Bulking Control Jae K. (Jim) Park, Professor Dept. of Civil and Environmental Engineering University of Wisconsin-Madison

Activated Sludge Separation Problems (1) Dispersed growth Cause: Microorganisms do not form but are dispersed, forming only small clumps or single cells. Effect: Turbid effluent. No zone settling of sludge. Slime (jelly) viscous bulking; or non-filamentous bulking Cause: Microorganisms are present in large amounts of exocellular slime. In severe cases, slime imparts a jelly-like consistency to the activated sludge. Effect: Reduced settling and compaction rates. Virtually no solids separation in severe cases resulting in overflow of sludge blanket from secondary clarifier. In less severe cases a viscous foam often is present. Bulking Cause: Filamentous organisms extend from flocs into the bulk solution and interfere with compaction and settling of activated sludge. Effect: High SVI - very clear supernatant. Low RAS and WAS solids concentration. In severe cases, overflow of sludge blanket occurs. Solids handling processes become hydraulically overloaded.

Settling Problem in Activated Sludge Processes

Activated Sludge Separation Problems (2) Pin floc or pinpoint floc Cause: Small, compact, weak, roughly spherical flocs are formed, the larger of which settle rapidly. Smaller aggregates settle slowly. Effect: Low SVI - a cloudy, turbid effluent Blanket rising Cause: Denitrification in secondary clarifier releases poorly soluble N2 gas which attaches to activated sludge flocs and floats them to the secondary clarifier surface. Effect: A scum of activated sludge forms on surface of secondary clarifier. Foaming/scum formation Cause: Caused by non-degradable surfactants and by the presence of Nocardia spp. and sometimes by presence of Microthrix parvicellar. Effect: Foams float large amounts of activated sludge solids to surface of treatment units. Nocardia and Microthrix foams are persistent and difficult to break mechanically. Foams accumulate and can putrefy. Solids can overflow into secondary effluent or overflow tank free-board on to walkways.

Floc Formers

Foaming Nocardia spp.

Ideal, Non-Bulking Activated Sludge Floc Filamentous organisms and floc forming organisms in balance Strong, large floc Filaments do not interfere Clear supernatant Low SVI Filamentous backbone Filamentous Bulking Activated Sludge Floc Filamentous organisms predominant Strong, large floc Filaments interfere with settling, compaction Clear supernatant High SVI Pin Point Floc Low filamentous organisms Weak, small floc Turbid supernatant High SVI

Young Sludge White foam The floc is barely forming and does not settle. A large amount of floc particles are left in the supernatant. Fluffy floc that is left behind in the supernatant is called straggler floc.

A Typical Sludge Old Sludge A typical activated sludge system with a mixed liquor suspended solids (MLSS) color that would be described as light brown or "coffee with cream". The MLSS color is dark brown, the foam also has a darker color.

Too much treatment time in an activated sludge system can cause an old sludge condition. This is sometimes called over-aeration. Over-aeration causes the floc to be very compact and dense. It settles quickly and leaves very small floc particles, called pin-floc, in the supernatant. Some of the floc particles are so light that they float to the surface and spread out in the clarifier. This condition is called ashing.

Over Aeration Pin Point Floc (in a Clarifier) Surface ashing

Denitrification Good settling sludge Sludge blanket rising 2NO3- + 10e- + 12H+ → N2  + 6H2O Good settling sludge Sludge blanket rising N2 Activated sludge

Filamentous Organism Types Between 25~30 types observed Approx. 10~12 types commonly seen Indicative of causative conditions Grow as filaments (trichomes) rather than individual cells or cell aggregates (flocs) A necessary and beneficial component of the activated sludge floc community Beneficial effects: enhance floc structure and produce low substrate residuals Detrimental effects: pin floc, bulking, foaming Factors affecting filamentous organism growth Sludge age, DO, waste characteristics, reactor configuration (or waste feeding regime), and presence of initial unaerated zones

Causes of Bulking (1) DO concentration Four filamentous bacteria proliferate with low DO. At low to moderate sludge age: type 1701, S. natans and Haliscomenobacter hydrossis At high sludge age: Microthrix parvicella All of these microorganisms usually respond well to increases in dissolved oxygen. Nutrient deficiency Type 021N, Thiothrix spp., type 0041 and type 0675 grow with nitrogen and/or phosphorus deficiencies. Biological slime often accumulates as well with the growth of these microorganisms. Adding ammonia or phosphoric acid to the aeration tank is usually required to control these microorganisms.

