/m2-day) Overflow rate, gal/ft2-day(m3/hr-m2)

Choose the bigger computation from steps 6 and 7 to size the settling facilities. Rough sizing for a clarifier is 700 1000 gpd/ft2 (1.19 to 1.70 m3/m2-hr).

9.4 SAMPLE DESIGN PROBLEM Example 1: Trickling Filter Without Recirculation

A wastewater from a refinery has a flow rate of 2,000 gpm (454 m3/h) and a BOD concentration of 480 mg/L. Estimate the size of trickling filter required to reduce the BOD concentration to 200 mg/L. The filter is to be packed with Surfpac that has a packing characteristic, np, of 0.5. Use K′ = 0.028. Use Eq. 9.3-1a to determine the diameter of the trickling filter for different depth of filter media. 1/2

1/2

é 4 ⋅ 2000 gpm é ln(480/200) ù1/0.5 ù ö æ ú ÷ ê ú ú êè π ø ë 0.028 D û û ë

φ = êç

éæ 17.64 ⋅ 454 m3 /h ö ÷ φ = 0.305 êç ÷ êçè π ø ë

or simplified

1/0.5 ù

æ 0.305 ln(480/200) ö çç ÷÷ 0.028 D è ø

1/2

ú ú û

2 é æ 31.27 ö ù φ = ê2546.48 ç ÷ ú

ê ë

è

D

(Customary)

ø úû

1/2

or simplified

2 é æ 9.54 ö ù φ = 0.305 ê2549.2 ç ÷ ú ê è D ø úû ë

(Metric)

For different depth of packing, the filter diameter is as follows: DEPTH OF PACKING, D

DIAMETER OF TRICKLING FILTER, f

ft

m

ft

m

20

6.1

79

24.0

25

7.6

64

19.5

30

9.1

53

16.2

35

10.7

46

14.0

40

12.2

40

12.2

45

13.7

36

11.0

The most economical combination of packing depth and trickling filter diameter, consistent with the allowable height to which the packing can be stacked, the available distributors, and the spatial limitations, should be selected. Example 2: Trickling Filter With Recirculation

A wastewater from a refinery has an influent flow rate of 2,000 gpm (454 m3/h) and a BOD concentration of 1,050 mg/L. Estimate the size of trickling filter required to reduce the BOD concentration to 400 mg/L. The filter is to be packed with Surfpac that has a characteristic factor, np, of 0.5. It has been decided to recirculate effluent to dilute the influent BOD concentration to 500 mg/L or below to assure aerobic conditions at the top of the filter. Use K′ = 0.028.

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9.0 AEROBIC ATTACHED GROWTH (Cont) Determine the recirculation ratio based on the desired diluted BOD5 concentration in the influent using Eq. 9.3-2. Nrr =

1050 mg/L − 500 mg/L = 5 .5 500 mg/L − 400 mg/L

Using Eq. 9.3-1b, the diameter of the trickling filter for different depth of filter media is: 1/2

é ù ê ú ê ú ê ú ê ú ê 4 ⋅ 2000 gpm (1 + 5.5) ú ú φ = ê 2 ê æ ö ú ê ç ÷ ú ê ç ÷ ú 0.028 D êπ ç ÷ ú ê ç lnæç (1050/400) + 5.5 ö÷ ÷ ú ÷÷ ú ê ç ç 1 + 5.5 øø û ë è è é ê ê ê ê ê φ = 0.305 ⋅ ê ê ê ê êπ ê ê ë

or simplified

é

1/2

ù 2ú êë π (0.125 D) úû

φ = ê

52000

(Customary)

1/2

ù ú ú ú ú 17.64 ⋅ 454 (1 + 5.5) úú 2 é ù ú ê ú ú ê 3.28 ⋅ 0.028 ⋅ D ú ú ê æ (1050/400) + 5.5 ö ú ú ÷÷ ú ú ê lnçç ú 1 + 5.5 ø ûú û ëê è

or simplified

é 52055.64 ù φ = 0.305 ê ú 2 ëê π (0.0735 D) ûú

1/2

(Metric)

For different depth of packing, the filter diameter is as follows: DEPTH OF PACKING, D

DIAMETER OF TRICKLING FILTER, f

ft

m

ft

m

15

4.6

68

20.7

20

6.1

51

15.5

25

7.6

41

12.5

30

9.1

34

10.4

35

10.7

29

8.8

40

12.2

26

7.9

45

13.7

23

7.0

The most economical combination of recirculation ratio, packing depth, and trickling filter diameter consistent with the allowable height for the packing should be selected. Example 3: Rotating Biological Reactor

An RBC is to be designed for an effluent soluble BOD of 20 mg/L for a wastewater with an initial soluble BOD of 300 mg/L. The flow is 347 gpm (79 m3/hr). Compute the total number of stages and the required area. 1. Let organic loading factor, LF, be 1.5 lb soluble BOD5/103 ft2-day (0.0074 kg/m2-day). 2. The required surface area of RBC from Eq. 9.3-3 is: A rbc =

3.

12 ⋅ 347 gpm ⋅ 300 mg/L 1.5 lb. SBOD5 / 103 ft 2 ⋅ day

= 832,800 ft2 (76865 m2)

Let LPF = 2 ⋅ LF = 2 ⋅ 1.5 = 3.0 lb SBOD5/103 ft2-day or 0.0148 kg/m2-day. ExxonMobil Research and Engineering Company – Fairfax, VA

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9.0 AEROBIC ATTACHED GROWTH (Cont) 4.

The loading per unit area is per Eq. 9.3-4 Max. Loading 12 ⋅ 347 gpm ⋅ 300 mg/L ⋅ 3.0 lbs SBOD5 / 103 ft 2 ⋅ day = Unit Area 832,800 ft 2

Max. Loading = 4.5 lb SBOD5 / 103 ft 2 ⋅ day (0.0221 kg/m 2 ⋅ day ) Unit Area 5. 6.

The calculated max. loading unit is within the allowable maximum loading on first stage of 4 to 6 lb SBOD5/103 ft2/d (0.0196 to 0.0294 kg SBOD5/m2/d). The surface area based on average flow using Eq. 9.3-5 is: Use average overflow rate of 850 gpd/ft2. A avg flow =

7.

