Activated Sludge Process and Its Design, Operation and Control

Activated Sludge Process and its Components

Activated Sludge Process Most commonly used aerobic biological treatment process • Microorganisms (mostly bacteria including nitrifiers, denitrifiers and phosophate assimilators) are involved in the treatment process Used for secondary treatment of medium and low strength industrial and municipal wastewaters • Designed and used mainly for the removal of biodegradable organic matter • Often designed and used even for the removal of nutrients (nitrogen and phosphorus) • Removal of nutrients, TSS, pathogens and heavy metals can be coincidental Generates waste activated sludge

Activated Sludge Process Usually employed in conjunction with physical and chemical treatment processes meant for • Preliminary & primary treatment (primary clarifier/ clariflocculator) • Post/ advanced/ tertiary treatment (disinfection, filtration, etc.)

Usually receives clarified wastewaters • Primary clarification is often omitted (in case of small

communities for small flows and low TSS levels and in hot climates for avoiding/controlling odour problems

• Certain modifications of ASP (sequencing batch reactors, oxidation ditches, aerated lagoons, contact-stabilization process) do not require primary clarification of wastewater

Raw sewage

Bar screen

Screenings

Sewage Sump & pumping Grit chamber

ASP

air

Equalization Tank

scum

Primary clarifier

air

Aeration tank

Treated effluent

Secondary clarifier

Grit

Exhaust gases (CO2 and H2O) Biogas flaring unit Biogas if not flared (supplied as fuel or emitted)

Stabilization tank

Sludge drying beds

Dried sludge for disposal

Clarified effluent to sewage sump Filtrate to sewage sump

Sewage Treatment Plant Incorporating Activated Sludge Process

Air or oxygen supply Nutrients and Alkalinity (if needed) Influent

Secondary clarifier

Aeration basin

Sludge recycling Wasted activated sludge

Activated Sludge Process

Effluent

Components of ASP Facilities and equipment of an ASP • Aeration basin(s) • Air/Oxygen supply/transfer system(s) • Secondary clarifier(s) • Return activated sludge system(s) • Waste activated sludge handling facilities/system • Chemical (nutrients and/or alkalinity) feed systems

Components of ASP Aeration basin • Wastewater is brought in contact with active microbial biomass for treatment (through bioflocculation, biosoprtion, and biooxidation including nitrification) • Favourable conditions for biological treatment are maintained in the aeration basin through aeration (for oxygen supply and mixing) and chemicals addition • Aeration basin may often include – An anoxic section (for denitrification) – A selector section for tackling bulking (often along with denitrification) and even for phosphate removal

Components of ASP Air/Oxygen supply and transfer systems • Mainly two types: diffused aeration and mechanical aeration systems • Diffused air system includes diffusers, air headers, air mains, and other piping and fixers, and blowers • Mechanical aerators – Surface mechanical aerators (fixed and floating aerators) with or without draft tubes – Submerged turbine aerators – Horizontal axis aerators (brush aerators)

• Aeration system should be capable of – Supplying enough oxygen to meet the demands – Mixing of the aeration basin contents to the desired level

Components of ASP Secondary Clarifier/ Secondary Settling Tank • Meant to remove biological flocs from mixed liquor and allow clarified secondary effluent out – Sludge thickening to desired level to facilitate both sludge recycling and wasting

• These are center-feed circular tanks of side wall liquid depth of 3.7 to 6 m and radius of < 5 times liquid depth – Rim feed circular clarifiers & rectangular clarif. are also used

Secondary clarifier has • Cylindrical baffle of diameter 30-35% of tank diameter • A central well (or mixed liquor inlet section) designed to – dissipate the influent energy – evenly distribute flow – promote flocculation

Components of ASP Secondary clarifier has • Revolving mechanism for scrapping (transport & remove) the settled sludge and for the removal of floating scum – The sludge is either plowed to the central hopper for removal or it is removed directly from the tank bottom by suction orifices either hydrostatically or by pumping – Very little scum is usually formed - removal becomes necessary when primary clarifier is not used

• Overflow weirs and collection troughs – placed at 2/3rd to 3/4th radial distance from the center in larger tanks and at the perimeter in the smaller tanks – baffles may be provided to deflect density currents and to avoid scum overflow

Components of ASP Return activated sludge system • Underdrain of the secondary clarifier • Reliable pumping and piping • Appurtenances for regulating return sludge pumping rate • Return sludge may pass through a selector (aerobic, anaerobic or anoxic) • Return sludge (bioflocculated organic matter!) stabilization prior to mixing with the influent

Components of ASP Waste activated sludge system • Wasting can be either from the secondary clarifier or from the aeration basin directly • Better regulation if wasted directly from the aeration basin – but volume wasted is higher • Wasted sludge needs handling and disposal – stabilization, thickening, dewatering and drying – Aerobic or anaerobic stabilization – Chemical or thermal stabilization – Thickeners and sludge drying beds – Simultaneous thickening and aerobic or anaerobic stabilization

Biodegradable fraction of the secondary sludge

Components of ASP Chemical feed systems • Nutrients and alkalinity addition may be required if the influent is deficient in them – Urea and Diammonium phosphate are usually used – phosphoric acid/phosphate rock (can these be used in place of the DAP?) – Nitrification and denitrification levels can influence the alkalinity addition required

• Chemical addition may also be made for the chemical precipitation removal of phosphorus from wastewater • Polymers may often be added for improving the settling characteristics of the mixed liquor solids • Chemical dosing often for temporary tackling of bulking sludge problem

Mechanisms of Treatment

Mechanisms of treatment: Organic matter removal Aerobic microorganisms (activated sludge), specially bacteria, are responsible and treatment involves bioflocculation, biosorption and biooxidation • Suspended & colloidal organic matter becomes integral part of biological sludge by bioflocculation and biosorption • Soluble organic matter is removed by biosorption (adsorption and absorption) • Bioflocculated & biosorbed organic matter is solubilized through hydrolysis and absorbed by microbes as food • Absorbed matter is biooxidized (partly respired and rest is used in synthesizing new microbial biomass) • Through wasting excess activated sludge (at secondary clarifier) organic matter is removed as biological flocs

