Evaluation of alternative technologies for upgrading wastewater treatment plants in Minnesota for new phosphorus limits H. David Stensel University of Washington Gary M. Grey George J. Kehrberger Hydroqual 11th Annual Education Seminar Central States Water Environment Association April 4, 2006

Upgrading WWTPs for phosphorus removal Phosphorus effluent limits of <1.0 mg/L P expected in general for state of Minnesota Minnesota Science and Economic Review Board Identify the most appropriate cost effective phosphorus reduction strategies for retrofitting existing treatment plants for different types of biological treatment processes Developed a protocol to systematically evaluate the effectiveness of phosphorus removal alternatives for various types of plants Apply protocol to evaluate phosphorus removal alternatives for different types of WWTPs

Summary of Treatment Plant Characteristics Different Sizes 0.5 – 19.1 MGD 8 Different Biological Treatment Processes 3 Activated sludge 2 Biological nutrient removal 2 Oxidation ditches 2 High purity oxygen 1 Trickling filter 4 Combined trickling filter & activated sludge 2 Lagoon systems 1 Rotating biological contactor

Continued Summary of Treatment Plant Characteristics 5 Plants with tertiary treatment (filters) 5 Plants dewater sludge 14 Plants land apply waste sludge Nutrient Requirements 11 Plants monitor only for phosphorus 7 Plants monitor only for ammonia nitrogen 14 Plants receive industrial wastewater 4 Plants have 1 mg/L phosphorus discharge limit 8 Plants have discharge limit for ammonia

Protocol used for phosphorus Removal plternatives evaluation Ehanced Biological Phosphorus Removal (EBPR) Only Chemical precipitation only EBPR +chemical addition Key site-specific information was obtained for evaluation

Goals of retrofit process evaluations for P removal Tank volumes required for process configuration selected – i.e. anaerobic, anoxic, aerobic Feasible retrofit modifications within existing facility Sludge production and P recycle Effect of pre-activated sludge processes on design and performance Primary treatment Trickling filter P removal possible with EBPR Chemical dose required for P removal and alkalinity control

Impact of wastewater characteristics Influent Parameter On Chemical Treatment On EBPR >BOD >Tank volume >Sludge production >Tank Volume >Sludge Production >TSS >Total P >Chemical dose >Effluent P conc. >BOD/P <Effluent P conc. >rbCOD/P <Effluent P conc <Temperature rbCOD=soluble readily biodegradable COD-VFA source

Impact of wastewater characteristics (continued) Influent Parameter On Chemical Treatment On EBPR >TKN, NH3 >Effluent P conc. <BOD/TKN >Loading variations

EBPR Protocol Select SRT Chemical addition option Size & Locate Anaerobic Tank WWT Character. Select SRT If nitrification locate and size anoxic tank Determine amount of P removed Evaluate Costs Chemical addition option

Chemical Addition Only Protocol Determine P used in biotreatment WWT Character. Identify dose points Determine chemical Sludge produced Determine chemical dose Evaluate Costs

EBPR Protocol Size – 1.0 Hour detention time Size & Locate Anaerobic Tank Size – 1.0 Hour detention time Ability to add to Existing System depends On existing design, capacity, and layout Easier to Add to Plug flow Tanks with Enough capacity PC AT SC Anaerobic PC AT SC

Oxidation Ditch and High Purity Oxygen (HPO) require an external tank Size & Locate Anaerobic Tank For some systems layout is not Compatible for fitting into existing tanks AN SC AT Oxidation Ditch and High Purity Oxygen (HPO) require an external tank HPO tanks PC AN SC AT

EBPR Protocol EBPR Select SRT Nitrification: NH3 to NO3 SRT@100C Function of temperature SRT@100C SRT@150C EBPR 5.1 days 4.1 days Nitrification 15.1 days 9.3 days

Tank volume needed is related to SRT and BOD removed Primary treatment lowers Yn Primary treatment with chemicals lowers Yn more Use of anaerobic zone in EBPR produces lower SVI and thus allows higher MLSS concentration 3500 mg/L possible

EBPR Protocol Nitrate reduced to N2 in anoxic tank Less nitrate to anaerobic zone 1 mg/L NO3-N robs 0.70 mg/L P removal Saves energy – use NO3 produced Improves sludge settling If nitrification locate and size anoxic tank ANAEROBIC INFL. EFFL. ANOXIC AEROBIC WAS

EBPR Protocol Typically 10-20% of aerobic volume More influent TKN, more nitrate; larger tank Less influent BOD/TKN; larger tank Less soluble BOD; larger tank If nitrification locate and size anoxic tank ANAEROBIC INFL. EFFL. ANOXIC AEROBIC WAS

EBPR Protocol P is removed by phosphorus accumulating organisms (PAOs) Determine amount of P removed P is removed by phosphorus accumulating organisms (PAOs) and exits system in waste sludge Anaerobic Anoxic and/or Aerobic Waste sludge NO3 P release P uptake Influent rbCOD Influent particulate BOD -Carbon storage-PHB -poly P storage

How much P is removed by microbes? P removal =f(PAO growth from rbCOD, % P in cells) + P removed for cell synthesis Assumes 25% dry wgt of PAOs=P

Processes that deprive PAOs of rbCOD Denitrification in anaerobic zone Nitrate (NO3) may be present in return activated sludge 1 mg/L NO3-N uses ~ 7 mg/L equivalent to 0.70 mg/L P removal by EBPR Trickling filter treatment prior to activated sludge in combined systems Effluent rbCOD can be at very low concentration Depends on influent rbCOD concentration and trickling filter loading