Sphaerotilus natans, 1000x Microthrix parvicella, 1000x Haliscomenobacter hydrossis, 1000x

Type 021N, 1000x Thiothrix II, 1000x Thiothrix I, 1000x

Causes of Bulking (2) Low pH Fungi can proliferate under low pH conditions. The condition is most often found when the influent pH is low. However, it may also be observed in nitrifying systems or oxygen-activated sludge systems where the natural alkalinity of the wastewater is low. Elimination of the low influent pH or addition of alkalinity to the system is required to subdue the fungi. Sulfide Thiothrix spp., type 021N, Beggiatoa spp. and type 0914 can all oxidize sulfide to elemental sulfur and incorporate the sulfur into the cell. The sulfur can be seen in the cell when observed under a microscope. The sulfide's source must be eliminated or the sulfide "tied up" chemically.

Causes of Bulking (3) Readily metabolized food Some organisms grow well on easily broken-down, soluble materials. Sugars and short carbon-chain materials are examples. These organisms are S. natans, type 021N, Thiothrix spp., H. hydrossis, Nostocoida limicola and type 1851. Reducing the sludge age and installing a selector often help to control N. limicola and type1851. Slowly metabolized food Types 0041, 0092, and 0675 and M. parvicella are able to grow on slowly broken-down foods and are prevalent in systems that biologically remove nitrogen and phosphorus. Their growth is also augmented by the use of complete mixing in the aeration tank. Control is attained by reducing the sludge age, using plug-flow if possible, and maintaining a uniform DO. Anoxic selectors do not help because some of these microorganisms may be able to denitrify.

Not a filamentous organism Nostocoida limicola, 1000x Nitrosomonas, 1000x Not a filamentous organism Nocardia, 1000x

Causes of Bulking (4) Reactor configuration At short to moderate sludge age, properly designed selectors (aerobic, anoxic, and anaerobic) can control the following organisms: Type 021N, Thiothrix spp., S. natans, type 1701, H. Hydrossis, and Nocardia Unaerated zones At long sludge age (e.g., nitrifying BNR plants), the presence of unaerated zones allows the growth of the following organisms that are not controlled by any type of selector: M. parvicella, type 0092, type 0041, and type 0675 Nature of substrate Soluble and readily biodegradable vs particulate and slowly biodegradable Organisms preferring soluble readily biodegradable substrates: S. natans, type 021N, Thiothrix spp., H. hydrossis, N. limicola, and type 1851

Control of Bulking (1) Low DO aerobic zone growers Features: readily biodegradable substrate low DO low moderate sludge age Organisms: S. natans, type 1701, H. hydrossis Control: aerobic, anoxic, or anaerobic selectors Mixotrophic aerobic zone growers moderate to high sludge age sulfide oxidized to stored sulfur rapid nutrient uptake rates under nutrient deficiency Organisms: Type 021N, Thiothrix nutrient addition eliminate sulfide

Control of Bulking (2) Other aerobic zone growers Features: readily biodegradable substrate moderate to high sludge age Organisms: Type 1851, N. limicola Control: aerobic, anoxic, or anaerobic selectors Aerobic, anoxic, and anaerobic zone growers Features: grow in aerobic, anoxic, or anaerobic (BioP) systems high sludge age grow on hydrolysis products of particulate (?) Organisms: Type 0041, type 0675, type 0092, M. parvicella Control: ? Because a wide variety of causes for filamentous organism growth exist, a wide range of control measures are required.