Mechanisms of treatment: Organic matter removal

Inorganic end products (CO2, H2O, NH3, Energy, etc.)

io n rp t bi os o

bi

lo f o

ul c c

n it o a ion t a id io n x o t bio spira re

Soluble organic matter

Activated sludge bi b o-o i o s xid y n ati t h e on s is

Recycled sludge

biosorption bioflocculation

Colloidal organic matter

Suspended organic matter

Residual sludge

New microbial biomass

Mixed liquor solids

Mechanisms of treatment: Organic matter removal Bioflocculation • Microbes of the aeration basin produce biopolymers that bring about flocculation and form biological flocs • Biological flocs are constituted of – Microorganisms – Cell debris – Suspended and colloidal organic and inorganic constituents of the wastewater

• Bioflocculated organic material can be hydrolysed into soluble organic matter and biosorbed

Mechanisms of treatment: Organic matter removal Biosorption • More rapid than biooxidation and involves both adsorption and absorption by microbes • Adsorbed matter can be solubilized by hydrolysis and partly absorbed by microbes and rest is bled into effluent

Biooxidation • Represents actual removal of biodegradable org. matter • Involves both aerobic respiration (including autooxidation) and biosynthesis • Respiration requires O2 (DO - 0.5 to 1.5 mg/L, 1.07 g/g) and produces H2O, CO2, NH3, etc. and energy • Biosynthesis produces new microbial biomass (1.42 g/g, yield coefficient) – requires nutrients (N and P)

Mechanisms of treatment: Nitrogen removal Organic-N decomposition and hydrolysis Ammonical-N oxygen

nitrification

assimilation

Organic-N

Organic-N (net growth)

lysis & auto-oxidation

Nitrite-N oxygen

nitrification

Nitrate-N

denitrification

Nitrogen gas Nitrous oxide gas

Organic carbon

Scheme of Biological Nitrogen Removal

Mechanisms of treatment: Nitrogen removal • Biological nitrogen removal occurs through nitrification and denitrification • Nitrification is aerobic 2-step process (NH3-N to NO2N to NO3-N) by aerobic autotrophic bacteria • Nitroso bacteria (Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus, Nitrosorobrio, etc.) are responsible for step-1 • Nitro bacteria (Nitrobacter, Nitrococcus, Nitrosdpira, Nitrospina, Nitroeystis, etc.) are responsible for step-2

• Can be achieved along with BOD removal in the same biological treatment unit

Mechanisms of treatment: Nitrogen removal - Nitrification When concentration of potentially toxic and inhibitory substances is high, two-sludge systems, each with an aeration tank and a clarifier, in series are used • 1st system is for BOD removal • Nitrification occurs in the 2nd system • Raw influent is partially bypassed into the 2nd system to facilitate sufficient flocculation & clarification Bypassed influent Influent

BOD removal

Clarifier

Wasted activated sludge

Nitrification

Clarifier

Effluent

Wasted activated sludge

Mechanisms of treatment: Nitrogen removal - Nitrification • Oxygen is required - 4.57 g/g of ammonical nitrogen (3.43 for nitrite and 1.14 g for nitrate) • Alkalinity is required - 7.14 g/g as CaCO3 • Nitrification is inhibited by – Low DO levels (2 mg/L is favourable) – pH below 6 is inhibitory and 7.5 to 8 is optimal – Sensitive to a multitude of organic toxicants (solvents, amines, proteins, tannins, phenols, alcohols, cyanates, ethers, carbamates, benzene, etc.) – Metals are inhibitory (complete inhibition at 0.25 mg/L for nickel and chromium, and 0.1 mg/L for copper) – Unionized ammonia can also be inhibitory

Mechanisms of treatment: Nitrogen removal - Denitrification • Denitrification involves reduction of nitrate by heterotrophic bacteria to nitrous oxide, and nitrogen • Wide range of bacteria (but not algae and fungi) are capable

• Coupled with respiratory electron transport chain – under anoxic conditions nitrate replaces oxygen • O2 equivalence of using nitrate or nitrite in place of oxygen is 2.86 g/g and 1.71 g/g respectively • BOD demand is 4 g/g NO3 reduced

• Alkalinity is produced in the process - 3.57 g (as CaCO3) per gram of nitrate reduced • Higher DO levels (>0.2 mg/l for pseudomonas & >0.13 mg/l for highly dispersed growth) are inhibitory

Mechanisms of treatment: Nitrogen removal - Denitrification • Two basic schemes, pre-anoxic (Substrate) and postanoxic, are used for the denitrification • In the pre-anoxic scheme the anoxic tank is followed by the aeration basin of the ASP and organic matter of the influent acts as an electron donor • In the post-anoxic scheme endogenous decay of microbial mass (also exogenous sources like methanol/acetate) provides electron donor • Simultaneous nitrification & denitrification is possible – Nitrification on the floc surface (if DO in the bulk liquid is high enough) and denitrification in the floc interior (if DO in the interior is low enough) – Depending on the mixing and aeration conditions, nitrification & denitrification can occur in the same tank

Mechanisms of treatment: Nitrogen removal - Denitrification Mixed liquor recycling

Influent

Anoxic section

Aerobic section

Treated effluent

Sludge recycling Wasted sludge

Pre-anoxic Denitrification

Influent

Aerobic section

Anoxic section

Treated effluent

Sludge recycling Wasted sludge

Post-anoxic Denitrification

Mechanisms of treatment: Phosphorus Removal Phosphorus Accumulating Organisms (PAO) bring about the removal in an anaerobic – aerobic system • Phosphorus is incorporated into sludge (as polyphosphate) in volutin granules and removed through sludge wastage

In the anaerobic tank of the system • Proliferation of PAOs occur and assimilate fermentation products (specially acetate) into storage products (polyhydroxybutyrate-PHB) • Concomitantly the stored polyphosphate is released as orthro phosphate

• Acetate is essential for forming PHB and for providing competitive advantage to the PAOs • Presence of nitrate can be inhibitory (acetate can be depleted and become not available to PAOs)