EBPR Protocol Evaluate Costs Preliminary costs only external tankage needed retrofit existing tanks for A2O process recycle lines and pumps mixers chemical feed equipment and storage O&M for mixing, pumping, labor Some things not included? site specific issues aeration design solids processing

General Preliminary Capital Costs Curves Used Figure 4.9 – Preliminary Budgetary Retrofit Capital Costs – Enhanced Biological Phosphorus Removal

General Preliminary Operating Cost Curves Used Figure 4.10 – Preliminary Budgetary O&M Costs – Enhanced Biological

What if EBPR does not provide enough P removal? Provide chemical addition at secondary effluent or in primary treatment Provide additional rbCOD (volatile fatty acid) by purchase of organics or produce VFA by on-site fermentation primary AN Aerobic SC WPS VFA WAS Fermenter To digester or other

Fermenter Design Assumptions Primary clarifier solids removal Influent TSS = 200 mg/L 65% TSS removal Primary sludge = 3% solids SRT = 3 days in gravity thickener fermenter VFA production = 0.15 g VFA/g TSS applied Elutriation returns 70% of VFA produced Additional P removal = 1 g P/ 12 g VFA added Sugar cost = $0.18 per lb COD

Impact of obtaining rbCOD (VFA) from on-site primary sludge fermenter

Chemical Addition Only Protocol Determine P used in biotreatment WWT Character. Identify dose points Al or Fe Al or Fe PC Biological Process SC waste waste

Effect of dose point on chemical requirement

Chemical Addition Only Protocol Determine chemical dose At dose point select effluent P From curve get Al/P ratio (Infl P- Effl P)Al/P ratio = Al dose, mg/L For primary step select effluent P so that Al/P ratio ~ 1.0 M/M Evaluate alkalinity consumed by alum/ferric addition 0.45 g alkalinity (as CaCO3) used per g alum

Chemical Addition Only Protocol Determine chemical Sludge produced 3 modes of sludge production AlPO4 Precip. Al(OH3) Precip. Increased Primary sludge removal 3.93 g/g P 0.23 g/g P %TSSr=65(0.0021*Al+1.0)

Impact of chemical addition to primary clarification step Decreases overall chemical dose Removes more suspended solids % TSS removal from 65 to 90% Removes more BOD % BOD removal from 35 to 65% Removes more on non degradable VSS More primary sludge production Decreases load to activated sludge process Increases capacity of activated sludge process More volume available for retrofit to biological phosphorus and nitrogen removal

Chemical Addition Only Protocol Cost Factors $0.10 per lb of Alum as Al2(SO4)3.18H2O $0.30 per lb of alkalinity as soda ash $180 per dry ton of solids processing and disposal $0.08 per kilowatt-hr Labor at $20/hour Present worth at 20 years and 5% interest rate

Capital cost for chemical feeding Figure 4.11 – Preliminary Budgetary Retrofit Capital Costs – Chemical Precipitation

Alternative Analyses at Level of Facility Planning Costs not included in analyses Specific site conditions Land availability for expansion Layout constraints for addition of tanks and piping Needed improvements to existing system Hydraulic profile limitations Additional sludge handling and disposal equipment

EBPR Retrofit Analysis Add anaerobic contact tank – 1.0 hour HRT Is nitrification required or occurrring? If yes, provide anoxic tank and recycle for A2O process If no, use lower SRT and A/O process Does activated sludge follow a trickling filter Determine if sufficient rbCOD remains after trickling filter to allow EBPR If not, bypass trickling filter Or use chemical treatment only Evaluate with single or two point chemical addition

Chemical Addition Retrofit Analysis Determine possible chemical dose points Evaluate chemical dose for different dose point options Determine sludge production Determine alkalinity depletion If nitrification system, consider alkalinity addition to maintain system pH

Summary of P Removal Alternatives Selection for Attached Growth Systems Selected Alternative Feed BOD/P ratio Trickling Filter Detroit Lakes RBC Brainerd Lagoons Redwood Falls Thief River Falls Chemical (no action) Chemical NA Chemical NA

Summary of P Removal Alternative Selection for Combined Systems Selected Alternative Feed BOD/P ratio Trickling/Activated Sludge Faribault Marshall Glencoe (w/ Industry) Glencoe (w/o Industry) Little Falls Chemical EBPR + Chemical 12 28 20 40 36

Summary of P Removal Alternative Selection for Activated Sludge Systems Selected Alternative Feed BOD/P ratio Conventional Alexandria Lake New Ulm Grand Rapids Chemical EBPR + Chemical Nutrient limited 27 23 >100 BNR St. Cloud Fergus Falls** EBPR 26 ** - demonstrated and proven for P < 1.0 mg/L

Summary of P Removal Alternative Selection for Activated Sludge Systems Selected Alternative Feed BOD/P ratio Oxidation Ditch Wadena Whitewater River Chemical EBPR + Chemical 22 46 High Purity Oxygen Moorhead Rochester** EBPR 32 30 ** - demonstrated and proven for P < 1.0 mg/L

Major Factors Effecting EBPR Selection EBPR higher capital – lower operating costs Influent wastewater Characteristics BOD/P ratio and soluble BOD fraction (rbCOD) Aeration tank configuration that is easily retrofitted for anaerobic tank addition and anoxic tanks by baffles Less sludge production with EBPR Recycle flows from digesters or anaerobic unit processes less favorable for EBPR Sludge processing and disposal methods Sludge holding and land application with minimal recycle good for EBPR Aerobic thickening processes Chemical treatment easier to implement and quicker Most EBPR applications also require chemical equipment and addition