Mechanisms of Filamentous Organism Bulking (1) 1. Surface to volume ratio Filamentous organisms Floc formers (FF) Unfavorable conditions Substrate (low) DO (low) Nutrient deficiency More frequently found in CSTR PFR Low substrate gradient due to dilution after instantaneous mixing High substrate gradient due to less dilution CSTR: continuously-stirred tank reactor Prone to bulking PFR: plug flow reactor

Mechanisms of Filamentous Organism Bulking (2) 2. Kinetic hypothesis µ Floc formers (FF) Bulking Filamentous organism (FO) µm Ks FO low low FF high high Low substrate S Failed to explain microbial population dynamics with temporal and spatial substrate transients

Mechanisms of Filamentous Organism Bulking (3) 3. Accumulation-regeneration hypothesis FF FO Capable of rapidly accumulating and storing substrate when exposed to a high substrate concentration compared with filamentous organisms When high substrate gradient exists, i.e., PFR, FO cannot compete; thus, no bulking PFR: high substrate gradient CSTR: low substrate gradient

Methods of Creating Substrate Gradient Temporal Batch operated Example: sequencing batch reactor (SBR) Spatial Plug flow reactor (PFR) Example: conventional activated sludge process Selector (a separate reactor ahead of the aeration basin) (see the detail later) Example: biological nutrient removal processes

Changes in Soluble COD and Respiration Rate over Time PFR (batch operated): rapid COD uptake due to limited food available CSTR: slow COD uptake due to unlimited supply of food

Mechanisms of Filamentous Organism Bulking (4) 4. Starvation resistance hypothesis Slow growing FO with high starvation resistance Fast growing FF with moderate starvation resistance (low DO) Fast growing FO with limited starvation resistance Fast growing FF with moderate starvation resistance (high DO) Fast growing FF with high DO µ At low DO, Fast growing FO wins Fast growing FF with low DO Slow growing FO wins S S* S**

Physical Characteristics of FF and FO Characteristics FF FO Max. substrate uptake rate High Low Max. specific growth rate High Low Endogenous decay rate Low High Decrease in µ from low S conc. Significant Moderate Resistance to starvation Low High Decrease in µ from low DO Significant Moderate Organic sorbability in excess S High Low Denitrification capability Yes No Luxury uptake of P Yes No

DO SVI Bulking Bulking

Design DO and Substrate Utilization Rate At higher q, more DO needed A typical municipal WWTP

Selectors Microorganisms with rapid sol. COD uptake/storage sequestrate substrate in selector and use it to survive in aeration basin (starvation phase) Microorganisms require energy to take up and store sol. COD in selector. Energy is obtained from Oxidation of COD using O2: Aerobic selector Oxidation of ammonia to NO3-: Anoxic selector Hydrolysis of stored poly-P to PO43-: Anaerobic selector Aerobic: kinetic only Anoxic: kinetic + ability to denitrify Anaerobic: kinetic + ability to store polyphosphate

Selector Types Selectors Secondary Aeration clarifier basin Effluent Influent RAS WAS Influent Secondary clarifier Aeration basin Effluent Selectors

Comparison of Biological Selectors (1) Aerobic Advantages Simple process, no additional internal recycle streams other than RAS Relies on basin geometry, not nitrification Disadvantages Does not reduce O2 requirements Requires more complex aeration system design to meet maximum O2 uptake rate in the initial high F/M zone May require patent fee if operated within a certain range of DO conditions

Comparison of Biological Selectors (2) Anoxic Advantages Tends to buffer nitrification (recovers approx. 3.5 lb alkalinity as CaCO3 per lb of NO3--N denitrified) Lowers O2 demand in a nitrification process (recovers approx. 2.86 lb O2 per lb of NO3- reduced) The initial high F/M region occurs in the anoxic zone with the high O2 demand met by NO3- instead of O2 Disadvantages Cannot be used with a process that does not nitrify Uses an additional recycle stream Requires care in design and operation to minimize the introduction of O2 in the anoxic zone. Poor system design could induce low DO bulking

Comparison of Biological Selectors (3) Anaerobic Advantages Simple design, no internal recycle other than RAS The simplest of selector systems to operate Can be used for biological phosphorus removal Disadvantages A patented process and require a licensing fee Does not reduce O2 requirements May not be compatible with long SRTs Requires care in design and operation to minimize the introduction of NO3- and O2 in the anaerobic zone. Poor system design could induce low DO bulking

