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Activated sludge process

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Activated sludge process

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Deals with what is activated sludge, mechanisms and kinetics of treatment, design of activated sludge process, secondary clarifiers and their design and bulking sludge, raising sludge and foaming of ASP.

Deals with what is activated sludge, mechanisms and kinetics of treatment, design of activated sludge process, secondary clarifiers and their design and bulking sludge, raising sludge and foaming of ASP.

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Activated sludge process

  1. 1. Activated Sludge Process Dr. Akepati S. Reddy School of Energy and Environment Thapar University Patiala (PUNJAB), INDIA
  2. 2. Activated Sludge Process Most commonly used secondary treatment process • Microbes, mainly aerobic heterotrophic bacteria, are involved Designed to remove (soluble) biodegradable organic matter • Removal of nutrients, TSS, pathogens and heavy metals is coincidental Usually clarified sewage (primary effluents) is treated • Primary treatment is omitted in case of small flows and low TSS sewage, and in hot climates (to avoid/control odour problems) • SBR, oxidation ditches, aerated lagoons, contact-stabilization process, etc. may not require primary treatment Treatment involves conversion of soluble organic matter into biological flocs and their removal as secondary sludge Includes an aeration tank and a secondary sedimentation tank • Aeration and mixing, and sludge recycling are additional features
  3. 3. Grit chamber Primary clarifier Aeration tank Secondary clarifier Stabilization tank Sludge drying beds Sewage Sump & pumping Bar screen Biogas flaring unit Dried sludge for disposal Exhaust gases (CO2 and H2O) Filtrate to sewage sump Clarified effluent to sewage sump Treated effluent Raw sewage Screenings Biogas if not flared (supplied as fuel or emitted) Grit STP Incorporating ASP Equalization Tank air air scum ASP
  4. 4. Aeration basin Secondary clarifierInfluent Effluent Wasted activated sludge Sludge recycling Nutrients and Alkalinity (if needed) Air/oxygen for aeration & mixing Activated Sludge Process
  5. 5. Components of ASP Aeration basin • Wastewater comes in contact with active microbial biomass for treatment – Bioflocculation, biosoprtion and biooxidation occur • Organic matter is transformed into biological flocs – Suspended and colloidal solids become integral part of flocs Aeration and mixing • Aeration supplies enough oxygen for aerobic bio-oxidation of organic matter • Mixing keeps biological flocs suspended and ensures contact between wastewater and microorganisms • Two types of aeration/mixing systems: diffused and mechanical aeration systems – Diffused aeration (diffusers, piping and fittings, and blowers) – Mechanical aeration systems - Surface aerators (fixed or floating types) with or without draft tubes - Submerged turbine aerators - Horizontal axis aerators (brush aerators)
  6. 6. Components of ASP Secondary sedimentation tank • To clarify the out-flowing aeration tank contents (mixed liquor) • To separate and thicken the biological flocs from mixed liquor for recycling or wasting Sludge recycling • Thickened sludge is returned back from secondary clarifier to the aeration tank to maintain desired concentration of biological solids • Includes pumps and necessary piping and fittings Activated sludge wasting • Sludge wasting is either as mixed liquor or as thickened sludge Chemical feed systems • Addition of nutrients and alkalinity may be required if the wastewater is deficient in them – sewage is usually not deficient – Urea and DAP are usually used as nutrients (phosphoric acid or phosphate rock can also be used in place of DAP) – Hydrated lime is dosed for alkalinity
  7. 7. Mechanisms of Treatment Aerobic microorganisms (activated sludge), specially bacteria, are responsible • Suspended and colloidal solids of the wastewater becomes integral part of biological sludge through bioflocculation • Biological sludge is constituted of microorganisms, cell debris, and suspended and colloidal solids of the influent • Organic matter of wastewater is biosorbed (adsorbed and absorbed) by microorganisms • Adsorbed organic matter is solubilized through hydrolysis • Simple soluble organic compounds are absorbed by microbes as food • Absorbed organic matter is bio-oxidized (partly respired & rest is used in biological flocs - new microbial biomass - synthesis) • Involves biooxidation, biosynthesis and autooxidation • Net synthesized biological flocs (excess sludge) is wasted
