D21WW_lecture_PartA

HERIOT-WATT UNIVERSITY School of the Built Environment D21WW (Water & Wastewater Treatment)

Recommended Reading Materials Texts Gerard, K (1996). Environmental Engineering. McGraw-Hill. Haigh N (1987): EC Environmental Policy & Britain. 2/e, Longman (A loose leaf continuously updated edition is held in the library Horan, N J (1990). Biological wastewater treatment systems (theory and operation). J. Wiley. Metcalf and Eddy Inc. (1991). Wastewater Engineering: treatment, disposal, reuse. McGraw-Hill. Tchobanoglous, G and Schroeder, E D (1987). Water quality, Addison-Wesley. Gray, N F (1994). Drinking water quality. J. Wiley.

Journals Journal, Chartered Institution of Water and Environmental Management

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Syllabus Year 4

Semester 2

School of the Built Environment

Module Title(s)

Water and Wastewater Treatment

Module Code(s)

D20WW

Outline syllabus of module(s)

PART A: Introduction Need for water & wastewater treatment Aim of the module Overview of water and Wastewater quality characteristics

PART B: Engineered Systems for Water Treatment Introduction to different treatment combinations Aeration Solids separation- the theory Primary sedimentation & design Coagulation & flocculation Filtration Disinfection and disinfecting agents Removal of refractory organics, colour and odour

PART C: Engineered Systems for Wastewater Treatment Overview of available options Preliminary Treatment Design Primary Treatment: a re-statement of primary sedimentation Secondary Treatment Further Wastewater Treatment Sludge Handling, Treatment and Disposal Disposal of Wastewater Treatment Effluent

PART D: Legislative aspects of Water Pollution Control UK laws EU Directives Future direction

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D20WW: Water and Wastewater Treatment

Part A: Introduction and Water Quality Characteristics

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1.

Introduction

Raw water from the different sources- rivers, springs, aquifers, etc- contains several impurities that must be removed before the water becomes suitable for drinking. However, because of the variation in the nature and levels of pollution, the combination of treatment process applied differs. On the other hand, the wastewater resulting from applying water to various domestic, industrial, commercial and agricultural uses contains many impurities that must be removed before the wastewater is discharged into the environment. The essence of wastewater treatment therefore is the substantial removal of major pollutants from the wastewater before discharging it into the environment. It is impossible to remove all of the pollutants; however, if a substantial part, typically >80%, is removed before discharge, then little or no adverse effects will be caused in the environment. 2. Aim The aim of this module is to understand the basis of design of water and wastewater treatment plant processes and operations, including the safe disposal of final products, i.e. effluent and sludge, of the treatment. The module will conclude with a brief discussion of the legislative and other provisions for water pollution control as applied in the UK and elsewhere.

3.

Important Water and Wastewater Characteristics – an Overview

The pollution loading of wastewaters from individual trades, farms etc., may be expressed in terms of kg/day of particular compounds. However, for mainly domestic wastes, the three most common parameters used to describe the waste are biochemical oxygen demand (BOD), the suspended solids (SS) and the bacteriological quality.

3.1

Biochemical oxygen demand (BOD)

(a)

Carbonaceous BOD (CBOD)

This is the oxygen used up by the bacterial oxidation of organic pollutants (Carbonaceous BOD) or ammonia (nitrogenous BOD), usually determined under standard conditions of incubation of 20 oC over 5 days. (Generally in this case, the nitrogenous BOD will be small, simply because those bacteria which oxidise ammonia would not have developed in sufficient numbers by the fifth day. The nitrogenous bacteria will become very abundant after about 7-8 days when most of the carbonaceous organic pollutants have been removed.) A BOD of 10mg/l means that 1 litre of waste will require 10 mg of oxygen when incubated at 20 oC for 5 days. The sample is incubated in a sealed bottle (250 - 300 mls) ; the initial dissolved oxygen concentration (measured on a duplicate sample ) less the final dissolved oxygen concentration (measured on the incubated sample) gives a measure of the BOD. The necessary bacteria and nutrients may be present naturally or may be added.

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Often it is necessary to predict the BOD without actual measurement. This can be achieved by modelling the BOD decay process. The kinetics of BOD oxidation depend upon the particular organics present; however, for domestic waste a first-order reaction rate is assumed:

dL = − K1L dt

(1)

where L is the BOD remaining or unoxidized (mg/l), t is time (d) and K1 is a constant with units of T -1 (day -1). Integration of the above equation using Lo as the initial unoxidized BOD gives the unoxidized BOD after time t as

Lt = e − K1t Lo

(2)

The oxidized BOD then becomes

BOD t = L o − L t = L o  1 − e − K1t 

(3)

Lo is often referred to as the ultimate BOD (mg/l), and approximates to the result of a BOD test carried out at 20oC over >20 days. Also, K1 depends on temperature in the form:

K T = K 20θ T − 20

(4)

where KT and K20 represent the values of K1 at T oC and 20 oC, respectively and θ is a constant.

