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 CO2 from air
Temperature sensitive
Require excess DO
Consumes 7.1 mg/L of Alkalinity per mg/L of NH3-N
Nutrient Removal - Denitrification
Denitrifying bacteria remove BOD but use NO3 instead of O2
Denitrification requires anoxic conditions….. NO3 but no O2
Recover 3.6 mg/L of Alkalinity per mg/L of NO3-N
Nitrification Factors - Mean Cell Residence Time (MCRT)
MCRTactual = MLVSS × Volume of Aeration Tank
QEFF× 𝑋EFF+QWAS× 𝑋WAS
Need to maintain minimum MCRT for healthy nitrifying population
MCRTmin =1
µ𝑚𝑎𝑥𝑁 −𝑏𝑁
If actual MCRT > minimum MCRT then complete nitrification can occur
MCRT is temperature dependent:
10-14˚C MCRTmin of 12-16 days
16-20˚C MCRTmin of 6-10 days
Reference: Metcalf and Eddy’s Wastewater Engineering: Treatment, Disposal, and Reuse 5th edition, 2014.
Nitrification Factors - Temperature
Optimum temperature for nitrification is 77 to 95oF
Growth rate decreases by 50% at 64oF
Growth rate decreases by 75% at 46 to 50oF
No activity at 39oF
MinimumMCRT
DesignMCRT
Ammonia(mg/L)
MCRT (days)
Treatment Technologies
Trickling Filters and RBCs (BOD and NH3) Activated Sludge (BOD, NH3, and BNR)• MLE• e-MLE• Bardenpho
Vendor Activated Sludge (BOD, NH3, and BNR) – No Primaries• Sequencing Batch Reactor• Oxidation Ditch• Aeromod Sequox™• D-ditch• Counter Current Aeration (Schreiber)
Modified Activated Sludge (BOD, NH3, and BNR)• Moving Bed Biofilm Reactor• Integrated Fixed Film• Membrane Bioreactor
Hybrids (BOD, NH3, 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 39oF
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 BNRPerformance 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
Activated Sludge
Headworks
Primary Clarifiers
Secondary ClarifiersDisinfection
Dewatering
Trickling Filters
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 0oF
• Trickling Filter WW temperature greater than 11oF
City of Kearney BioWin Data
Steady State BioWin Model ResultsSeason 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
Current Performance:
• Average Daily Flow – 3.5 MGD
• Winter NH3 Performance less than 1 mg/L-N
• Future NH3 Limit – 1.2 mg/L at 5.9 MGD
Town of Christiansburg WWTP
Activated SludgeInfluent EQ
Headworks
Primary Clarifiers
Secondary Clarifiers
Anaerobic Digestion
Disinfection
Effluent PS
Dewatering
History and Design
Expansion to 4 MGD completed in 2000• For future growth• Due to CWWE NH3 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 • NH3 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
Town of Christiansburg BioWin Data
Influent:
• 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 NH3 in one day
Town of Christiansburg BioWin Data
Steady State BioWin Model ResultsSeason 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
1-Anoxic
Effluent
Sludge
Recycle Pump
RAS Pump
WAS Pump
Primary Clarifier
2-Aerobic 3A-Aerobic 4-Aerobic3B-Aerobic 5-Anoxic
Secondary Clarifier
Anaerobic DigesterGravity Belt Thickener
PS Pump
Effluent NitrogenWinter 10⁰C, 1600 MLVSS, No Filtrate Storage
12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM
CO
NC
(m
gN
/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 PFDWith Filtrate Storage
Screened Influent
1-Anoxic
Effluent
Sludge
Recycle Pump
RAS Pump
WAS Pump
Primary Clarifier
2-Aerobic 3A-Aerobic 4-Aerobic3B-Aerobic 5-Anoxic
Secondary Clarifier
Anaerobic DigesterGravity Belt Thickener
PS Pump
Filtrate Storage
Effluent NitrogenWinter 10⁰C, 1600 MLVSS, With Filtrate Storage
12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM
CO
NC
(m
gN
/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 NitrogenWinter 10⁰C, 2400 MLVSS, With Filtrate Storage
12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM12 AM
CO
NC
(m
gN
/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
Activated Sludge Influent EQ
Headworks
Primary Clarifiers
Secondary Clarifiers
Anaerobic Digestion
Disinfection
Dewatering
Town of Abingdon WCWRFHistory 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
1-Anoxic Effluent
Sludge
Recycle Pump
RAS Pump
WAS Pump
Primary Clarifier2-Aerobic 3-Aerobic Secondary Clarifier
Anaerobic DigesterCentrifugeCentrate Storage
PS Pump
4-Aerobic
Thickener
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 NH4+/NO3-
• Continuous DO and ORPmonitoring
Case Study Improved Process and Control
ANOXIC MIX
AERATE
Aeration Basin Control Ammonia Based Aeration Control (ABAC)
DO
PLCVFD
NH4
1. Ammonia feedback control2. 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
Anoxic Oxic
Nitrate → Nitrogen NH4→ Nitrite → Nitrate
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
Questions and Comments