  8. 8. Soluble organic matter Nb soluble organic matter Nb. suspended organic matter Oxygen (1-1.42Y) CO2, H2O, NH3, Energy, etc. New heterotrophic Microbial biomass Auto-oxidation kd CO2, H2O, NH3, Energy, etc. Carbonaceous BOD is the sum of oxygen utilized during biooxidation of the organic matter and during autooxidation of the microbial biomass Oxygen (1.42Kd) Residual biomassBio-oxidation B io-synthesis Y Suspended organic matter Hydrolysis Residual biodegradable organic matter What happens to organic matter in Activated Sludge Process? Bioflocculation and Biosorption are much faster than bio-oxidation • Hydrolysis and bio-oxidation are slower processes • Bio-oxidation requires O2 (DO - 0.5 to 1.5 mg/L)
  9. 9. Nutrient removal occurs through • Ammonical-N from organic-N, nitrification and denitrification • Assimilation of ammonical-N and conversion into organic-N Nitrification • Aerobic 2-step process (ammonia to nitrite and then to nitrate by autotrophic bacteria • Becomes significant if DO levels are higher (>2.0 mg/L) and oxygen requirement is 4.57 g/g of NH3-N (3.43 to nitrite-N and 1.14 g to nitrate-N) • Demands alkalinity (7.14 g/g as CaCO3) Denitrification (respiration where nitrate is electron acceptor) • Reduction of NO3 by heterotrophic bacteria into N2O and N2 • Coupled with respiratory electron transport chain and demands respiration of 4 g BOD per g of NO3 • 1 gram of O2 can be replaced by 2.86 g of nitrite or 1.71 g of nitrate • Produces alkalinity (3.57 g (as CaCO3)/g nitrate denitrified) • DO levels >0.1 or 0.2 mg/l are inhibitory Mechanisms of Treatment
  10. 10. Mechanisms of Treatment Phosphorus removal • Phosphorus Accumulating Organisms (PAO) in an anaerobic – aerobic system are involved • Phosphorus is incorporated into sludge (as polyphosphate/ volutin granules) and removed through sludge wastage • PAOs have 20-30% of the biomass as phosphorus • PAOs form very dense, good settling flocs • In the anaerobic tank of the system • proliferation of PAOs occurs • fermentation products (acetate) are assimilated and poly- hydroxy-butyrate (PHB) is stored – concomitantly polyphosphate is released as ortho phosphate • In the aerobic tank • PHB is oxidized and concomitantly phosphate of the effluent is stored within the cell • Stoichiometrically about 10 grams of bCOD is needed for the removal of one gram of phosphate
  11. 11. Substrate Utilization Rate Aeration tank       + −= − = − = es e a eiei su SK Sq x SS V SSQ r .max)()( τ V Xa Q Si Q Se Se qmax. qmax./2 Ks rsu is substrate or 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 ) τ is hydraulic residence time (HRT) q
  12. 12. Net Biomass Synthesis Rate adsug xkYrr −= d kxYx SK Sq r aa es e g −      + = )( .max da ei gda ei g kx YSS rkx V YSSQ r − − =− − = τ )()( rg is net biomass production rate (g VSS/m3 .day) Kd is endogenous decay coefficient (g VSS/g VSS. Day) Y is yield coefficient d a ei k x YSS SRT − − = τ. ).(1 d es e k SK SqY SRT − + = ..1 .max
  13. 13. Oxygen Utilization Rate adsuO xkrYr .42.1)42.11(2 +−= d kx SK Sq xYr a es e aO .42.1 )( . )42.11( .max 2 +      + −= da ei O kx SS Yr .42.1 )( )42.11(2 + − −= τ gsuO rrr 42.12 −= (1-1.42Y) is the fraction of utilized organic matter bio-oxidized 1.42kd is auto-oxidation rate in terms of oxygen or bCOD
  14. 14. 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 typical values for domestic sewage Kinetic parameters values vary with the wastewater, with the Microbial population and with Temperature Kinetic parameter values can be determined from bench scale testing or full-scale plant test results Temperature correction to the kinetic parameter values is done by ASP kinetics Parameters and typical parameter values for the sewage )20( 20 − = T T kk θ 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
  15. 15. Aeration tank Se,Xa,V Settling tank Q,Si,Xi Qr,Xr,Se Qw,Xr,Se Qe or (Q-Qw) Xe,Se Aeration tank Se,Xa,V Settling tank Q,Si,Xi Qr,Xr,Se Qw,Xa,Se Qe or (Q-Qw) Xe,Se Xi is considered negligible All biodegradable suspended organic solids of influent are hydrolyzed into soluble organic matter Inorganic and non-biodegradable organic SS remain unaffected and no new SS of these categories are formed Only clarification & sludge thickening occurs in the clarifier