850 gpd/ft 2

= 587.9 ft 2 (54.6 m2 )

The surface area based on peak flow using Eq. 9.3-6 is: Use maximum overflow rate of 1000 gpd/ft2. A peak

8.

1440 ⋅ 347 gpm

flow

=

1440 ⋅ 347 gpm ⋅ 3.0 1000 gpd/ft 2

= 1499 ft 2 (139 m2 )

Choose 1499 ft2 (139 m2) to size the settling facilities.

9.5 OPERATING STRATEGIES AND ENHANCEMENTS In case of overload, poor performance may be observed in RBCs such as low DO, H2S odors, and poor first-stage removals, if multi-staging exists. Under these conditions, filamentous organisms such as Beggiatoa, a sulfate-reducing organism, may develop. Overloading problems can be overcome by removing baffles between first and second stages to reduce surface loading and increase oxygen-transfer capability. Other approaches include supplemental air systems, step feed, or recycle from the last stage. Once an RBC system is in place, there is nothing that can be altered except the speed of rotation and degree of submergence of the discs. Regular maintenance of RBC such as greasing the bearing and checking the level of lubricant in the chain guards will be required on a weekly basis. On a quarterly or semiannual basis, maintenance includes changing the lubricant in the gear reducer and inspecting the chains and sprockets for wear and slack.

10.0 ANAEROBIC SYSTEMS The anaerobic treatment process is a biological treatment process that occurs in the absence of dissolved oxygen. The suspended or attached-growth microorganisms “consume" the soluble organic material by converting it, in the presence of nutrients, to a gas mixture consisting of CH4, CO2, H2S, water vapor, and more microorganisms. The microorganisms obtain their energy from organic (i.e., alcohols and ketones) or inorganic (i.e., carbonates and sulfates) compounds in the feed. In this biological process, wastewater is mixed with recycled bio-sludge and then digested in a reactor sealed off from the entry of air. The contents of the digester are mixed completely. The anaerobic sludge is recycled back to mix with the influent wastewater while the effluent is discharged for further treatment. The anaerobic process has a low microorganism growth rate, and therefore, the disposal of excess bio-sludge is minimized. There are several types of anaerobic reactors: fixed bed, fluidized bed, expanded bed, conventional digester, recycled bed, recycle flocs, and upflow anaerobic sludge blanket reactor. Some of these reactors contain fixed films that supply a surface area for the microorganisms to grow. The anaerobic treatment process becomes stable and economical when the influent BOD wastewater concentration is greater than 3000 ppm (mg/L) and is mainly soluble organics. However, this process is sensitive to excessive oil, pH, and salt variations. The BOD removal efficiency ranges from 60 to 90 percent. The effluent from the anaerobic digester is not suitable for discharge to a receiving water body and therefore must be treated further usually with aerobic technology. Anaerobic (without oxygen addition) biological treatment is known to have been applied to only one ExxonMobil location on a large scale, for a wastewater from a crude oil production treatment facility. This type of system was applied because of the concentrated nature of the produced water. In this particular location, conventional aerobic activated sludge treatment followed the anaerobic treatment unit to ensure effluent quality requirements were met.

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11.0 ANOXIC SYSTEMS An anoxic system is one where an oxidized inorganic compound such as nitrate ( NO3− ) or nitrite ( NO2− ) functions as an electron acceptor during certain types of microorganism activity in the absence of free molecular oxygen. The main application for anoxic systems is denitrification; the removal of nitrates from wastewater by fostering the growth of microorganisms that convert NO3− to nitrogen gas. Organics + NO3− → New Cells + N2 + CO2 + H2O + OH– + Energy

from Eq. (6.1-5)

Denitrification is achieved by the action of a number of species of heterotrophic bacteria, e.g., Bacillus, Pseudomonas, Achromobacter, Micrococcus, etc., under conditions of oxygen deprival. Denitrification occurs preferentially in the presence of a carbonaceous substrate but can still occur without carbon, although at a much lower rate. Due to the relatively short residence times in industrial biological treatment systems, adequate carbonaceous substrate is generally required. Denitrification may be required at sites that have stringent total nitrogen effluent specifications. The application of anoxic systems to denitrificiation is further discussed in Section 12.0 NITROGEN MANAGEMENT.

12.0 NITROGEN MANAGEMENT 12.1 DESCRIPTION Biological control and removal of nitrogen from wastewater is based on four biological processes: (1) ammonification; (2) cell synthesis: (3) nitrification; and (4) denitrification. The interrelationship of these processes constitutes the nitrogen cycle in aqueous systems. A representation of the nitrogen cycle in wastewater treatment systems is depicted on Figure 12.1-1. Ammonification is the biological conversion of organic nitrogen to the ammonium form. It can be expressed as: Organic nitrogen + Microorganisms → NH3 / NH+4

Eq. (12.1-1)

Nitrogen is an essential nutrient for microbial growth. Cell matter, generally described in simplified form as C5H7NO2, contains nominally 12 wt. % nitrogen. Waste activated sludge serves as a removal mechanism based on both bound nitrogen within the cell matter and also dissolved nitrogen compounds in water associated with the wasted sludge not recycled to the treatment plant. Nitrification, as described in Eqs. 6.1-3 and 6.1-4, is the biological oxidation of ammonia. It occurs in two steps, first to the nitrite form, then to the nitrate form. Overall, 2 moles of oxygen are required to oxidize 1 mole ammonium ion, or 4.57 lb oxygen/lb N. Denitrification, as described in Sections 6.1 and 11, is the process by which microorganisms convert nitrate or nitrite into nitrogen gas. The principal biochemical pathway for denitrification involves using oxygen bound up in the nitrate or nitrite, and not free or dissolved oxygen. A simplification of Eq. (6.1-5) can be expressed as: 4 NO3− + 2 H2O → 2 N2 + 4 OH– + 5 O2

Eq. (12.1-2)

Accompanying the formation of nitrogen gas, note that 1 mole of hydroxyl ions are produced for each mole of nitrate reduced. Further, 1.25 mole of oxygen is produced for each mole of nitrate reduced, or 2.86 lb oxygen per lb N. Theoretically, 63% of the oxygen consumed in the nitrification reaction is produced during denitrification. This produced oxygen is available to supply the oxygen demand of organic substrate and microorganisms present in the anoxic reaction zone. Nitrogen Forms