Mechanisms of treatment: Phosphorus Removal In the aerobic tank of the system • Stored products (PHB) are oxidized to release energy and concomitantly phosphate of the effluent is stored within the cell as polyphosphate – Mg, K and Ca ratios of 0.71, 0.5 and 0.25 to phosphorus respectively are believed to facilitate polyphosphate storage – pH and DO should be >6.5 and >1.0 mg/L respectively

Typical microbial biomass has 1.5 to 2% phosphorus - in PAOs phosphorus content can be as high as 20-30% Stoichiometrically about 10 grams of bCOD is needed for the removal of one gram of phosphate from wastewater PAOs form very dense, good settling flocs

Mechanisms of treatment: Phosphorus Removal

Influent

Anaerobic system

Aerobic system

Clarifier

Treated effluent

Recycled sludge

Wasted sludge containing phosphorus

Reactor for phosphorus removal includes an anaerobic tank with HRT 0.5 to 1 hour and placed ahead of the aeration tank Return activated sludge and influent are brought in contact in the anaerobic tank Requires >2.5 days SRT in anaerobic and aerobic systems

ASP and its Modifications

Plug flow reactor Long narrow aeration basins (length:width = >10:1) with plug flow regime True plug flow does not exist – extent of longitudinal mixing depends on the type of aeration system used Degree of longitudinal mixing can be described by Dispersion Number (ND) as D/(UL) or Dt/L2 D – coefficient of axial dispersion (m2/sec.) – for diffused aeration system it increases by a factor of 2 with air flow increase from 20 to 100 ft3/min.1000 ft3 tank volume U – mean velocity of flow (m/Sec.) L – length of the tank t – HRT of the tank (L/U) for Q+QR flow

For good plug flow condition ND value should be 6 mg/L F/M ratio 0.6-1/day SRT: 1-4 days MLSS 2000-9000 mg/L BOD loading: 1.3-3.2 kg/m3.day HRT: 1-3 hours Sludge recycling ratio: 0.25-0.5

Disadvantages More complicated equipment and complex installation, operation and maintenance Peak flows can disrupt operation by sludge washout Has limited capacity for nitrification Nocardia foaming is possible

Activated Sludge Process: Other Modifications Deep shaft aeration • Shaft depths can be as high as 400 feet • F/M ratio of 1 to 2/day, mixed liquor DO level of 10-20 mg/L and MLSS of 8000 to 12000 mg/L are possible • Solid liquid separation may be by dissolved air flotation or by vacuum degasification and conventional gravity clarification

Integrated fixed film activated sludge process • Can enhance nitrification specially at low temperatures • SVI and solids loading to clarifier are decreased • Long sludge age assists removal of refractory organics

Activated Sludge Process: Other Modifications Thermophilic aerobic activated sludge • Optimum temperatures are 55-60°C (>45°C) • Autoheating to maintain temperature may be possible at 20,00040,000 mg/L COD removal and 10-20% O2 transfer efficiency • Advantages: Rapid degradation rates (3-10 times greater and 10 times greater autooxidation) and low sludge yield • Thermophilic bacteria fail to flocculate - hence difficult to settle

Membrane filtration • Hollow fiber membranes of 0.1 micron pore size and 13kPa (1.3 m water column) suction is possible • Permits operation at high MLSS levels (10,000-40,000 mg/L) and make sludge quality unimportant • High quality effluent (directly disinfectable) is possible • Membrane clogging is however inevitable

ASP Kinetics

What happens to organic matter in Activated Sludge Process?

-o

Bi o

Nb. suspended organic matter Soluble organic matter

Residual biomass Auto-oxidation kd New heterotrophic CO2, H2O, NH3, Microbial biomass Energy, etc.

is es th yn -s Y

Residual biodegradable organic matter

Oxygen (1-1.42Y)

io B

Nb soluble organic matter

CO2, H2O, NH3, Energy, etc.

xi d

Hydrolysis

at io

n

Suspended organic matter

Oxygen (1.42Kd)

Carbonaceous BOD is the sum of oxygen utilized during biooxidation of the organic matter and during autooxidation of the microbial biomass ammonia Oxygen (3,43 g/g)

nitrite

nitrate

Oxygen (1.14 g)

Nitrogenous BOD is the sum oxygen utilized during nitrification of Ammonical-N to nitrite-N and nitrite-N to Nitrate-N

ASP kinetics Rate of utilization of soluble substrate

Q( Si − S e ) xa qmax . S e rsu = =− V K s + Se

rsu is organic matter utilization rate (g/m3.day) qmax is maximum specific organic matter utilization rate (g/g microbial mass) Xa is microbial biomass concentration (g/m3) Se is organic matter concentration (g/m3) in the ASP Ks is half-velocity constant (organic matter concentration in g/m3 at which organic matter utilization rate is qmax./2 )

qmax. Q & Si

Aeration tank V & Xa

QSe

qmax. /2 Ks

S or Se

ASP kinetics Biomass growth rate • Microbial biomass growth rate is proportional to the organic matter utilization rate and biomass decay rate

rg = Yrsu − k d xa rg is net biomass production rate (g VSS/m3.day) Kd is endogenous decay coefficient (g VSS/g VSS. Day) Y is yield coefficient

• Can also be shown as

Y .xa .qmax . .S e rg = − xa .k d ( K s + Se )

Y .Q( Si − S e ) rg = − xa .kd V

ASP kinetics Oxygen utilization rate • Oxygen utilization rate is sum of oxygen utilization for biooxidation of organic matter and for autooxidation of biomass

rO2 = rsu − 1.42rg

rO2 = (1 − 1.42Y )rsu + 1.42k d .xa

– (1-1.42Y) is the fraction of utilized organic matter bio-oxidized – 1.42kd is auto-oxidation rate in terms of oxygen or bCOD •

Oxygen utilization rate can also be expressed as

(1 − 1.42Y ) xa .qmax . .S e rO2 = + 1.42 xa .k d ( K s + Se )

(1 − 1.42Y )Q ( Si − S e ) rO2 = + 1.42 xa .kd V

ASP kinetics ASP kinetic parameters qmax. (2-10 g of bCOD per g VSS day, 5 is typical) Ks (10-60 mg/l of bCOD, 40 is typical) Y (0.3 to 0.6 mg VSS per mg bCOD, 0.4 is typical) kd (0.06 to 0.15 g VSS per g VSS.day, 0.1 is typical) Values in parentheses are for domestic sewage