  16. 16. Treated effluent BODU (Se) Use of this equation requires – Primary variable SRT (assumed) (typical values are 5 to 15 days) – Ks, kd, qmax and Y are ASP kinetic parameters Obtained from the following through solving for Se Note that the Se is independent of influent bCOD (or BODu) [ ] ( ) 1. )(1 .max −− + = d ds e kYqSRT SRTkK S d es e k SK Sq Y SRT −      + = max1 )()( )( ratewastagesludgeorrategenerationsludgenet systemtheofsludgetotal SRT =
  17. 17. Active Biomass Concentration (xa) Mixed Liquor Active Biomass Concentration Use of this equation requires – Primary variables SRT and τ (or HRT) typical values are 4 to 12 hours – ASP kinetics parameters Y and kd – Si and Se are influent and effluent bCOD values Obtained from the following basic equation Here xa depends on kd, Y, SRT, τ and bCOD removal ( ) )(1 SRTk YSSSRT x d ei a + − = τ ( ) d a ei k x SSY SRT − − = . 1 τ
  18. 18. Sludge Generation and Wastage Rates Net biomass synthesis rate (NBSR): Estimated by Obtained through simplification of the following material balance equation )(1 )(. SRTk SSQY NBSR d ei + − =       −       =       rateionautooxidat Biomass ratesynthesis biomassGross ratesynthesis biomassNet daei kVxSSQYNBSR ..)(. −−= ( ) )(1 SRTk YSSSRT x d ei a + − = τ Here V is replaced by Q.τ For xa the following equation is used
  19. 19. Sludge Generation and Wastage Rates Secondary sludge generation rate is comprised of – Net biomass synthesis rate – Cell debris generation rate from biomass autooxidation – Nonbiodegradable VSS contributed by the influent (Nb.VSS) – Inorganic suspended solids contributed by the influent (In.SS) GRSSInGRVSSNbCDGRNBSRSSGR .... +++= )..(.. VSSNbQGRVSSNb = )..(.. SSInQGRSSIn = ( )       + −−= −= SRTk SSQYfCDGR NBSRratesynthesisGrossfCDGR d eid d .1 1 1)( Here fd is the fraction of the auto-oxidized biomass left behind as cell debris (usually taken as 0.15)
  20. 20. MLSS x GRSSInGRVSSNbCDGRNBSR NBSR a = +++ .... MLSS MLVSS GRSSInGRVSSNbCDGRNBSR GRVSSNbCDGRNBSR = +++ ++ .... .. can be obtained from MLSS and MLVSS Sludge Wastage – it can be • From the return sludge line – Lesser volume of sludge is wasted – Control is difficult (may need measurement of MLSS and TSS level in clarifier underflow) • From the aeration tank in the form of mixed liquor – Volume wasted is large – Can be wasted either into a primary clarifier or a thickener – Control is much easier (may need only TSSe measurement) Sludge Generation and Wastage Rate
  21. 21. Sludge wasting rate • Determined on the basis of SRT – Due consideration is given to sludge washout (TSSe in the clarified effluent) • Depends on secondary sludge generation rate (SSGR) minus secondary sludge washout rate (SWOR) SSWR = SSGR – SWOR Where SWOR is Q.TSSe Volumetric sludge wastage rate SSWR/MLSSu (when wasted from the secondary clarifier underflow) SSWR/MLSSa (when wasted from the aeration tank) Observed SRT = (V.MLSSa)/SSWR It is greater than the SRT chosen as primary variable TSS of the clarified secondary effluent influences its value At TSS = 0, observed SRT is equal to primary variable SRT Sludge Generation and Wastage Rates
  22. 22. Determined by writing material balance around secondary clarifier • Mass balance for secondary clarifier • Assuming Xe as negligible and taking QwXr as VXa/SRT and taking V as Qτ one can find Qr as Determined by writing material balance around the aeration basin • Assuming new biomass growth and influent biomass (Xi) concentration as negligible, material balance for aeration tank is Determined by the sludge settlability characteristic (SVI) eerwrrra XQXQXQQQX ++=+ )( ar a r XX SRT QX Q −       − = τ 1 )( rarr QQXXQ += ar a r XX QX Q − = Sludge Recycling 1 100 100 − = SVIP r w Pw is MLSS as % (3000 mg/L is 0.3%) SVI is in mL/g r is sludge recycle ratio RQ Q r =
  23. 23. Oxygen Demand Rate 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 (SOTE/SOTR) – These rates require correction to actual operating conditions (AOTE/AOTR)       −       =       CDGRplusNBSR ofequivalentOxygen substrateloadedof equivalentOxygen demand Oxygen ( ) [ ]CDGRNBSRnSSQdemandO ei +−−=2
  24. 24. Actual Oxygen Transfer Efficiency/Rate • AOTR is actual oxygen transfer rate under field conditions – it is influenced by – 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 ( ) F C CC SOTEorSOTRAOTRorAOTE T s LTHs ..024.1 . 20 20, α β −         − =