Nitrogen in aqueous treatment systems is present in a variety of forms. Petroleum and petrochemical raw wastewater will typically contain ammonia nitrogen, and depending on onsite manufacturing processes, organically bound nitrogen can also be present. During aerobic biological treatment, both ammonification of organic nitrogen and nitrification of ammonia nitrogen can occur, yielding oxidized nitrogen compounds. These oxidized species, predominantly nitrates, but some nitrites, can be reduced under anoxic conditions to yield nitrogen gas. A simplified representation of the nitrogen cycle within a wastewater treatment system is depicted in Figure 12.1-1. Controlling nitrogen in effluents requires both an understanding of applicable specific discharge limits as they relate to nitrogen and analytical test methods and reported results. Discharge limits may be specified as one or more of the following: ammonia; ammonia nitrogen; Total Kjeldahl Nitrogen; organic nitrogen; nitrates; nitrate nitrogen; nitrite; or nitrite nitrogen . Wastewater nitrogen measurement parameters, Standard Test Methods, and the relationship of these parameters are summarized in Table 12.1-1.

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12.0 NITROGEN MANAGEMENT (Cont) Process Microbiology

Unlike nitrification, a broad range of bacteria can accomplish denitrification. Denitrifiers are commonplace in most natural environments. Denitrifiers can be readily sustained in aerobic systems because of their ability to use oxygen and efficiently oxidize organic matter. This is due in part to the fact that they are facultative: they can use either oxygen or nitrate as their terminal electron acceptor. The process where microbial cells generate energy involves transferring electrons from a reduced electron donor (e.g., an organic substrate) to an oxidized electron acceptor (e.g., oxygen, nitrate, or sulfate). Microbial metabolism ensures that the most efficient form of energy production is utilized. Thus, if oxygen is present, it will be used preferentially over nitrate. Likewise, if oxygen is not present, nitrate will be preferentially used over sulfate. Sulfate reduction to sulfide and resulting odor production are not likely to occur within a treatment system that is anoxic (i.e., nitrate is present). Nitrate depletion through carbon overdosing, and thus leading to short term anaerobic conditions is also not likely to lead to sulfate reduction since the sulfate reducers will not have time to proliferate in numbers capable of significant sulfate reduction. Further, sulfate reducers are subject to poisoning in the aerobic zones of combined anoxic-aerobic treatment systems. Process / Reactor Variations For Biological Denitrification

All biological processes for total nitrogen removal require that ammonification (of organic nitrogen) and nitrification precede actual denitrification. For denitrification to occur, nitrates must be present. Various reactor configurations, however, can be employed to achieve denitrification with and without recycle of the effluent from the nitrification reactor. For each configuration, suspended or attached growth systems can be utilized. ExxonMobil does not have an attached growth nitrification & denitrification system in operation, but non-ExxonMobil refineries, literature, and pilot testing by EMRE indicate that attached growth can be considered for cost effective denitrification operations. There are three primary process configurations for Nitrification / Denitrification. Descriptions of these primary configurations are as follows: 1. Separate Sludge Post-Denitrification (Figure 12.1-2a) - Separate sludge denitrification is a system where a clarifier is provided for each reactor with its own sludge recycle system. Multiple sludge or separate sludge systems operate at higher unit removal rates and consequently require lower reactor volumes. However, this advantage is normally overshadowed by several disadvantages relative to single sludge systems, including increased clarification requirements, inferior sludge properties, increased need for pH control, increased aeration requirements, and increased organic carbon requirements (as compared to pre-denitrification mode). 2. Single Sludge Post-Denitrification (Figure 12.1-2b) - In this configuration, the aerobic reactor is followed by the anoxic reactor and then by the post aeration reactor. Most of the organic carbon in the wastewater is consumed in the nitrification / organic removal stage. For denitrification to occur, an adequate amount of organic carbon must be present in the wastewater. Therefore, addition of an external carbon source is typically required. 3.

Single Sludge Pre-Denitrification (Figure 12.1-2c) - In this single sludge configuration, the anoxic reactor precedes the aerobic reactor. A large portion (typically 4 to 6 times the influent rate) of the fully nitrified aerobic mixed liquor is recycled to the anoxic reactor to promote rapid denitrification. Depending on the desired total nitrogen removal, the organic material in the influent is used as a carbon source for the denitrifying bacteria. This configuration is applicable for refineries / chemical plants that have a fairly high influent TOC concentration. If the site does not have a sufficient amount of biodegradable TOC in the influent, an external carbon source such as methanol or sodium acetate must be added. Except for synthesis during denitrification in the anoxic zone, ammonia and organic nitrogen in the influent are untouched as they pass from anoxic to aerobic stage. The advantage of the pre-denitrification mode is that the wastewater organics can serve as the electron donor (carbon source) for the denitrification reaction. This type of denitrification system is found at the Slagen and Ingolstadt Refineries.

Alternative Reactor Designs

A variety of reactor designs and operating strategies have been applied to the nitrification / denitrification process. These are described below: 1. Concentric Reactors - This reactor within a reactor approach saves on both plot space and pumping of internal recycle, and is readily applied to a single sludge pre-denitrification system. The smaller inner circle volume acts as the anoxic zone, while the outer annulus volume serves as the aerobic nitrification reactor. Raw wastewater is fed to the anoxic zone where it is mixed with internal recycle from the aerobic zone. Effluent from the aerobic zone flows by gravity to a clarifier from where biomass is separated for return to the anoxic zone. This type of reactor is found at Slagen Refinery.

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12.0 NITROGEN MANAGEMENT (Cont) 2.

3.

4.