Kinetic coefficient values vary with the wastewater, with the Microbial population and with Temperature Kinetic coefficient values can be determined from bench scale testing or full-scale plant test results Temperature correction to the kinetic coefficients is done by

kT = k 20θ

(T − 20 )

θ is temperature activity coefficient (typical value 1.02 to 1.25) kT and k20 are k values at T°C and 20°C respectively

ASP Design

ASP design: Inputs Quantities and characteristics (and their diurnal, seasonal and wet-weather variations) of the wastewater to be treated – Carbonaceous substrates (bCOD, sbCOD, nb suspended COD) – Nutrients: Nitrogen (TKN and nitrate-N (plus nitrite-N)) and Phosphorous (total and orthro phosphorus) – Suspended solids (Total, volatile, biodegradable volatile and non-biodegradable volatile) – Alkalinity – Flow rates and variations (average flow and peaking factor) – Temperature (winter and summer critical temperatures of both wastewater and ambient air)

ASP design: Inputs •

• • • • •

Purposes to be served by the ASP – removal of bCOD – bCOD removal and nitrification – Removal of bCOD and nitrogen (nitrification-denitrification) – bCOD and phosphorus removal Treated effluent characteristics required (only bCOD or both bCOD and nutrient levels desired) ASP kinetics parameters (qmax. , Ks, Y and kd) Settling characteristics of bio-solids (SVI and zone settling velocities of the mixed liquor solids) Solids retention time (SRT) and loading criteria (F/M ratio and volumetric organic loading) to be used for good sludge settling properties Selection of reactor type (including use of selectors)

• SRT for BOD removal is typically 3 to 5 days – shorter SRT discourages nitrification. • Typical F/M ratio may range from 0.04/day for extended aeration units to 1.0/day for high rate process. • Volumetric organic loading rate typically varies from 0.3 to 3.0 kg/m3.day. • Expansion to meet the future treatment needs is an important consideration in the design. • Type and size of reactors and solid separation facilities influence both construction and operation costs. • Selectors may be needed for nutrient removal and for limiting the filamentous growth. • Staged reactor or plug flow reactor may be appropriate for nitrification – toxic or inhibitory substances can depress the nitrification rates.

What is included in the ASP design Design of ASP requires determination of •

Aeration basin volume

•

Aeration requirements

•

Chemical (nutrients and alkalinity) dosing requirements

•

Sizing and detailing of the secondary clarifier

•

Sludge recycling requirements

•

Activated sludge wastage rates required

•

Treated effluent characterization

Q,Si,Xi

Settling tank

Aeration tank Se,Xa,V

Qe or (Q-Qw) Xe,Se

Qr,Xr,Se Xi is considered negligible All biodegradable suspended organic solids of influent Qw,Xr,Se are hydrolyzed into soluble organic matter Inorganic and non-biodegradable organic SS remain unaffected and no new SS of these categories formed Nothing except settling & thickening occurs in clarifier

Q,Si,Xi

Settling tank

Aeration tank Se,Xa,V

Qr,Xr,Se Qw,Xa,Se

Qe or (Q-Qw) Xe,Se

Treated Effluent Soluble bCOD K s [1 + k d ( SRT )] Se = SRT ( qmax . .Y − k d ) − 1 Use of this equation requires – Primary variable SRT (assumed) – ASP kinetic parameters Ks, kd, qmax and Y

Obtained from the following through solving for Se

Y .qmax .S e 1 = − kd SRT K s + Se Independent of the influent bCOD

Treated Effluent Soluble bCOD Specific substrate use for ASP V Q( S i − Se ) Si − Se q= = τ= xaV xaτ Q Specific substrate utilization rate according to Michaelis-Menten equation

qmax S e q= K s + Se

Si − S e qmax . S e = xaτ K s + Se ( Si − S e ).Y 1 = − kd SRT xa .τ

Y .qmax . .S e 1 = − kd SRT K s + Se

Mixed Liquor Active Biomass Concentration SRT ( S i − S e )Y xa = τ 1 + k d ( SRT ) Use of this equation requires – Primary variables SRT and τ – ASP kinetics parameters Y and kd

Obtained from the following basic equation through solving for xa

Y ( Si − Se ) 1 = − kd SRT τ . xa Here xa depends on kd, Y, SRT, τ and bCOD removal

Mixed Liquor Active Biomass Concentration Net activated sludge synthesis rate is equal to activated sludge wastage rate

Q( Si − S e )Y − k d .xa .V = ∆x Sludge age or mean cell residence time is (total sludge of the system) SRT = (net sludge generation rate) or ( sludge wastage rate)

Vxa SRT = Q( S i − S e )Y − k d .V .xa

( S i − S e )Y 1 = − kd SRT xaτ

Net Biomass Synthesis Rate Net biomass synthesis rate (NBSR) is estimated by

Y .Q ( S i − S e ) NBSR = 1 + k d ( SRT ) Use of this equation requires – Primary variable SRT – ASP kinetics parameters Y and kd

Obtained through simplification of the following material balance equation

Biomass  Net biomass  Gross biomass    = − synthesis rate   synthesis rate  autooxidation rate       

NBSR = Y .Q( Si − S e ) − xa .V .k d

Here V is replaced by Q.τ and the expression for xa is used

Net Biomass Synthesis Rate NBSR = YQ( Si − S e ) − xaVk d

In the above by replacing

SRT ( S i − S e )Y Qτ for V and for xa τ 1+ k d ( SRT ) And on simplification

YQ( Si − S e ) NBSR = 1 + k d ( SRT ) Here net synthesis of nitrifying microbes is not considered Depends on the TKN nitrified (Influent and effluent TKN difference minus nitrogen assimilated into biomass)

Cell Debris Generation Rate Cell debris generation rate (CDGR) is estimated by

Y .Q( Si − S e ).k d .SRT CDGR = f d 1 + k d .SRT Use of this equation requires – Primary variable SRT – ASP kinetics parameters Y and kd – other constant fd

Obtained from multiplication of the expression for xa with V, kd and fd (V is replaced by Q.τ ) – Here xa.V.kd indicates the biomass autooxidation rate

