Presentation Outline Nitrification/denitrification refresher Treatment technologies available for nitrification and BNR/ENR What is the problem? BNR/ENR VPDES permitting Causes of reduced BNR performance during winter Cold wet weather strategies Case Studies Kearney, NE; Christiansburg, Abingdon Advances in process control
Nutrient Removal - Nitrification Nitrifying bacteria (AOB and NOB) use alkalinity for cell growth (autotrophic); do not remove BOD; and utilize CO 2 from air Temperature sensitive Require excess DO Consumes 7.1 mg/l of Alkalinity per mg/l of NH 3 -N
Nutrient Removal - Denitrification Denitrifying bacteria remove BOD but use NO 3 instead of O 2 Denitrification requires anoxic conditions.. NO 3 but no O 2 Recover 3.6 mg/l of Alkalinity per mg/l of NO 3 -N
Nitrification Factors - Mean Cell Residence Time (MCRT) MCRT actual = MLVSS Volume of Aeration Tank Q EFF X EFF +Q WAS X WAS Need to maintain minimum MCRT for healthy nitrifying population MCRT min = 1 µ maxn bn If actual MCRT > minimum MCRT then complete nitrification can occur MCRT is temperature dependent: 10-14 C MCRT min of 12-16 days 16-20 C MCRT min of 6-10 days Reference: Metcalf and Eddy s Wastewater Engineering: Treatment, Disposal, and Reuse 5 th edition, 2014.
Nitrification Factors - Temperature Optimum temperature for nitrification is 77 to 95 o F Growth rate decreases by 50% at 64 o F Growth rate decreases by 75% at 46 to 50 o F No activity at 39 o F Ammonia (mg/l) Minimum MCRT Design MCRT MCRT (days)
Treatment Technologies Trickling Filters and RBCs (BOD and NH 3 ) Activated Sludge (BOD, NH 3, and BNR) MLE e-mle Bardenpho Vendor Activated Sludge (BOD, NH 3, and BNR) No Primaries Sequencing Batch Reactor Oxidation Ditch Aeromod Sequox D-ditch Counter Current Aeration (Schreiber) Modified Activated Sludge (BOD, NH 3, and BNR) Moving Bed Biofilm Reactor Integrated Fixed Film Membrane Bioreactor Hybrids (BOD, NH 3, and BNR) Biological Aerated Filters Filters (nitrifying and denitrifying)
What is the Problem? Nitrification and BNR are significantly impacted by cold wastewater temperatures Cold Wet Weather Events (CWWE) The problem is compounded in the northeast and mid-atlantic during winter months because 40+ inches of annual precipitation Wet weather events produce peak day flows that are 5-10 Q These flows are likely due to inflow of snow melt, cold rain inflow, and/or river inflow In Virginia, we have seen influent WW temperatures as low as 39 o F
VPDES Permitting Nutrient limits (TN & TP) in Virginia are based upon annual concentration and loading (saving grace) NH3 or TKN limits are weekly maximum and monthly average When in doubt: maintain aeration and maintain solids inventory (avoid washout) After the CWWE, return basins to established SOP and BNR will quickly return
VPDES Permitting
Primary Causes of Reduced BNR Performance During CWWE Solids inventory management Reduced influent BOD Reduced hydraulic retention time Elevated influent dissolved oxygen Lack of side stream nutrient management TN and TP effluent concentrations are low due to dilution
Cold Wet Weather Strategies Focus collection system on identification and correction of inflow sources Make sure that all tanks, clarifiers, and filters are in service If you have equalization, plan for it to be empty in advance of CWWE Equalize nutrient-laden sidestreams Provide more biomass in winter; minimize wasting if you can Minimize sludge blankets in clarifier Increase aerobic HRT anoxic/aerobic swing zones go aerobic Be prepared to add metal salt for P removal Enhanced process control - consider installing ion specific electrode to monitor nitrogen In Virginia, BNR during CWWE is not as important; maintain biomass and nitrification - TN removal will recover quickly after CWWE
City of Kearney TF/SC WWTP Headworks Primary Clarifiers Dewatering Activated Sludge Trickling Filters Secondary Clarifiers Disinfection
City of Kearney WWTP
City of Kearney BioWin Data Influent: Average Daily Flow 3.5 MGD Current Design Flow = 4.8 MGD Future Design Flow = 5.8 MGD Total cbod = 220 mg/l TSS = 237 mg/l Ammonia = 24.7 mg/l Nebraska Winter: Routine air temperatures less than 0 o F Trickling Filter WW temperature greater than 11 o F
City of Kearney BioWin Data Current Performance: Average Daily Flow 3.5 MGD Winter NH 3 Performance less than 1 mg/l-n Future NH 3 Limit 1.2 mg/l at 5.9 MGD Steady State BioWin Model Results Season Summer Winter Summer Winter Flow (mgd) 4.8 4.8 5.9 5.9 Total cbod (mg/l) 2.32 2.89 2.90 4.74 TSS (mg/l) 5.70 6.31 6.27 7.58 Ammonia (mg/l) 0.19 1.10 0.27 4.21 Total Nitrogen (mg/l) 13.93 14.83 12.05 16.73
Town of Christiansburg WWTP Effluent PS Disinfection Dewatering Secondary Clarifiers Anaerobic Digestion Primary Clarifiers Headworks Influent EQ Activated Sludge