Cyclic Aeration - Alternating aerobic and anoxic zones can be achieved in a continuous-flow, activated sludge system by cycling the aerators on and off to create anoxic and aerobic zones. This approach, termed cyclical nitrogen removal (CNR) can be most effectively applied at existing treatment plants that must meet new or revised nitrogen discharge limits. Modifications may be as minimal as installing baffles or timers to cycle the aeration equipment, but can also include providing internal recycle pumps and piping. If several alternating zones are used, raw wastewater may be step-fed to those downstream zones in which organic carbon or COD has been depleted and denitrification reaction rates have become carbon limited. Oxidation Ditches or Racetrack Reactor - An oxidation ditch reactor uses looped trenches that provide a continuous circulation path for the wastewater. Aerators within the flow path supply both oxygen and motive force to the wastewater. Conceptually, the oxidation ditch is an endless channel. Because only a portion of the mixed liquor is withdrawn each cycle, a high internal recycle ratio is achieved. Nitrification occurs in that portion of the loop immediately downstream of the aerators; oxygen deficient conditions prevail immediately upstream of the aerators, thereby providing anoxic conditions for the denitrification reaction. Oxidation ditch reactors are more typical in municipal service and have not been used in the ExxonMobil circuit. Sequencing Batch Reactors - A variant of the fill-and-draw SBR described in Section 7.0 involves pulsing the aeration equipment in the activated sludge treatment unit on a timed cycle. This results in alternating aerobic and anoxic conditions being achieved on a temporal basis within the same reactor (as opposed to multiple reactors or zones in series). SBR denitrification systems have not been applied at ExxonMobil manufacturing sites.

Alternative Biological Processes

Biological processes less widely applied, or still in development, include facultative lagoons with algae harvesting and constructed wetlands. Facultative lagoons achieve nitrogen removal by both ammonia stripping and by algae synthesis. Carbon dioxide produced from aerobic (surface layers) and anaerobic stabilization (bottom layers) is the carbon source for the algae, which photosynthetically produce biomass and oxygen. CO2 depletion associated with algae synthesis will result in an increase in pH; this can limit ammonia stripping. Significant ammonia stripping does not occur at a pH of greater than 8.5. Surface constructed wetlands (characterized by a free water surface) rely on higher forms of plant life (relative to algae), e.g., duckweed, water hyacinths, to achieve nitrogen removal. This is effectively an attached growth system that relies on synthesis of new plant matter for nitrogen removal. Systems that rely on synthesis for nitrogen removal ultimately must plan for harvest and disposal of the resultant biomass.

12.2 DESIGN CONSIDERATIONS Similar to conventional and nitrifying activated sludge systems, the design of an anoxic denitrification process must consider the following: 1) compliance with regulatory effluent requirements, 2) feed wastewater characteristics, variability, and pretreatment requirements, 3) selection of process configuration, 4) selection of reactor type, and 5) need for pilot plant data. The design considerations for all these items are discussed in Section 6.2 of this Design Practice. In addition, there are specific considerations that apply for the denitrification process, which are discussed below. Effluent Nitrogen Limits

Discharge requirements may be specified or expressed as one or more of the following: ammonia; ammonia-nitrogen; Total Kjeldahl Nitrogen; organic nitrogen; nitrates; nitrate-nitrogen; nitrite; or nitrite-nitrogen. Wastewater nitrogen measurement parameters, Standard Test Methods, and the relationship of these parameters are summarized in Table 12.1-1. Actual design must address the different forms of nitrogen present in the effluent, and their impact on meeting specified limits. For example, if only ammonia or ammonia and nitrite (due to toxicity) are limited, then only nitrification is needed. If nitrate is limited, or a combination of ammonia, nitrite and nitrate, then nitrification alone is not sufficient and denitrification is also needed. Although not typical, if organic nitrogen (TKN) is limited then one must also take into account volatile suspended solids (microbial matter) in the effluent. Cell matter, generally described in simplified form as C5H7NO2, contains nominally 12 wt. % nitrogen. At 20 mg TSS/L and a VSS/TSS ratio of 0.8, the organic nitrogen in the suspended cell matter will contribute 2 wppm nitrogen in the effluent. pH

Denitrification is a net producer of alkalinity. The theoretical alkalinity production is 3.57 mg of CaCO3 per mg of nitrogen reduced. See Table 12.2-1. The production of alkalinity will raise system pH and offset some of the loss of alkalinity associated with nitrification in combined systems. Nitrification consumes 7.15 mg of alkalinity (expressed as CaCO3) per mg of ammonia-nitrogen nitrified. The pH in the anoxic reactor should be kept between 6.5 and 8.5. The specific optimum will vary depending on bacteria present, wastewater components, and denitrification reactor configuration. In a single sludge system, pH is typically controlled in the aerobic reactor only.

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12.0 NITROGEN MANAGEMENT (Cont) Temperature

The operating temperature for the denitrification process is approximately the same as in the nitrification process. The maximum allowed temperature is approximately 99°F (37°C) due to sensitivity of Nitrobacter. The optimum temperature for the nitrification / denitrification system is approximately 86°F (30°C). Temperatures below 68°F (20°C) will substantially decrease the rate of substrate removal such that nitrification may not occur at all, resulting in no denitrification. Dissolved Oxygen (D.O.)

D.O. level is very critical for both nitrifying and denitrifying reactors since it directly controls the growth rate of the nitrifiers. For a combined nitrification and denitrification process, the D.O. in the aerobic reactor should be established just above 2 mg/L and a D.O. of 0 mg/L should be targeted for the anoxic reactor. Care should be taken not to allow the D.O. to rise much above 2 mg/L in the aerobic reactor because it tends to carry over to the anoxic reactor. This D.O. will preferentially be consumed over that available from the nitrate. Dissolved oxygen will inhibit the activity of denitrifying enzymes. A dissolved oxygen level of ≥ 0.2 mg/L has been reported to inhibit the initiation of denitrification. Once initiated, the rate of denitrification will be significantly reduced (relative to that at zero D.O.) if the system dissolved oxygen level is increased. Relative denitrification rates (relative to those at zero D.O) of 50 % at 0.2 mg/L and 10 % at 2 mg/L have been reported. (Reference- Randall, page 51) Mixed Liquor Recycle Rate and Recycle Ratio

Effluent nitrogen levels are related to the mixed liquor recycle in single sludge pre-denitrification systems. Typical mixed liquor recycle ratios of 4-6 are common; higher ratios of 10-20 may be required to achieve low residual nitrogen in the effluent. Energy consumption and pumping costs can be significant. Plant layout should take into consideration such energy costs and look to minimize distances and hydraulic gradients between the anoxic and aerobic zones. Assuming complete denitrification of the NO3––N recycled to the anoxic stage and neglecting nitrogen assimilation, the required recycle ratio (internal mixed liquor + return sludge) is given by Eq. 12.2-1 (Reference 5) R =

(NH4+ − N)o − (NH4+ − N)e (NO3− − N)e

where: R

–1

Eq. (12.2-1)

=

Recycle ratio (multiples of raw influent wastewater)

( NH+4 –N)o

=

Influent ammonium-nitrogen, mg/L

( NH+4 –N)e

=

Effluent ammonium-nitrogen, mg/L

( NO3−

=

Effluent nitrate-nitrogen, mg/L

–N)e

If partial denitrification is required, R = % denitrification/(100-% Denitrification). The following demonstrates the impact of increased denitrification targets on recycle rates: PERCENT DENITRIFICATION REQUIRED TO ACHIEVE EFFLUENT LIMIT

MINIMUM RECYCLE RATIO

70

2.3

80

4

85

5.7

90

9

95

19

Based on practical considerations, and a desire to limit pumping requirements, maximum design denitrification targets are typically in the 80 – 85 % range. Higher percent denitrification can be achieved, but at significantly higher capital and operating cost.