Secondary Sludge Generation Rate Secondary sludge generation rate is comprised of – Net biomass synthesis rate – Cell debris generation rate from biomass autooxidation – Rate of contribution of Nonbiodegradable VSS by the influent (Nb.VSS) – Rate of contribution of Inorganic suspended solids by the influent (In.SS)

Secondary sludge generation rate (SSGR) is SSGR = NBSR + CDGR + Nb.VSS .GR + In.SS .GR

Here

Nb.VSS .GR = Q.( Nb.VSS )

In.SS .GR = Q.( In.SS )

Active biomass, MLSS and MLVSS Active biomass to MLSS ratio NBSR NBSR + CDGR + Nb.VSS .GR + In.SS .GR MLVSS to MLSS ratio NBSR + CDGR + Nb.VSS .GR NBSR + CDGR + Nb.VSS .GR + In.SS .GR MLSS value

NBSR + CDGR + Nb.VSS .GR + In.SS .GR xa . NBSR

MLVSS value MLSS .

NBSR + CDGR + Nb.VSS .GR NBSR + CDSR + Nb.VSS .GR + In.SS .GR

Sludge Wastage Rate Sludge wastage can be from the – Secondary clarifier under flow line – Aeration tank or its outlet prior to sec. clarifier as mixed liquor

• Rate of wastage depends on secondary sludge generation rate (SSGR) minus secondary sludge washout rate (SWOR)

SSWR = SSGR − SWOR Where SWOR is Q.TSSe

Sludge Wastage Rate Volumetric sludge wastage rate is – SSWR/MLSSu (when wasted from the secondary clarifier underflow) – SSWR/MLSSa (when wasted from the aeration tank or its outlet prior to the secondary clarifier)

Observed SRT is (V.MLSSa)/SSWR SRT chosen as the primary variable is (V.MLSSa)/SSGR

Observed SRT is greater than the SRT chosen as the primary variable – Difference between the two will depend on the TSS of the clarified secondary effluent

Oxygen Demand Rate  Oxygen   Oxygen equivalent  Oxygen equivalent of  demand  = of loaded substrate  −  NBSR plus CDGR        O2 demand = Q( Si − S e ) − n[ NBSR + CDGR ]

Here ‘n’ is oxygen equivalence of microbial biomass(1.42!)

The oxygen demanded is supplied by – Surface (floating or fixed) aerators – Diffused aeration systems (introduce oxygen/air into liquid) • Turbine mixers can disperse introduced air bubbles • Hydraulic shear devices can reduce bubble size

Suppliers of aeration systems indicate oxygen transfer rates of their equipment at standard conditions – Rates require correction to actual operating conditions

Actual Oxygen Transfer Rate  β .Cs TH − C L  (1.024T − 20 ).α .F AOTR = SOTR  C s , 20   • AOTR is actual oxygen transfer rate under field conditions – Salinity-surface tension of the wastewater (β ) – Operating temperature of the wastewater – Atmospheric pressure (related to altitude) – Average depth of aeration (diffused aeration system) – Operating DO of the aeration tank – Oxygen transfer coefficient of wastewater compared to that of clean tap water (α ) – Degree of fouling of the diffusers (diffused aeration system)

• SOTR is standard oxygen transfer rate in tap water at 20°C and zero dissolved oxygen level • Applicable even for oxygen transfer efficiencies

Actual Oxygen Transfer Rate β is salinity – surface tension factor • Taken as ratio of saturation DO wastewater to clean water • Typical value is 0.92 to 0.98 (0.95 is commonly used)

α is oxygen transfer correction factor for the wastewater • Typical range for diffused aeration systems is 0.4-0.8 • Typical range for mechanical aerators is 0.6-1.2

F is fouling factor accounting for both internal and external fouling of diffusers • Impurities of compressed air cause internal fouling • Biological slimes and inorganic precipitants cause external fouling • Typical value is 0.65 to 0.9

Actual Oxygen Transfer Rate Csֿ,T,H is average saturation of clean water at the operating temperature, altitude and aerator depth

• For surface aerators

C s ,T , H = Cs ,T , H

• Can be obtained from literature (for the atmospheric pressure at the altitude in question)

• For diffused aerators it can be obtained by  Patm , H + Pw,mid depth  Patm , H  9.81× 28.97( H − 0)    C s ,T , H = Cs ,T , H  = exp −   P ( ) P 8314 273 . 15 + T atm , H   atm , 0   • Applicable if biological oxygen uptake is not considered Ot  1  Pd Cs ,T , H = Cs ,T , H  +  2  Patm.H 21  • Ot is % O2 in air leaving aeration basin (typically 18-20%)

Air Requirements of Diffused Aeration {Oxygen demand }  Air  = required     Actual oxygen  × oxygen fraction  transfer efficiency   in the air      Expressed in kg/hr. and Nm3/hr Actual temperature depends on the level of compression (Ambient temperature + pressure (in kg/cm2 gauge) X 10°C)!

Filtered air can minimize internal fouling Consider air flow velocity and temperature while sizing ducting Relate air delivery pressure to the water column over the diffuser Consider head losses in the diffused aeration system in estimating the air pressure required Find the number of diffusers on the basis of typical air delivery per diffuser (consider internal and external fouling)

Nutrient Requirements Inflow of nitrogen Influent may have TKN (organic-N+ammonical-N) and nitrate-N (nitrate+nitrite) Nutrient addition (in the form of Urea and DAP)

Fate of nitrogen in the ASP Organic-N is converted into ammonical-N Ammonical-N can nitrified into nitrate-N Nitrate-N can be denitrified and lost in the gaseous from (as N2O and N2) Ammonical-N and Nitrate-N can be assimilation by active biomass and stored within as organic-N

Outflow of nitrogen Loss in the treated effluent either as TKN or as nitrate-N or as both Loss as organic-N in wasted activated sludge

Nutrient Requirements  N in the  N in the   N lost through   N in the  N requirement =  +  wasted sludge  + denitrification  − inf luent  effluent        