Design Parameters Parameter Recommended value Source Initial substrate concentration > 80 mg COD/L Lee et al. (1982) Initial F/M 20~25 g COD/g MLVSS·day Lee et al. (1982) 8~12 g COD/g MLVSS·day Jenkins (1988) Initial floc loading 50~150 g COD/g MLSS Eikelboom (1977&82) No significance Lee et al. (1982) Fractional substrate removal 50~70% Eikelboom (1977&82) in all selectors (soluble and 80% Chudoba et al. (1987) degradable) Soluble substrate leaving selector 60 mg COD/L Jenkins (1988) Substrate gradient between first 25 mg/L Chudoba et al. (1973) selector and main aeration basin Aeration basin dispersion number < 0.2 Tomlinson (1979) Number of compartments 1~5 van Niekerk (1986) Hydraulic retention time 12~25 min van Niekerk (1986) 5~20 min Casey et al. (1975)

Initial Substrate Concentration Design Guideline (1) Initial Substrate Concentration 80 Initial substrate concentration: > 80 mg COD/L (Lee et al., 1982)

Design Guideline (2) Initial F/M (COD) Initial F/M: 20~25 g COD/g MLVSS·day (Lee et al., 1982)

Soluble COD Leaving Selector Design Guideline (3) Soluble COD Leaving Selector 60 Soluble COD leaving selector: < 60 mg/L (Jenkins, 1988)

Control of Activated Sludge Bulking Using Substrate Gradients Temporal - intermittently fed A.S., e.g., SBR Spatial Dispersion number (D/uL): < 0.1 or # of CSTR in series: > 6 Floc loading (F/L): 50~150 g COD/g MLSS PFR: CSTR:

Estimation of # of CSTRs in Series and Dispersion Number, D/uL Levenspiel, O. (1972). Chemical Reaction Engineering, 2nd Ed., pp. 272-279.

Dispersion Model u L D = 0 D = 0

Example (1) Determine dispersion number and # of CSTRs in series. Time, t (min) Tracer conc., C (g/L) 5 10 15 20 25 30 35 3 4 2 1

Example (2)

Relationship between Sludge Settleability and Dispersion Number Dispersion number (D/uL): < 0.1

# of CSTR in series: > 6 Relationship between Sludge Settleability and Number of Equivalent Tanks # of CSTR in series: > 6 6

Bulking Control - Summary 1. Change of the mixing configuration in the aeration basin (low dispersion number) 2. Selector installation - promotes growth of floc- forming bacteria 3. Chlorine injection (hydrogen peroxide also an option) 4. Increase in the aeration basin DO - low DO bulking 5. Nutrient addition - nutrient deficiency bulking

Bulking Control by Chlorination Dose based on g Cl2/kg MLVSS/day (lb Cl2/1000 lb/day) Low dose: 2~3 g Cl2/kg MLVSS/day Moderate dose: 5~6 g Cl2/kg MLVSS/day High dose: 10~12 g Cl2/kg MLVSS/day Dose controlled by monitoring trend in SVI and microscopic examination. Dose point selected to achieve adequate dosing frequency of 3 time/day or more. Excellent mixing at dose point required to avoid turbidity formation. Cl2 Cl2 Secondary clarifier Aeration basin Cl2 RAS WAS Most common

Aerobic Selector Design Staged reactor with F/M gradient. First stage loading important to control viscous bulking. (F/M)I = 8~12 g COD/g MLVSS·day SOUR 50~60 mg O2/g MLSS·hr, but only 15~25% of substrate oxidized. DO 1~2 mg/L. Selector HRT often 10~20 min for municipal wastewater Three to four compartments (1/1/1/2 or 1/1/2) F/M=12 kg COD/kg MLSS·hr F/M=6 kg COD/kg MLSS·hr F/M=3 kg COD/kg MLSS·hr Wastewater RAS

Anoxic Selector Design Reactor staging not necessary. F/M gradient less than aerobic selector (6/3/1.5 g COD/g MLSS·day) Provide mixers rather than aeration. About 8 g NO3--N req./g COD oxidized (5 g NO3--N req./g BOD oxidized). NO3- supplied by RAS and mixed liquor recycle Rapid denitrification rates (5~10 mg NO3--N/g MLSS·hr at 20°C). Anoxic zone HRT often about 1 hr for municipal wastewaters. Three compartments (1/1/2) Anoxic selector Secondary clarifier Aeration basin Internal recycle RAS WAS