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12.0 NITROGEN MANAGEMENT (Cont) Power Input to Anoxic Zone

The purpose of mixing within the anoxic zone is simply to keep the mixed liquor solids suspended while minimizing surface turbulence, and thus minimize transfer of oxygen from the atmosphere. Accordingly, significantly less power input (HP/Mgal) is needed than for aerobic zones as shown in Table 12.2-2. As noted above, mixed liquor recycle flow is substantially greater than the raw wastewater influent rate. The energy required, however, to properly mix this recycle flow with the influent is likely to be greater than that required for solids suspension. In that case, it is recommended that these streams be mixed immediately upon entering the anoxic zone by adding them in close proximity to each other.. Organic Substrate to Nitrogen Ratio

Carbonaceous matter in the wastewater is used as the energy source and electron donor for the denitrification reaction. When there is an insufficient amount of carbon in the denitrification system, denitrification can be inhibited. Should there be a lack of carbonaceous matter in the system, a readily biodegradable external carbon source such as methanol, acetic acid, sodium acetate, or certain raw wastewaters can be used. Factors to consider when choosing an external substrate include cost, low sludge yield, and toxicity / handling issues. The quantity of organic substrate required is dependent on the specific organic selected. Postulated denitrification oxidationreduction reactions for common external carbon sources are given in the table below: SUBSTRATE

DENITRIFICATION REACTION

EQUATION NO.

5 CH3OH + 6 NO3− → 3 N2 + 8 H2O + 1 CO3= + 4 HCO3−

Eq. (12.2-2)

Acetic Acid

5 CH3COOH + 8 NO3− → 4 N2 + 6 H2O + 2 CO2 + 8 HCO3−

Eq. (12.2-3)

Sodium Acetate

5 CH3COO− + 8 NO3− → 4 N2 + 4 H2O + 3 CO3= + 7 HCO3−

Eq. (12.2-4)

Methanol

Based upon these reactions, minimum theoretical substrate requirements can be determined based upon the oxygen demand exerted by the specific organic substrate employed. See Table 12.2-3. These requirements are the minimum needed to consume free oxygen produced from denitrification, and should be increased by a nominal factor of 1.3 - 1.5 to assure substrate requirements for nitrogen reduction and cell synthesis are satisfied. One must add to this value any additional substrate needed to deplete dissolved oxygen present and to account for the reduction of any nitrite-nitrogen present. For example, the denitrification equation using methanol and including cell synthesis is as follows: NO− + 1.08 CH OH + H+ → 0.065 C H O N + 0.47 N + 0.76 CO + 2.44 H O Eq. (12.2-5) 3

3

5 7 2

2

2

2

The following empirically derived equation shows the amount of methanol required where nitrate, nitrite and dissolved oxygen are present: æ 2.47 g methanol ö æ ö ÷ + (NO2− − N)o ⋅ ç 1.53 g methanol ÷ + DO ⋅ æç g methanol ö÷ Cm = (NO3− − N)o ⋅ ç ç 1.15 g COD ÷ − − ç ÷ ç g NO3 − N g NO2 − N ÷ø è ø è ø è

Eq. (12.2-6)

Using the theoretical values in Table 12.2-3, (excluding nitrite) the equation would be as follows: æ 1.91 g methanol ö ÷ + DO ⋅ æç g methanol ö÷ Cm = (NO3− − N)o ⋅ ç ç 1.5 g COD ÷ − ç ÷ − g NO N è ø 3 è ø where: Cm (NO3–N)o (NO2–N)o DO

= = = =

Eq. (12.2-7)

Required methanol amount, grams Initial Nitrate-N amount, grams Initial Nitrite-N amount, grams Initial dissolved oxygen amount, grams

Since most refinery applications will use carbon in the feed wastewater as part or all of the carbon source for denitrification, the carbon to nitrate-nitrogen ratio is needed for refinery wastewaters to determine if sufficient carbon is available. Because the source of carbon in all refinery wastewaters is different, bench scale testing is needed to be sure of the carbon to nitrate ratio needed. Bench testing for Ingolstadt Refinery indicated a carbon to nitrate ratio of at least 2 was needed. For that specific wastewater, this corresponds to a BOD to nitrate-nitrogen ratio of 4 and a COD to nitrate-nitrogen ratio of 7. For comparison, a literature source also found a BOD to nitrate-nitrogen ratio of 4 to be needed for denitrification.

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12.0 NITROGEN MANAGEMENT (Cont) Solids Residence Time (SRT)

SRT is one of the most critical design parameters of the nitrification / denitrification system. For a single sludge combined nitrification / denitrification system, the SRT is defined as: æ ö Volume of Nitrication Reactor ÷÷ ⋅ Total System SRT (designed for minimum operating temperatur e) SRT = çç Total Volume of the Biological System è ø

The microbial species responsible for nitrification have relatively low specific growth rates and only grow in the aerobic zone. Since nitrification must precede any denitrification reaction, it is necessary to maintain a SRT sufficiently large to insure a suitable nitrifier population. The recommended design SRT for a nitrification / denitrification system is 25 days. Hydraulic Retention Time (HRT)

The hydraulic retention time is the length of time the raw influent wastewater is retained in the BIOX. Typically, the HRT for the aerobic reactor should be long enough to obtain the desirable SRT. Per Section 6.3, Standard Procedures - Design Conditions, at least 18 hours is recommended for nitrifying systems (aerobic zone). The anoxic zone in an activated sludge nitrification-denitrification system should have an HRT of at least 4 hours. The anoxic volume should not be more than 20 to 33 % of the total reactor volume. Acceptability of the design should be demonstrated by pilot testing. Shorter HRT's may be acceptable. Actual detention time (ADT) within the anoxic and aerobic reactors is a fraction of the HRT and is a strong function of the internal recycle and sludge recycle. It is the quotient obtained by dividing reactor volume by the actual total volumetric flowrate to the reactor. A minimum actual detention time of one hour is recommended for the anoxic zone; lower values should only be considered for retrofit applications where existing reactor volume is fixed.