Nitrate-N in the influent is usually negligible influent mainly has TKN Nitrogen in the treated effluent can be ammonical-N or nitrate-N or organic-N (in the TSSe) Nitrogen in the wasted activated sludge is 12.23% - obtained from empirical formula of the activated sludge (C60H87O23N12P) Denitrification loss of nitrogen can be significant if the ASP is designed for nitrification and denitrification to occur When concentration is 4days is 2 to 4 mg/L • Ammonical nitrogen and total phosphorus (soluble form) are >0.1 and >0.3 mg/L respectively • For properly functioning secondary clarifier in case of mixed liquor solids with good settling characteristics TSS is 5-15 mg/L

F/M Ratio, BOD Volumetric Loading Aeration tank volume Food to microorganisms ratio

V = HRT .Q

• In terms of active biomass

F QSi = M xaV

• In terms of MLVSS

F QSi = M MLVSS .V

BOD loading

QSi BOD loading = V

Total bCOD of the effluent

xa S e + 1.42 × TSS e × MLSS

Design of Secondary Clarifier

Design of Secondary Clarifier Secondary clarifier includes – Inlet section or central well – Sludge settling zone – Sludge thickening and storage zone – Clarified effluent overflow weir and collection trough

Requires sludge settling zone surface area, π (D2-d2)/4 • Area required for clarification and area required for thickening are found out and the larger of the two is used • Either of the following two design approaches can be followed – Talmadge and Fitch method - uses data derived from a single batch settling test – Solids flux method - uses data obtained from a series settling tests conducted at different solids concentration

All other details of the clarifier are either assumed or obtained through hydraulic and mechanical design

Secondary Clarifier: Talmadge and Fitch method Final overflow rate for a secondary clarifier is selected based on the consideration of – Area for clarification – Area for thickening – Rate of sludge withdrawal

Data from a single settling test is used for finding both area required for thickening and for clarification and greater of the two is considered for design Area required for clarification is usually greater than the area required for thickening

Secondary Clarifier: Talmadge and Fitch method Area required for thickening • Tu corresponds to Hu and obtained through • Co is initial TSS and Ho column height • Cu is underflow sludge concentration

Qtu At = Ho

H o Co Hu = Cu

Critical concentration controlling sludge handling capability – Draw tangents to initial and final legs of settling curve – Bisect the angle of intersection and extend to settling curve to get Cc

Find tu (time at which sludge concentration is Cu) • Draw tangent through Cc • Locate Hu on y-axis, extend horizontal line to the tangent through Cc - draw vertical from the intersection to obtain Tu

Secondary Clarifier: Talmadge and Fitch method Area for clarification – Here Qc is clarification rate – V is interface subsidence velocity

Qc Ac = v

Interface subsidence velocity • Slope of the tangent on the initial leg of the settling curve is taken as subsidence velocity Clarification rate • Taken as proportional to the liquid volume above Hu H0 − Hu and computed as Qc = Q Ho – Here Hu is sludge depth curresponding to tu – Q is flow rate of mixed liquor into the clarifier

Secondary Clarifier: Solids flux method Area required for thickening depends on the limiting solids flux that can be transported to the bottom of the settling tank Data obtained from a series of column settling tests conducted at different solids concentration is used Solids flux depends on the characteristics of the sludge (relationship between sludge concentration and settling rate and solids flux)

Secondary Clarifier: Solids flux method Downward flux of solids in a settling tank occurs due – gravity settling

SFt = SFg + SFu

– bulk transport from sludge withdrawal – Here SFg is solids flux due to gravity – SFu is solids flux by bulk transport

Solids flux due to gravity

SFg = CiVi

– Ci is concentration of solids at the point in question – Vi is settling velocity of the solids at Ci concentration – Vi of sludge at different concentrations is obtained from multiple settling tests - Slope of the initial portion of the curve is Vi

Secondary Clarifier: Solids flux method Solids flux by bulk transport

Ci Qu SFu = CiU b = A – Ub is bulk underflow velocity – Qu is underflow rate of sludge – A cross sectional area of the sludge – Flux by bulk transport linearly increases with increasing withdrawal rate

Total flux increases initially, then drops to limiting solids flux (SFL)and then increases with increasing withdrawal rate

Secondary Clarifier: Solids flux method Alternative graphical method for limiting solids flux (SFL) • Uses only the gravity flux curve • Decide the underflow sludge concentration and draw tangent to gravity flux curve through Cu on X-axis and extend to Y-axis • Point of intersection on Y-axis gives SFL

Secondary Clarifier: Solids flux method Area for thickening • Area required for thickening will that area at which actual solids is lower than equal to limiting solids flux (SFL) – If solids loading is greater than limiting solids flux then solids will build up in the settling basin and ultimately overflow

• Area required for thickening

A=

( Q + Qu ) Cu SFL

Q is overflow Qu is underflow SFL is limiting solids flux

• For a desired underflow concentration one can increase or decrease the slope of the underflow flux line

Design of Secondary clarifier on the basis of SVI and ZSV Settling and thickening characteristics of the mixed liquor measured by either SVI or ZSV can be used as basis SVI below 100 is desired and above 150 typically indicates filamentous growth Surface over flow rate for a secondary clarifier is related to zone settling velocity as shown below

Surface overflow rate = ZSV (Vi) can be estimated by Here Vi is zone settling velocity (SVI)

Vi SF Vi = Vmax exp(− K ) x

SF is safety factor and taken as 1.75 to 2.5 Vmax is maximum zone settling velocity taken as 7 m/h K is a constant with value 600 l/mg for ML with SVI 150 X if MLSS concentration

Design of Secondary clarifier on the basis of SVI and ZSV MLSS, ZSV and SVI/DSVI are related

ln (Vi ) = 1.871 − (0.1646 + 0.001586 SVI ) x ln (Vi ) = 2.028 − (0.103 + 0.002555 DSVI ) x Here x is MLSS concentration in g/l DSVI and SVI in ml/g

Fluctuations in wastewater and return sludge flow rates and MLSS concentration should be considered in the design – Safety factor used is meant for this purpose

Solids loading rate is a limiting parameter and affects effluent concentration of TSS – Effluent quality remains unaffected over a wide range of surface overflow rates (upto 3-4 m/h)