Anaerobic Selector Design Design concepts similar to anoxic selector, except internal recycle. HRT typically 0.75 to 2 hrs. Three compartments (1/1/2) (F/M)i = < 6 g COD/g MLSS·day Design criteria still evolving. Anaerobic selector Secondary clarifier Aeration basin RAS WAS

Selector Configuration at City of Hamilton, OH Original system T3 Modification to selector (HRT: 4 min.) Modification of T2A to selector (HRT: 7 min.) HRT: 8~12 min

SVI Change over Time, City of Hamilton, OH T3: Better settling (low SVI) due to selectors T2A & T2B: After selector installation, decreased SVI

Operational Data, City of Albany, GA Wastewater Treatment Plant SVI drop after chlorination Stable effluent SS and BOD

SVI drop after anoxic selector installation Effect of Anoxic Stage on the Settleability of Activated Sludge in a Completely-Mixed Plant SVI drop after anoxic selector installation

Upper Occoquan Sewage Authority Historically plagued by M. parvicella Mechanical surface aerators Denitrification expensive Aerobic selector

Performance Data

Tri-City, Oregon Historically poor plant stability Low alkalinity water Anoxic selector Mixed liquor recycle Baffle Primary effluent To secondary clarifiers RAS Submerged mixer Anoxic selector

Performance Data SVI, mL/g 300 Plug flow anoxic Plug flow anoxic Step feed Step feed Plug flow anoxic Aeration of selector 200 SVI, mL/g 100 1986 1987 1988

Fayetteville, Arkansas Effluent phosphorus requirement Readily biodegradable wastewater Anaerobic selector Surface mechanical aerator Submerged mixer Anaerobic selector Aeration basin Primary effluent To secondary clarifiers RAS

Anoxic Selector Design (1) 1. Determine COD uptake rate, denitrification rates, and nitrate requirements from a pilot study 2. Measure influent (primary effluent) and secondary effluent (or recycle) CODs to determine degradable substrate (CODinf - CODeff). 3. Determine flow rates, recycle rates, and recycle concentrations to be used for design. 4. Assume 50~80% of the biodegradable COD must be removed in the selectors or that the soluble COD in the selector effluent must be reduced to 60 mg/L. 5. From the above information, determine the required HRT of the selector which in turn can be used to determine the selector volume from the flow and recycle flow. 6. Compartmentalize (determine the number of selectors) to provide either a high soluble substrate concentration or a high F/M in the initial compartment.

Anoxic Selector Design (2) Design conditions Flow rate, Q = 8 mgd Recycle, R = 0.5Q = 4 mgd Required selector soluble effluent COD = 60 mg/L Total COD of primary sedimentation tank effluent, CODinf = 250 mg/L Soluble COD of primary sed. tank effluent, sol. CODinf = 150 mg/L Nitrate requirements = 8 (typically 6~10) mg sol. COD removed/mg NO3--N (note that complete denitrification is not the goal in bulking control but merely uptake of COD.) Denitrification rate = 4 mg NO3--N/g VSS·hr MLSS in recycle = 7,000 mg/L MLVSS = 0.75 MLSS in recycle; thus, Xr = 5,250 mg/L NO3- in recycle = 15 mg N/L Substrate uptake in full-scale plant = 65 mg COD/g VSS·hr Substrate uptake in pilot-scale batch reactor = 140 mg COD/g VSS·hr Substrate uptake in pilot-scale anoxic reactor = 120 mg COD/g VSS·hr

Anoxic Selector Design (3) Selectors Effluent Aeration tank Secondary clarifier Q = 8 mgd CODinf=250 mg/L Sol. CODinf=150 mg/L R = 4 mgd Xr=5,250 mg/L NO3-=150 mg N/L Sol. CODr=50 mg/L Determination of degradable COD influent to the selector after dilution with recycle Biodegradable sol. CODinf = 150 - 50 = 100 mg/L Mass balance: Q·Cinf + 0.5Q·Crec = 1.5 Q·Cmix (conc. in the selector) Thus, 8 mgd × 100 mg/L + 0.5 × 8 mgd × 0 mg/L = 1.5 × 8 mgd × Cmix Cmix = 66.7 mg/L readily biodegradable COD Assuming 65% of this must be removed in the selector, 43 mg/L must be removed.