(b)

Nitrification and Nitrogenous BOD (NBOD)

The term BOD in general calculations on loading, sewage treatment, effluent standards etc. usually means the 5 day 20oC BOD exerted by the carbonaceous material. In most circumstances this is acceptable because, as stated previously, the nitrosomonas and nitrobacter species of bacteria which are responsible for oxidizing

− (nitrite) and NO − → NO − (nitrate), respectively, by (ammonia) NH 4 → NO 2 2 3 NITRIFICATION are usually in very low concentrations in most samples of rivers, sewage, effluents etc. However, if the sample is taken after a lightly loaded

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percolating filter, the nitrogenous oxygen demand may be partially exerted within the 5 day period. Nitrogenous kinetics are also assumed to be first-order and the resulting BOD expression is

BOD N = L NO  1 − e − K N t 

(5)

where LNO is the ultimate nitrogenous BOD (mg O2/l), KN is the reaction rate constant (day-1), and t is time in days. KN, like K1, also depends on the temperature (see eq. (4)). For a combined carbonaceous BOD plus nitrogenous BOD, the total BOD is

 −K t − t  BOD = L o  1 − e − K1t  + L NO  1 − e N ( N )   

(6)

where tN is a delay time (day) before nitrifying organisms are effective.

Example 1 A BOD test is carried out for 15 days at 20oC. The reaction rate constants K1 and KN are 0.16 day -1 and 0.1 day -1, respectively, and Lo and LNO are 450 mg/l and 300 mg/l, respectively. The nitrifying organisms are assumed to be effective only from the time t = 8 days. Calculate the BOD exerted at days 5, 8, 12 and 15. Solution

Limitations of the BOD The problems with the BOD as a water quality parameter include that: i. its determination takes a long time- at least 5 days for current standard; ii. its determination may be affected by the presence of bactericidal substances in the sample;

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iii. it merely characterises the biodegradable organics; wastewater, particularly from industrial sources, contain a whole range of non-biodegradable organics which also need oxidising with dissolved oxygen. Hence, there is now the move away from BOD to using the chemical oxygen demand (COD). The determination of the COD is much faster (3 hours) and characterises all oxidisable substances in the waste, whether or not they are biodegradable.

Example 2 A BOD test was carried out on three samples of river water. All gave a dissolved oxygen initial reading of 7 mg/l. The final dissolved oxygen reading after 5 days incubation at 20 oC were 0, 3, 7 mg/l. Calculate the three BOD values. Solution

3.2

Suspended solids (SS)

The material in suspension in sewage commonly consists of inorganic grits, silts or clays, organic compounds such as bacteria, fats, greases, and a wide range of wastes from food cleaning and preparation processes. The total suspended material which will normally be removed by passage through a fine glass-fibre filter paper is classed as suspended solids. Both the BOD and SS concentrations in sewages are influenced by the daily per capita water usage. Present practice is therefore to assume a per capita production of BOD and SS in terms of kg/cap.day and to calculate expected concentrations in terms of local water usage. Example 3 A small sewage works serves a population of 500 people whose per capita BOD5 and SS production is 0.055 kg/day and 0.08kg/day, respectively. Water consumption is metered at 200 l/cap.day. A small industry discharges an effluent of 5 l/s containing 100 mg/l BOD5 and 200 mg/l SS over a period of 3 hours per day. Calculate the mean BOD5 and SS concentrations in the works inlet.

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Solution

In general, both the SS and BOD5 are the two main parameters for monitoring the performance of treatment works. In the UK for example, it is customary to expect that effluents from treatment works contain no more than 20 mg/l BOD5 and 30 mg/l SS for discharge to inland rivers, based on the Royal Commission Standards. These are referred to as the consent conditions.

3.3. Turbidity While the SS is used for wastewater, turbidity is used for examination of water. Turbidity is the extent to which light penetrates in the water sample. The main source of turbidity in water is the erosion of colloidal materials e.g. clay, silt. Other sources are soaps, detergents in household and in industrial wastewater.

Turbidity is measured photometrically using the Jackson Turbidimeter (Figure 1) and expressed JTU (Jackson Turbidity Unit). 1 JTU (Jackson Turbidity Unit) = Turbidity of 1 mg SiO2/litre of distilled water

More recent unit of measurement is the FTU (Formazin Turbidity Unit). Formazin provides more reproducible results/standards than SiO2; the standardised candle has also now been replaced by electric bulb.

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Sample

View

Long glass tube (calibrated against turbidity by silica, SiO2)

Black metal sheath

Standardised candle

Figure 1: Jackson Turbidimeter

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3.4

Bacteriological standards

Some countries use bacteria as an indication of the pollution state of a water sample. However, because of the very many types of bacteria and other micro-organisms that could be present, the practice is to use certain bacteria derived from the gut of warm blooded animals as an indication of pollution, e.g. faecal coliforms. The number of these micro-organisms is enumerated using the fermentation tube test. Five tubes containing 10ml, five tubes containing 1 ml, and five tubes containing 0.1 ml of sample together with the necessary growth medium are incubated for 24 hours and the number of tubes showing positive reactions, indicated by the giving off of gas, is determined. Then the number of bacteria is expressed as most probable number, MPN/100 ml sample which is read from tables prepared for the purpose. Note that for a strong sewage, extensive dilution of the sample may be necessary prior to the incubation.

3.5

Typical Sewage Analyses

An analysis of a typical municipal sewage will produce characteristics as shown below. The wastewater will also contain other quality parameters as shown in Table 2. Treatment Stage

Quality characteristics (mg/l) BOD

300

Settled sewage (After primary clarifier) 175

20

COD

700

400

90

SS

400

200

30

Ammonium-N (NH4-N)

40

40

5

Nitrate-N (NO3-N)