Other information for the design of Secondary Clarifiers Side wall liquid depth can be as low as 3.5 m for large clarifiers and as high as 6 m for smaller clarifiers – Deeper clarifiers have greater flexibility of operation and larger margin of safety

Tank inlet section or central well – Jetting of influent (cause for density currents) should be avoided through dissipate influent energy – Distribution of flow should be even in horizontal and vertical directions and should not disturb the sludge blanket – Design of central well should promote flocculation – Cylindrical baffle of diameter 30-35% of the tank diameter can be used as central well – Bottom of the feed well should end well above the sludge blanket interface

Other information for the design of Secondary Clarifiers Weir placement and loading • In larger clarifiers circular overflow weir with trough can be placed at 2/3rd to 3/4th radial distance from the center – For smaller clarifiers it can at the perimeter – A baffle can be provided to deflect density currents away from the overflow weir and avoid scum overflow – Up-flow velocity in the vicinity of weir should be 3.5-7 m/hr

• Weir loading rates should be < 375 m3/m.day – Should be 2 mg/l under normal loading conditions

– Selector processes (aerobic, anoxic and anaerobic) in place of complete mix systems can be a solution for bulking from longer SRT and low F/M ratios – Internal plant overloading can be avoided through recycling centrate and filtrate during the periods of minimal hydraulic and organic loading – Not retaining the sludge for more than 30 minutes can avoid septic conditions and subsequent bulking

Bulking Sludge Bulking can be temporarily controlled by Cl2 and H2O2 – 0.002-0.008 kg per day of Cl2 per kg of MLVSS for 5-10 hr HRT systems – Chlorination can produce turbid effluent and kill nitrifiers – Trihalomethanes and other compounds with potential health and environmental effects can be formed – Dose of H2O2 depends on extent of filamentous development

Rising Sludge Differentiated from bulking sludge by presence of small gas bubbles in the sludge Common in systems with conditions favourable for nitrification Nitrification is the common cause • Nitrification in the aeration basins produces nitrite and nitrate • Denitrification in the clarifiers converts produces nitrogen gas • Trapping of nitrogen gas makes the sludge buoyant

Solutions may include • Reduced sludge detention in the clarifier (increasing the speed of sludge collection and withdrawal) • Reduced mixed liquor flow to the clarifier (decreases sludge depth) • Decrease SRT and/or aeration for controlling nitrification • Post-aeration anoxic process prevents denitrification in clarifiers

Foaming Usually associated with Nocardia and Microthrix parvicella – Hydrophobic cell surfaces allow attachment of bacteria to and stabilization of air bubbles to cause foaming (0.5 to 1.0 m thick)

The foaming can go beyond the ASP and get into aerobic and anaerobic sludge digesters Control measures – Avoid foam trapping aeration basins (baffles with flow under can trap foam in the basin) – Reduce oil and grease (Nocardia and Microthrix are usually associated with these) flow into the aeration basin – Avoid recycling of skimmings of clarifiers to aeration basins – Use of selectors can discourage foaming – Addition of small concentrations of cationic polymers and chlorine spray over the surface of foam can also reduce foaming

Selector Processes A small tank or a series of small tanks are used for mixing the incoming wastewater with the return sludge under aerobic or anoxic/anaerobic conditions • Controls filamentous bulking and improves sludge settling characteristics • High rbCOD F/M ratio discourages filamentous growth but encourages floc forming non-filamentous bacterial growth

Selector process designs are two types • kinetic or high F/M selectors – Higher substrate concentrations result in faster substrate uptake by floc forming bacteria – High DO (6 -8 mg/L) is needed for maintaining aerobic floc – Recommended F/M ratios are 12, 6 and 3 per day COD F/M ratios for a 3 tank selector – too high F/M ratios, >8 BOD/day ) can cause viscous bulking

Selector Processes Metabolic or anoxic or anaerobic processes selectors • Improved sludge settling characteristics and minimal filamentous bacteria are observed with the biological nutrient removal processes – Filamentous bacteria can not use nitrate or nitrite as electron acceptor under anoxic conditions – Filamentous bacteria do not store polyphosphates and hence can not consume acetate under anaerobic conditions

• Anoxic or anaerobic metabolic conditions are used – Anaerobic selector can be used before the aeration tank (phosphorus removal can occur) – If nitrification is used, then anoxic selectors can be used

• For high F/M anoxic/anerobic selectors SVI of mixed liquor can be as low as 65-90 mL/g (common SVI is 100-120 mL/g)

ASP monitoring

Monitoring Monitoring including sampling frequency and parameters to be analyzed should be need based Monitoring may be required for – Carrying out treatability studies and design of the ASP – Facilitating operation and control of the ASP – Assessing compliance with the requirements (achievement of set and design efficiencies) – Performance evaluation of the ASP

Monitoring involves sampling and analysis of samples of – – – – –

Influent wastewater of the ASP Aeration tank contents Mixed liquor being fed to the secondary clarifier Clarified secondary effluent (secondary clarifier overflows) Return sludge

Monitoring Monitoring can be online (appropriate for process control !) – Monitoring involves collection of samples and analysis in – Industrial units own ETP or central laboratory (for routine parameters) – Outside laboratory (sometimes 3rd party laboratory) for advanced analysis (for non-routine parameters requiring sophisticated instruments) 3rd party laboratory is depended on specially for compliance assessment/ monitoring

Sampling location should judiciously chosen

Monitoring Method of sampling chosen should ensure collection of representative samples – Usually grab sampling can serve the purpose – Occationally composite (flow proportionated?) sampling may be required

Frequency of sampling and parameters to be analysed for should be decided on the basis of the monitoring objective/purpose Typical sampling locations and comprehensive list of parameters for analysis for an ASP are identified here Sampling at all the locations, sampling by any specific method of analysis, and analysis for all the identified parameters may not always be required Sampling frequency can be at fixed intervals and whenever a need arises

Streams to be monitored and parameter Influent (including internal recycle flows): Assess both quantity and characteristics – Flow rate and variations (if needed the flow should be managed) – Internal recycle flows – Organic matter concentration – BOD, bCOD (BOD kinetics) and COD – Nutrients – TKN (ammonical and organic nitrogen), nitrate plus nitrite nitrogen, and total phosphorus – Suspended solids: TSS, VSS, nbVSS (indirect method) – Total alkalinity, Temperature and pH