Anoxic Selector Design (4) Determination of degradable COD influent to the selector after dilution with recycle - continued Alternatively, using the criterion of 60 mg/L sol. COD in the selector effluent, 8 mgd × 150 mg/L + 0.5 × 8 mgd × 50 mg/L = 1.5 × 8 mgd × Cmix Cmix=117 mg/L total sol. COD Amount to be removed to attain a residual of 60 mg/L sol. COD would be: 117 mg/L - 60 mg/L = 57 mg/L This criterion of 60 mg/L seems over conservative for this case since 50 mg/L of this is not readily biodegradable. Determination of selector biomass Q·Xinf + 0.5 Q·Xr = 1.5 Q·Xmix 8 mgd × 0 mg/L + 0.5 × 8 mgd × 5,250 mg/L = 1.5 × 8 mgd × Xmix Xmix = 1,750 mg VSS/L

Anoxic Selector Design (5) Check whether there is sufficient nitrate in recycle Q·Ninf + 0.5 Q·Nr = 1.5 Q·Nmix 8 mgd × 0 mg/L + 0.5 × 8 mgd × 15 mg/L = 1.5 × 8 mgd × Xmix Nmix = 5 mg NO3--N/L available NO3--N req. = 43 mg/L COD removed  8 mg COD/mg NO3--N = 5.3 mg NO3--N/L Thus, 0.3 mg/L NO3--N is needed. Potential remedies 1. Recycle more RAS. 2. Recycle directly from end of nitrification tank. 3. Remove less substrate in the selector and check whether acceptable. 4. Add additional nitrate to the selector. For simplicity, option 4 will be chosen. Calculate HRT in the selector Denitrification limited:

Anoxic Selector Design (6) Calculate HRT in the selector - continued Need to remove 5.3 mg NO3--N/L; thus, HRT = 5.3 mg NO3--N/L  7 mg NO3--N/L·hr = 0.757 hr = 46 min Substrate limited: Need to remove 43 mg COD/L; thus, HRT = 43 mg COD/L  210 mg COD/L·hr = 0.20 hr = 12 min Since the time required for denitrification is greater than for substrate removal, the necessary HRT is controlled by denitrification. Determine total selector volume V = (Q+Qr) × t = 1.5 × 8 mgd × 0.757 hr × 1day  24 hr = 0.3785 Mgal = 0.3785 × 106 gal × 1 ft3/7.48 gal = 50,602 ft3

Anoxic Selector Design (7) Check initial substrate concentration Assuming three equally-sized completely-mixed compartments each of 1/3 × 46 min = 15.3 min = 1.06 × 10-2 day Thus, COD removed = 1.79 mg NO3--N/L × 8 mg COD/mg NO3--N = 14.3 mg/L Therefore, the concentration of sol. COD will be 117-14.3 = 102.7 mg COD/L. This is greater than the recommended 80 mg COD/L; thus, three compartments may be sufficient. This check is based on the assumption that principles important to design of aerobic selector should also be applied to anoxic selectors. Jenkins (1988): 8~12 mg COD/g MLVSS·day; thus, acceptable.

References Eikelboom, D. H. and H. J. J. van Buijsen, Microscopic Sludge Investigation, Manual, TNO Res. Inst. for Env. Hygiene, Delft, The Netherlands, 1981. Hegg, B.A., K.L. Rakness and J.R. Schultz, Evaluation of Operations and Maintenance Factors Limiting Municipal Wastewater Treatment Plant Performance, paper presented at 41st annual meeting of the RMWPCA, Albuquerque, NM, 1977. Jenkins, D., M.G. Richard and G.T. Daigger, Manual on the Causes and Control of Activated-Sludge Bulking and Foaming, Lewis Publishers, Ann Arbor, 193 p., 1993. U.S. General Accounting Office, Report to the Administrator, Environmental Protection Agency, Wastewater Dischargers Are Not Complying with EPA Pollution Control Permits, GAO/RCED-84-53, 61 p., 1983.