12.3 DESIGN PROCEDURE Currently, Slagen and Ingolstadt Refineries have full scale Nitrification / Denitrification processes in ExxonMobil. Slagen is a vendor design and denitrification was not required, so design HRT's less than DP guidelines were accepted. Ingolstadt is an inhouse design. Table 12.3-1 lists the dimensions and major design parameters for the Slagen and Ingolstadt Nitrification / Denitrification systems. The Slagen influent and effluent compositions are average values over a month of normal operation. Ingolstadt Refinery’s Nitrification / Denitrification facility is a revamp of an existing activated sludge system to meet new permit requirements limiting nitrogen discharges. Included in Table 12.3-1 are the design parameters. The influent and effluent compositions are the results obtained from the first few months after startup. An external carbon substrate was required at Ingolstadt. Table 12.3-2 provides a sample equipment list for nitrification / denitrification system. To develop a comprehensive DBM for a specific application, it is recommended that a pilot test be conducted at the site. The pilot testing will enable the engineer to understand the feed variability, define the optimal hydraulic and organic loadings, and assess the need for an external carbon substrate. Also, the applicability of the various process and reactor configurations available will be dependent on site specific facilities, wastewater quality, and discharge limits. This is especially true for those situations where retrofits of existing activated sludge treatment facilities are contemplated. If an alternative reactor design, for example concentric reactors, is selected for a project, then a duty spec may be prepared instead of a DBM. A sample duty specification sheet for Nitrification / Denitrification is provided in Table 12.3-3 to aid in discussions with the vendor or consultant. The design procedure for a single-sludge, single-anoxic zone nitrification-denitrification system consists of sizing the aerobic zone to nitrify the influent oxidizable TKN completely, and then sizing the anoxic zone and determining the required recycle rate. The procedure for sizing the aerobic zone is addressed in Section 6.3 of this practice. Steps 1-13 inclusively of the extended aeration-nitrification design procedure apply to nitrification-denitrification systems. In theory a reduction in the theoretical oxygen requirements can be taken to account for nitrate oxygen recycled to the anoxic zone for use in partially satisfying the raw wastewater oxygen demand within the anoxic zone. In systems where an external substrate is added to the anoxic zone to maintain required C/N (or COD/N) ratios, theoretical oxygen requirements can be greater than for an extended aerationnitrification system only. This is especially true when excess substrate is added to drive the denitrification reaction to completion. The size of the anoxic zone will be the larger of that calculated based on the HRT (feed based) and the ADT (including recycle). This requires knowing the feed rate, the internal mixed liquor (nitrate) recycle rate and the return activated sludge recycle rate. The required nitrate recycle rate is determined by the design nitrogen removal efficiency.

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12.0 NITROGEN MANAGEMENT (Cont) 12.4 - SAMPLE DESIGN PROBLEM - PERFORMANCE REQUIREMENTS, EXTERNAL SUBSTRATE REQUIRED AND ROUGH SIZING For a single sludge nitrification/denitrification system with pre-anoxic zone, and the wastewater characterized in Section 6.4 Sample Problem Step 1: Step 1: Determine the Level of Denitrification Required to Meet a Limit of 10 mg/L of ammonia, nitrite and nitrate as nitrogen.

The influent nitrogen level is given as 65 mg/L. Assume 100 %of the influent TKN is oxidized and /or converted to NH+4 –N, and100% nitrification is achieved in the aerobic reactor. Max. Effluent N present as NO3–N = 10 mg/L Denitrification required at design effluent limits = [ 65 - 10] mg/L / [65 ] mg/L x 100 = 85% Step 2: Determine if External Substrate is required in the Anoxic Zone.

The design feed COD is 550 mg/L. The required denitrification is [ 65 –10] mg/L = 55 mg/L NO3–N The applied COD/ NO3–N removed = 550 / 55 = 10 This exceeds the substrate requirements for a refinery wastewater (per Ingolstadt pilot data). No external substrate is required. Step 3: Determine the Recycle Ratio necessary to achieve this Percent Denitrification.

Per Eq. 12.2-1, based on NH+4 –N in the influent = 65mg/L; and NO3–N in the effluent = 10 mg/L At 100% nitrification, the effluent NH+4 –N = 0. The required recycle ratio is equal to (65mg/L / 10 mg/L) - 1 = 5.5 The return sludge recycle ratio is given as 1. 2 (Step 13) The internal mixed liquor recycle ratio is then = 5.5 – 1.2 = 4.3 Step 4: Determine the Volume of the Anoxic Zone.

Based on and HRT of 4 hours, the calculated volume is: V = (Q) HRT = (5.7 Mgal/d) (4 hours) (1 day/24 hours) = 0.95 Mgal Based on a recycle ratio of 5.5, and an ADT of 1 hour in the anoxic zone, the calculated volume is: V = Q (1 + RR) ADT = (5.7 Mgal/d) (5.5) (1 hour) ( 1 day/24 hours) = 1.3 Mgal The volume based on ADT is larger and would be selected. The total reactor volume is the sum of the anoxic zone and the aerobic zone (see pg. 30 for aerobic zone volume calculation). VTotal = 1.3 + 5.8 = 7.1 Mgal The anoxic zone represents 1.3/7.1 x 100 = 18 % of the total volume. Based on an HRT rule-of-thumb that the anoxic zone should not be more than 20 to 33 percent of the total reactor volume, so 1.3 Mgal is acceptable.