Aeration basin contents: – Temperature, pH and Dissolved oxygen – Oxygen uptake rate and specific oxygen uptake rate (Toxicity assessment)

Monitoring of ASP • pH in the aeration tank to check whether enough alkalinity is present • Nutrients in the treated effluent – TKN, Nitrate-N and Total-P • DO in the aeration tank (sample should not come in contact with air and biological flocs should be deactivated) • MLSS and MLVSS, TSS in the clarified secondary effluent and SVI of mixed liquor • Visual and microscopic examination of microbial flocs

Streams to be monitored and parameter Mixed liquor – – – –

MLSS and MLVSS Nitrate and nitrite nitrogen SVI and ZSV Microscopic examination of biological flocs

Return sludge – MLSS or consistency

Clarified secondary effluent – Soluble BOD, total BOD, COD – Nitrate plus nitrite nitrogen and TKN (ammonical and organic nitrogen separately) – Total phosphorus and TSS

Design of the ASP Characterization and quantification of the influent to be treated – Flow rate and variations (average flow and peaking factor) – flow equalization may be needed to dampen variations – Organic matter concentration – BOD, bCOD (BOD kinetics) and COD – Nutrients – TKN (ammonical and organic nitrogen), nitrate plus nitrite nitrogen, and total phosphorus – Suspended solids: TSS, nbVSS (indirect method) – Total alkalinity – Temperature (summer critical and winter critical temperature of the influent)

Ambient conditions, like, summer and winter critical temp., altitude and atmospheric pressure, may be needed

Operation and Control of the ASP Chemical dose requirements – Flow rate, BOD, TKN (even nitrate plus nitrite –N), Total –P, and Alkalinity of the influent – Residual TKN (and nitrate plus nitrite –N) and total –P of treated effluent – Frequency of monitoring can be weekly once or lesser

Aeration system – Suction pressure down stream to the air filter, compressed air pressure and air flow rate (online monitoring) • May indicate filter clogging, diffuser fouling and bursting of diffuser membrane

– DO level in the aeration basin (may be as a part of investigation of bulking sludge)

Operation and Control of the ASP Regulation of the return sludge system – Sludge blanket depth in the clarifier (continuous monitoring may be needed) – Consistency of the return sludge and MLSS in the mixed liquor – SVI of the mixed liquor – Influent flow rate

Sludge wasting system – Consistency of the clarifier underflow and MLSS of the mixed liquor – TSS in the clarified effluent of the clarifier

Presence of toxic/inhibitory subtances – OUR and SOUR of the mixed liquor in the aeration

Operation and Control of the ASP Secondary clarifier control • Influent flow rate (to assess hydraulic overloading) • Nitrate, temperature, SVI and ZSV of mixed liquor – (to indicate raising sludge, density currents, and settling properties of mixed liquor solids)

• Sludge blanket depth • TSS level and turbidity of the clarified effluent – Straggler floc: fluffy floc in the clarified effluent – associated with low SRT operation – often coexist with white foam – Pinpoint floc: pin-floc in the clarified effluent – associated with high SRT operation – coexist with quickly settling floc and often with darker foam – Surface ashing: condition of lighter pin-floc floating to the surface and spreading out in the clarifier

Analytical Methods Flow meaurement (online measurement) by – Pumping rate and pump runtime recording – Treated effluent flow rate by flow meters such as V- or rectangular notches, parshall flume, etc.

BOD, bCOD and COD – BOD: BOD bottle method, head-space BOD method, respirometric technique, or by BOD sensors – bCOD: through BOD kinetic experiments – COD: by closed or open reflux methods • Use BOD – COD – bCOD relationship for the operation and control of ASP

Analytical Methods Nutrients • TKN (organic and ammonical –N) by kjeldahl apparatus and either titrimetry or colorimetry • Nitrite and nitrate –N by using cadmium reduction column and colorimetry

Suspended solids (TSS, VSS, nbVSS, MLSS and MLVSS) • Filtration, gravimetry and ashing for MLSS and MLVSS • TS – TDS difference technique for TSS (and ashing for VSS) • Change in VSS over sufficiently long time through aeration by autooxidation can be basis for nbVSS measurement

SVI for mixed liquor: volume in mL occupied by one gram of mixed liquor solids after 30 minutes settling ZSV: subsidence velocity in m/hr. of the sludge blanket interface in the settling column)

Analytical Methods • Oxygen uptake rate measurement by recording DO depletion of the mixed liquor over a few minutes • Specific oxygen uptake rate from OUR by dividing with MLVSS or active biomass concentration • Microscopic examination of biological flocs of mixed liquor or of secondary clarifier for filamentous growth and for microscopic air bubbles • pH by pH meter (or indicator strip!) and alkalinity (in mg/L as CaCO3) by titrimetry • Temperature (of ambient air, influent and mixed liquor) • DO (of the aeration basin contents) by DO meter with submersible long leed probe

Data recording and analysis All monitoring data needs recording in log books – Data may be weekly/monthly/seasonally analysed for trends and performance reports may be generated

All incidents may be recorded and the recording can include the investigatory monitoring being carried out and corrective and preventive actions taken – Hydraulic or organic overloading – Loading of toxic or inhibitory substances – Bulking sludge, raising sludge, and foaming problems – Critical machinery failures

Performance Assessment

Compliance Assessment BOD and TSS (and even flow rate) are needed to be monitored – Comparison with statutory requirements may be needed – BOD is contributed by • Residual sBOD • Biodegradable fraction of the TSS (MLVSS to MLSS ratio!)

If operated for nutrient removal then TKN and nitrate and nitrite –N and total –P may also be needed – Nutrients are also present in the TSS

ASP Performance Evaluation ASP performance evaluation may involve • Measurement of actual performance of the ASP and comparison with the designed performance – May concentrate mainly on • Treatment efficiencies (BOD removal and nutrient removal) • Sludge generation rates and MLVSS/MLSS ratio • Oxygen consumption rates

• Back calculation of ASP kinetic parameters and comparison with values used in the design and improvement of equations used in the ASP performance assessment • Performance evaluation of the secondary clarifier