  25. 25. Actual Oxygen Transfer Rate or Efficiency β is salinity – surface tension factor • Taken as saturation DO ratio of 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 - accounts 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
  26. 26. Actual Oxygen Transfer Rate Cs _ ,T,H is average saturation DO of clean water at operating temp. and altitude at mid-water depth (aerator–surface) • For surface aerators Can be obtained from literature (for the atmospheric pressure at the altitude in question – Annexure C of MetCalf-Eddy) • For diffused aerators it can be obtained by Ot is volume % O2 in the air leaving the aeration basin (typically 18- 20%) HTsHTs CC ,,,, =       += 212 1 . ,,,, t Hatm d HTsHTs O P P CC ( ) ( )       + −× −= T H P P atm Hatm 15.2738314 097.2881.9 exp 0, ,
  27. 27. Air Requirements Air is also required for the mixing of aeration tank contents Typical air requirement for mixing is 0.01 to 0.02 m3 /m3 .min. Air required for mixing and for oxygenation, whichever is larger is used as design air requirement Air required is expressed in kg/hr. and Nm3 /hr Actual temperature of the air depends on the level of compression Ambient temperature + pressure (in kg/cm2 gauge) X 10°C! { }       ×       =       airthein fractionoxygen efficiencytransfer oxygenActual demandOxygen required Air
  28. 28. 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
  29. 29. Nutrient Requirements Nitrate-N in the influent is 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 <0.3 mg/L nitrogen is believed to be limiting for the biooxidation removal of substrate       −       +       +       = luent theinN ationdenitrific throughlostN sludgewasted theinN effluent theinN trequiremenN inf ( )NNitrateTKNQxQ MLSS x TSSQtrequiremenN uw a e +−+      += 1223.01223.03.0
  30. 30. Phosphorus requirement can be assessed in a manner similar to the nitrogen requirement by N and P required can also be conservatively estimated as Here bCOD is in g/m3 Y is yield coefficient (0.4!) Nutrient requirement can also be expressed as the required bCOD:N:P ratio of the influent Nutrient Requirements )3.0.1223.0( iii NNitrateTKNYbCODQtrequiremenN −−+= )3.0.0226.0( ii PTotalYbCODQtrequiremenP −+= 2.1:2.5:100:: =PNbCOD ( )iuw a e PTotalQxQ MLSS x TSSQtrequiremenP −+      += 02263.00226.03.0
  31. 31. Alkalinity Requirements • 70-80 mg/L as CaCO3 for maintaining the pH at 6.8 to 7.4 • Nitrification if occurring requires 7.07 g as CaCO3 per g of NH3-N nitrified • Denitrification if occurring produces 3.57 g as CaCO3 per g of nitrate reduced Treated effluent quality • Characterized by soluble bCOD, TSS and VSS, and nutrients • Soluble bCOD for SRT >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 Others Aspects of ASP Design
  32. 32. Total bCOD of the effluent MLSS x TSSS a ee ××+ 42.1 QHRTV .= Aeration tank volume Vx QS M F a i = VMLVSS QS M F i . = Food to microorganisms (F/M) ratio In terms of active biomass In terms of MLVSS V QS loadingBOD i = BOD loading Other Aspects of ASP Design
  33. 33. Center-feed circular tanks with side wall liquid depth of 3.7 to 6 m and radius of < 5 times liquid depth are used Includes – Inlet section or central well • Size is 30-35% of tank diameter • It is separated from the sludge settling zone by a cylindrical baffle • It is meant to dissipate the influent energy, to evenly distribute flow and to promote flocculation – Sludge settling zone – Sludge thickening and storage zone – Peripheral overflow weir and collection trough • Baffles are often provided to deflect density currents and avoid scum overflow (scum baffles) Has a central rotating mechanism to scrap, transport and remove the thickened sludge (and also the floating scum) – Sludge is removed directly from tank bottom by suction orifices Secondary Clarifier
  34. 34. Design of Secondary Clarifier Very similar to primary sedimentation tank • Rather than just clarification both clarification and sludge thickening occur • Sludge blanket is maintained for thickening to occur and hence depth is >3.7 m • Larger central well, density currents, relatively lower weir loading rates Area required for clarification and area required for thickening are found out and the larger of the two is used Design approaches for the secondary clarifiers – 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 Secondary clarifier is also designed on the basis of SVI and ZSV
  35. 35. 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 lesser than the area required for thickening