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12.0 NITROGEN MANAGEMENT (Cont) 12.5 OPERATING STRATEGIES AND ENHANCEMENTS In addition to those activated sludge operating strategies and enhancements described in Section 6.5, biological systems for total nitrogen management will benefit from the following practices: Monitor Feed Carbon and Nitrogen - An on-line total nitrogen analyzer is recommended on the raw untreated wastewater to the biological treatment system. Increases in feed nitrogen can be compensated for by external substrate addition to the anoxic reactor, increased alkalinity addition to the aerobic reactor, and increased oxygen supply to the nitrification reactor. Recycle rates to the anoxic reactor can also be stepped up in order to maintain compliance with discharge limits. pH - The pH should be monitored in each reactor. Provision to control the pH in the aerobic reactor at 6.5 - 8.5 through alkali addition should be provided. Carbon Substrate Source - Where possible raw wastewater should be utilized as the source of carbon substrate for the denitrification reaction. This allows savings on external substrate purchases, and reduces overall waste sludge production. Additionally, it offers the opportunity to reduce power consumed in delivering oxygen to the aerobic reactor by utilizing nitrate derived oxygen to treat influent BOD and COD. Where external substrate is required, any excess beyond that required to reduce nitrates should be minimized in order to limit sludge production and power consumption. Control of Oxygen Supply – Dissolved oxygen levels should be monitored continuously in each reactor. The nitrification reactor should be operated at 2 mg/L. Higher levels are to be avoided because free oxygen recycled to the anoxic reactor will be preferentially consumed over that available from nitrates. Variable speed blowers/compressors should be considered for diffused air systems. If mechanical aerators are used, the capability to cycle aerators on/off to control oxygen delivered to the system should be considered.

13.0 CHEMICAL ADDITION SYSTEMS An overview of nutrient addition, flocculant addition, and pH control for biological treatment applications is discussed below. However, Design Practice Section XIX-A9 Chemicals / Chemical Feeders for Wastewater Treatment and Design Guide DG 11-6-3 should be consulted for the actual design procedures and equipment criteria.

13.1 NUTRIENTS Nutrients are chemical elements such as nitrogen, potassium, phosphorous, sulfur, cobalt, zinc, and copper, which are essential for plant or animal growth. As a rule-of-thumb, a ratio of BOD5, nitrogen, and phosphorus (BOD5:N:P) = (100:5:1) is needed to provide the major nutrients. Most refinery / petrochemical plant wastewaters have necessary micronutrients, such as iron and zinc, so these do not normally need to be added for biological activity. Chemicals added to supply the nutrient requirements and the addition systems are discussed in the Design Practice Section XIX-A9 Chemicals / Chemical Feeders for Wastewater Treatment and Design Guide DG 11-6-3.

13.2 FLOCCULANTS Flocculants (polymer) are sometimes added to improve the settling of difficult sludges in gravity clarifiers. Injection is downstream of the aeration basin, but upstream of the gravity clarifier. Polymers are also used for the chemical conditioning of wastewater treatment sludge to enhance both the sludge thickening and dewatering process. The Standard Jar Test Procedure is recommended to determine what type and dosage of polymer should be used for a particular application. This procedure as well as the design procedure for polymer addition systems can be found in Design Practice Section XIX-A9 Chemicals / Chemical Feeders for Wastewater Treatment. Other references such as the Guidance Manual for Polymer Selection in Wastewater Treatment Plants (Reference 27) and ER&E Report No. EE.20E.84 Polyelectrolyte Guide (Reference 26) should also be consulted.

13.3 pH CONTROL pH - The pH should be monitored continuously in each aeration basin. The pH should be maintained at 6.5 to 8.5 at all times. The nitrifying bacteria are especially sensitive to pHs below 6.5. If the influent to the BIOX system is not in the range of 6.5 to 8.5, neutralization by the addition of caustic or acid is recommended. Nitrifying systems can produce large pH drops in the aeration basin from the feed pH, therefore it is recommended to pH control the basin, which is much easier to control than the feed. The Design Practice on Chemicals / Chemical Feeders for Wastewater Treatment discusses the design procedures of the addition systems.

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14.0 NOMENCLATURE 1.08 4.57 8.34

Temperature activity coefficient for activated sludge processes Conversion factor for amount of oxygen required for complete oxidation of TKN Conversion factor [lb/Mgal-(mg/L)]

a′

Oxygen utilization coefficient for synthesis, lb (kg) O2/lb (kg) organics removed

A Aavg flow Apeak flow Arbc Av AOR b

Clarifier Area in an activated sludge unit, or settling basin area in an aerated lagoon, ft2 (m2) Surface area based on average flow using overflow rate, ft2 (m2) Surface area based on peak flow using overflow rate, ft2 (m2) Surface area, ft2 (m2) Specific surface, ft2/ft3 (m2/m3) Actual Oxygen Required, lbs O2/d Endogenous decay coefficient, d-1

b′

Oxygen utilization coefficient for endogenous respiration, lb (kg) O2/lb (kg) VSS-d

C C20 CL Cm

Actual oxygen concentration, mg/L Surface dissolved oxygen saturation concentration at standard conditions, mg/L Concentration of dissolved oxygen to be maintained in the aeration basin, mg/L Required methanol concentration, mg/L

C*∞ 20

Aeration system equilibrium dissolved oxygen saturation concentration at standard conditions, mg/L

Cs Csw

Saturation oxygen concentration, mg/L Saturation oxygen solubility in the mixed liquor at operating temperature and design altitude, mg/L

C*walt

Aeration system dissolved oxygen saturation at operating conditions, mg/L

D Dcm Dip Do e f fd fp F/M G GL HRT kd

Depth of filter media, ft (m) Circle of influence - diameter for complete mixing, ft (m) Circle of influence - diameter for impingement pattern, ft (m) Initial dissolved oxygen concentration, mg/L Efficiency Conversion factor for converting BOD5 to BODULT (0.45 - 0.7) Supersaturation depth correction factor Pressure factor for oxygen solubility Food to microorganism ratio Solids flux, lb/ft2-d (kg/m2-d) Limiting solids flux, lb/ft2-d (kg/m2-d) Hydraulic residence time, d Fraction of biosolids degraded in an aerated lagoon

K′

Constant for a particular packing, wastewater, and temperature, dimensionless

Ks KLa LF LFmax LPF MLSS MLVSS

Substrate concentration at one-half the maximum growth rate, mg/L Overall oxygen transfer coefficient, d-1 Organic loading factor, lb BOD5/103 ft2/d (kg/m2/d) Maximum Organic loading factor, lb BOD5/103 ft2/d (kg/m2/d) Organic load peaking factor, lb BOD5/103 ft2/d (kg/m2/d) Mixed liquor suspended solids, mg/L Mixed liquor volatile suspended solids, mg/L