History and Design Expansion to 4 MGD completed in 2000 For future growth Due to CWWE NH 3 Violations Rerated WWTP to 6 MGD in 2008 ADF 2.5 MGD; PDF 10+ MGD Plug Flow Activated Sludge configured for 5-stage process in future High influent Nitrogen: TKN of 40 mg/l NH 3 of 30 mg/l Converted to MLE over 2014 and 2015 Primarily to recover alkalinity and avoid the use of lime Anoxic selector to enhance floc-forming bacteria and improved settling TN has averaged: 10 mg/l in summer 11 mg/l in winter
Town of Christiansburg WWTP
Strategies at Christiansburg Make sure that all tanks, clarifiers, and filters are in service Equalization (2.6 MG) Equalize nutrient-laden sidestreams (not planned) Minimize sludge blankets in clarifier Maintain aeration in aerobic zones
Influent: Town of Christiansburg BioWin Data Flow = 2.5 MGD Total cbod = 204 mg/l TSS = 255 mg/l Ammonia = 30 mg/l-n BioWin Simulation Example of GBT slug loading Anaerobically digested sludge campaign thickened 1 to 2 times per week. Can double the NH 3 in one day
Town of Christiansburg BioWin Data Steady State BioWin Model Results Season Summer Winter Winter Winter Temperature (⁰C) 20.0 10.0 10.0 10.0 Filtrate Storage No No* Yes* Yes MLVSS (mg/l) 1,600 1,600 1,600 2,400 Recycle Rate (%) 400% 400% 400% 400% Flow (mgd) 2.5 2.5 2.5 2.5 Total cbod (mg/l) 1.75 2.45 2.45 1.73 TSS (mg/l) 7.52 7.17 7.17 7.17 Ammonia (mg/l) 0.34 4.97* 4.97* 2.30 Total Nitrogen (mg/l) 11.2 13.2 13.2 12.2 * Steady State BioWin Modeling Based Average Conditions. See Time Series Charts For Plant Reaction To Campaign Thickening.
Town of Christiansburg BioWin PFD Screened Influent Recycle Pump 1-Anoxic 2-Aerobic 3A-Aerobic 3B-Aerobic 4-Aerobic 5-Anoxic Primary Clarifier Secondary Clarifier Effluent RAS Pump PS Pump Sludge Gravity Belt Thickener Anaerobic Digester WAS Pump
Effluent Nitrogen Winter 10⁰C, 1600 MLVSS, No Filtrate Storage CONC (mgn/l) 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 Effluent Total N Effluent Ammonia N Effluent Nitrite + Nitrate
Christiansburg BioWin PFD With Filtrate Storage Screened Influent Recycle Pump 1-Anoxic 2-Aerobic 3A-Aerobic 3B-Aerobic 4-Aerobic 5-Anoxic Primary Clarifier Secondary Clarifier Effluent RAS Pump PS Pump Sludge Filtrate Storage Gravity Belt Thickener Anaerobic Digester WAS Pump
Effluent Nitrogen Winter 10⁰C, 1600 MLVSS, With Filtrate Storage CONC (mgn/l) 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 Effluent Total N Effluent Ammonia N Effluent Nitrite + Nitrate
Effluent Nitrogen Winter 10⁰C, 2400 MLVSS, With Filtrate Storage CONC (mgn/l) 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 Effluent Total N Effluent Ammonia N Effluent Nitrite + Nitrate
Town of Abingdon WCWRF Headworks Dewatering Activated Sludge Influent EQ Primary Clarifiers Disinfection Secondary Clarifiers Anaerobic Digestion
Town of Abingdon WCWRF History and Design Expansion from 2.75 MGD to 4.95 MGD in 2007 For future growth Wet weather treatment Ammonia limit due to discharge to Wolf Creek Provide treatment above and beyond VPDES requirements Existing CMAS converted to MLE Anoxic selector to enhance settling TN removal without expanding tank volume Cloth media filtration Converted secondary clarifier to sidestream storage
Town of Abingdon WWTP
Town of Abingdon BioWin PFD Screened Influent Recycle Pump Primary Clarifier 1-Anoxic 2-Aerobic 3-Aerobic 4-Aerobic Secondary Clarifier Effluent RAS Pump PS Pump Sludge Thickener WAS Pump Centrate Storage Centrifuge Anaerobic Digester NH4 Correction
Strategies at Town of Abingdon s WCWRF Equalization (1.6 MG) Equalize nutrient-laden sidestreams (conversion of abandon secondary clarifier) Minimize sludge blankets in clarifier Maintain aeration in aerobic zones Cloth media filter provides limited protection from washout
Case Study Improved Process and Control CHA SBR Pilot Study for high TKN waste stream AERATE and MIX controlled by ion selective sensor NH 4 +/NO 3 - Continuous DO and ORP monitoring
Case Study Improved Process and Control AERATE ANOXIC MIX
Aeration Basin Control Ammonia Based Aeration Control (ABAC) 1. Ammonia feedback control 2. Cascade DO Control NH4 1.5 mg/l then DO = 0.5 mg/l NH4 1.5 mg/l then DO = 2.0 mg/l VFD DO PLC NH4 Nitrate Nitrogen NH 4 Nitrite Nitrate Anoxic Oxic
In Conclusion Focus on maintaining nitrification Focus on maintaining solids inventory Prepare for the next CWWE by: Utilizing all tanks Managing high nutrient return sidestreams Empty equalization storage After the CWWE: Provide environmental conditions for BNR, and it will recover quickly
Acknowledgements Ryan Hendrix Town of Christiansburg Sarita Moore Town of Abingdon Charles Bott, PhD, PE HRSD Stephanie Klaus, PE - HRSD
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