  36. 36. Area required for thickening • Tu corresponds to Hu and obtained through • Co is initial TSS and Ho column height • Cu is underflow sludge concentration 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 o u t H Qt A = u oo u C CH H = Secondary Clarifier: Talmadge and Fitch method
  37. 37. Secondary Clarifier: Talmadge and Fitch method Area for clarification – Here Qc is clarification rate – V is interface subsidence velocity 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 Hc and computed as – Here Hc is critical sludge depth – Q is flow rate of mixed liquor into the clarifier v Q A c c = o c c H HH QQ − = 0
  38. 38. 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)
  39. 39. Downward flux of solids in a settling tank occurs due – gravity settling – 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 – 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 ugt SFSFSF += iig VCSF =
  40. 40. Solids flux by bulk transport – 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 A QC UCSF ui biu ==
  41. 41. 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
  42. 42. 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 • For a desired underflow concentration one can increase or decrease the slope of the underflow flux line ( ) L u SF CQQ A 0+ = Q is overflow Qu is underflow SFL is limiting solids flux
  43. 43. 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 ZSV (Vi) can be estimated by Here Vi is zone settling velocity (SVI) 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 SF V rateoverflowSurface i = xKVVi )exp(max −=
  44. 44. MLSS, ZSV and SVI/DSVI are related 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) xSVIVi )001586.01646.0(871.1)(ln +−= xDSVIVi )002555.0103.0(028.2)(ln +−= Design of Secondary clarifier on the basis of SVI and ZSV
  45. 45. 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
  46. 46. Operational Problems of ASP Common problems encountered in operating the ASP • Bulking sludge • Rising sludge • Nocardia foam Bulking sludge • Causes high suspended solids in the effluent Flocs do not compact and settle well and sludge blanket depth increases (beyond typical 10 to 30 cm) • Results in poor treatment performance Maintaining desired level of MLSS/MLVSS becomes difficult, effluent has suspended BOD, higher recycle rates reduce wastewater’s HRT Two types of bulking: Filamentous and Viscous bulking
  47. 47. Filamentous bulking • Filaments normally protrude out of the sludge floc • Surface area to mass ratio increases and sludge attains poor settling properties Viscous bulking • Caused by excessive amount of extracellular hydrophilic biopolymer • Makes the sludge highly water retentive (hydrous bulking) Bulking Sludge
  48. 48. Factors causing bulking – Wastewater characteristics, like, readily biodegradable organic matter and fermentation products, H2S and reduced sulfur compounds (septic water), nutrient deficiency and low pH – Flow variations and variations in pH – Design limitations, like complete mix reactor conditions, limited air supply, poor mixing, short circuiting, defective sludge collection and removal and limited return sludge pumping capacity – Operational issues, like, low DO, insufficient nutrients, longer SRT and subsequent low F/M, insufficient soluble BOD (for these filamentous organisms are very competitive), internal plant overloading (recycle loads of centrate and filtrate) Nutrient limiting systems and very high loading of wastewater with high levels of readily biodegradable COD can cause viscous bulking Bulking Sludge
  49. 49. Control of bulking may require investigation on – Wastewater characteristics – Process loading – Return and waste sludge pumping rates – Internal plant overloading – Clarifier operation Investigation is usually started with microscopic examination of mixed liquor Bulking Sludge
  50. 50. Solutions for bulking – Decreasing SRT or operating the aeration equipment at full capacity can take care of bulking from limiting DO • DO 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
  51. 51. 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 Bulking Sludge
  52. 52. 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 Rising Sludge
  53. 53. 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 Foaming
  54. 54. 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
  55. 55. 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)

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