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14.0 NOMENCLATURE (Cont) np N N1 N2 (NH4–N)o (NH4–N)e (NO3–N)e Ni Nio Nir

A constant characteristics of the packing, dimensionless Actual oxygen transfer rate of surface aerators under operating conditions, lb O2/bhp hr Initial Nitrate - N concentration, mg/L Initial Nitrite - N concentration, mg/L Influent ammonium nitrogen, mg/L Effluent ammonium nitrogen, mg/L Effluent nitrate nitrogen, mg/L Effluent TKN, mg/L Influent TKN, mg/L Nitrogen required, lb/d (kg/d)

No Nrr p P Pb Pe P1 P2 Px q qover qtp qtd Q Qr Qw R

Standard oxygen transfer rate of the surface aerators under standard condition, clean water, 20°C, O mg/L DO, l atm pressure, usually supplied by aeration vendor, lb O2/bhp hr Recirculation ratio, dimensionless Saturation water vapor pressure at temperature of the mixed liquor, mm Hg Phosphorous required, lb/d (kg/d) Barometric pressure, psia Power input for blower at specified efficiency, hp Compressor inlet absolute pressure, psia Compressor outlet absolute pressure, psia Waste sludge production, lb/d (kg/d) Specific rate of substrate removal, d-1 Overflow rate, gal/ft2·day (m3/m2-hr) q at the pilot temperature, d-1 q at the design (minimum) temperature, d-1 Wastewater flow rate, Mgal/d (m3/d) but in section 5.2, gpm (m3/hr) Recycle flow rate, Mgal/d (m3/d) Sludge wasting flow rate, Mgal/d (m3/d) The recirculation ratio

RT

Reaction rate at T°C

R20 Rr S S Sd So SS SSo SSe Sludge SOTE SOTR SRT

Reaction rate at 20°C Oxygen utilization rate, mg/L-d Effluent substrate concentration, mg/L Concentration of growth limiting substrate in solution, mg/L Desired BOD5 concentration in the diluted feed, mg/L Influent substrate concentration, mg/L Suspended solids in effluent of aeration basin in an aerated lagoon, mg/L Influent suspended solids in an aerated lagoon, mg/L Suspended solids in settling basin effluent in an aerated lagoon, mg/L Sludge accumulation in lb/yr in an aerated lagoon Standard oxygen transfer efficiency, % Standard oxygen transfer rate requirement, lbs O2/hr Sludge retention time, d

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14.0 NOMENCLATURE (Cont) td

Page

Design (minimum) temperature, °C

tp

Pilot plant temperature, °C

T

Temperature, °C

Ta

Compressor inlet absolute temperature, °R

u um V Vi W X Xe Xf Xi Xo Xr Xv Y Ym

Specific microbial growth rate, d-1 Maximum specific microbial growth rate, d-1 Aeration tank reactor volume, Mgal (m3) Initial settling velocity, ft/hr (m/hr) Weight of air, lb/s Concentration of solids under aeration (MLSS), mg/L Effluent suspended solids concentration, mg/L The number of filters in parallel operation, dimensionless MLSS (TSS) concentration converted to g/m3 or lbs/ft3 Influent solids, mg/L Concentration of TSS in the recycle stream = clarifier underflow solids concentration, mg/L Mixed liquor volatile suspended solids, mg/L Microbial yield coefficient, mass microorganisms/mass substrate removed Maximum yield coefficient, mass microorganisms/mass substrate removed

α

Alpha factor for aeration

β

Beta factor for aeration

π

Total pressure, mg Hg

θ

Theta Value for aeration

φ

Diameter, ft (m)

µ

Viscosity of water, cP

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TABLE 1-1.A CHARACTERISTICS OF EXXONMOBIL ACTIVATED SLUDGE UNITS LOCATION CHARACTERISTIC Pretreatment

Average Flow Rate, gpm (m3/hr)

ANTWERP

FOS

INGOLSTADT

Sour Water Stripping, API Sep., DAF

Sour Water Stripping, API Sep., DAF

Sour Water Stripping, API Sep., DAF

1760 (400)

1270 (290)

0.69 (2600)

0.62 (2340)

6

9

35

Mechanical

Mechanical

Mechanical

Typical SRT, d

20+

20

Nitrification

Yes

Operational Parameters Aeration Volume, Mgal (m3) Aeration Residence Time, hr Aerator Type

Denitrification Settling Basin Dimensions, ft (m)

175 (40)

0.35 (1321)

28 Yes

No

No

Yes

100 x 13 x 10

130 x 33 x 7

25 x 46 x 8

(30 x 4 x 3)

(40 x 10 x 2)

(7.5 x 14 x 2.5)

200

560

700

Performance Parameters Typical Influent Concentrations, mg/L BOD

700

COD TOC

380

Ammonia

15

73

Typical Effluent Concentrations, mg/L BOD COD

22 50

76

TOC Ammonia

24 2

58

Phenols

0

Oil and Grease TSS Post Treatment

97

0.4 20

18

43

None

Settling Pond

Settling pond

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TABLE 1-1.A CHARACTERISTICS OF EXXONMOBIL ACTIVATED SLUDGE UNITS (Cont) LOCATION CHARACTERISTIC KARLSRUHE

PORT JEROME

ROTTERDAM

Sour Water Stripping, API Sep., DAF

Sour Water Stripping, API Sep., DAF

Sour Water Stripping, API Sep., DAF

1100 (250)

880 (200)

2860 (650)

Aeration Volume, Mgal (m3)

0.11 (400)

1.2 (4500)

1.8 (7000)

Aeration Residence Time, hr

1.5

18

11

Mechanical

Diffused / Mechanical

Diffused

2

10 - 15

30+

No

Yes

Yes

Pretreatment

Average Flow Rate, gpm (m3/hr) Operational Parameters

Aerator Type Typical SRT, d Nitrification Denitrification Settling Basin Dimensions, ft (m)

No

No

No

115 x 33 x 7

5500 ft2

132 ∅ x 10

(35 x 10 x 2)

(500 m2)

(40 ∅ x 3)

250

1000

450

60

30

35

60

150

100

60