OBG PRESENTS: At the Mercy of the Process
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OBG PRESENTS: At the Mercy of the Process Impacts of Nitrogen Removal Performance on WWTP Disinfection Ned Talbot, PE | Tri-Association Conference 2018 | 8/30/18 | 9:00-9:30AM
Transcript of OBG PRESENTS: At the Mercy of the Process
OBG PRESENTS:
At the Mercy of the ProcessImpacts of Nitrogen Removal Performance on WWTP DisinfectionNed Talbot, PE | Tri-Association Conference 2018 | 8/30/18 | 9:00-9:30AM
AGENDAOverview of Plant Processes
Martinsburg WWTP
Biological Performance and Stresses
Impacts on Disinfection Performance
Operator Techniques for Disinfection Reliability
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OVERVIEW OF PLANT PROCESSESMartinsburg WWTP
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WRRF Project: Martinsburg, WV
3 MGD (12 MGD influent, 10 MGD peak to bio)
5 mg/L TN effluent limit
0.5 mg/L TP effluent limit
Project Summary
Demo:
Trickling filters
Nitrification tower
Modifications:
New headworks, biological, ballasted polishing
Improve clarification, disinfection, anaerobic digestion
New solids handling, operations building 4
Martinsburg WWTP Upgrade
Simplified Process Flow Diagram
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1. Headworks 2. MBBR 4. Co-Mag 3. Disinfection
5. Solids Handling
MBBR Overview
MMoving
BBed
BBiofilm
RReactor
Fixed Biofilm / IR / TN (and some TP) Removal Fixed Biofilm + SS / IR + RAS / TN + EBio-P
IIntegrated
FFixed-Film
AActivated
SSludge
Protected biomass on plastic media expands capacity and effluent quality
Nutrients, peak flows, cold temps
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MBBR for Martinsburg
Benefits Obstacles
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HeadworksNew Plant:MBBR ANR Process
Expanded and Optimized Chlorination
Status: Constructed, Optimized
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DISINFECTION PROCESSES
Chlorination
Sodium Hypochlorite (12.5% solution)
2NaOCl+2H2O →2NaOH+HOCl+OCl-+H
Goal: free chlorine residual
Optimization
CFD model of flow / mixing process upgrades
Online chlorine residual and ORP control
Dechlorination
Sodium bisulfite dosing at effluent v-notch
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Optimization Goals
CFD modeling
Eliminate low flow spots or short-circuiting
Create highly turbulent injection points
30 min plug flow contact time at peak flow
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Velocity Contour at El 380.75 (1.75’ above floor)
BIOLOGICAL PERFORMANCE AND STRESSESImpacts on Disinfection Performance
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Biological Treatment Stresses on Disinfection
Before Upgrade: Trickling Filter Plant with
Nitrification Tower
• Incomplete nitrification / residual ammonia swings
• Chlorine demand linked to tower performance
• Incomplete hypochlorite mixing / short circuiting
After Upgrade:Advanced Nutrient
Removal
• Ammonia fully converted; CCT optimized
• MBBR sometimes stressed
• Occasional effluent NH3 / NO2 NO3
• Swings in chlorine demand
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MBBR Stresses
Variable MBBR Influent Flow and Loading
Unexplained Influent BOD Loading Variability
Diurnal variations – high ammonia in dewatering filtrate – press 6am to 4pm, 5 days / week
Wet weather – 2018
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Influent BOD: NH3-N
Higher than Design
Interim Wastewater Treatment
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Design: 12:1 | Startup: 20:1
Parameter UnitDesign Mass
Balance
Startup Data
Apr 2014-Apr 2015
% of Design Load
July 2016-Oct 2016
% of Design Load
Flow MGD 3.0 2.46 82% 1.8 60%
Inf BOD5lb/d 5,655 2,724 48% 3,652 65%mg/L 226 132 246
Eff BOD5 mg/L 17 15.0 5.20
Inf TSSlb/d 4,579 1,942 42% 4,604 101%mg/L 183 95 310
Eff TSS mg/L 20 12.9 5.7
Inf NH3-Nlb/d 463 257 56% 180 39%mg/L 18.5 12.5 12.1
Eff NH3-N mg/L 0.6 1.3 0.23
MBBR Stresses
Variable Influent BOD: Ammonia Loading
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Change in Average BOD:NH3 ratio
MBBR Stresses
Wet Weather Flow Spikes
Stress on MBBR Challenges Disinfection
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MBBR Stresses
Variable MBBR Influent Flow and Loading
Unexplained Influent BOD Loading Variability
Diurnal variations – high ammonia in dewatering filtrate – press 6am to 4pm, 5 days / week
Wet weather – 2018
Winter Versus Summer
Temperature change
Zone loading – swing zone operation changes
Aeration optimization
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Stress on MBBR reactors led to nitrite spikes
MBBR Biofilm by Zone
Diurnal VariationSolids Treatment Recycles
Seasonal FlexibilitySummer, Winter
BOD - Heterotrophs (H)Denitrifiers – Heterotrophs-DNNitrifiers – Autotrophs (A)
IR ControlMatch NO3 to BOD5
Process Performance by Design
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Mix,BOD5, NO3-> Anoxic
BOD5?, NH3
Air,BOD5?, NH3-> Oxic
NH3
AirNH3, NO3Oxic
NH3?, NO3
AirNH3?, NO3Oxic
NO3
IR
Mix,NO3, Carbon-> Anoxic
TN < 5
Air
Mix,BOD5, NO3-> Anoxic
BOD5,NO3 NH3
Mix,BOD5, NO3-> Anoxic
NH3
AirNH3, NO3Oxic
NH3, NO3
AirNH3, NO3Oxic
NO3
IR
Mix,NO3, Carbon-> Anoxic
TN < 5
Air
“Summer” (more denitrification capacity)
“Winter” (more nitrification capacity)
MBBR Stresses
Operational and Temperature Changes
Increased Ammonia in Effluent
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Stress on MBBR reactors challenges disinfection
Wastewater Constituent Impacts on Chlorine Dose and Residual
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Wastewater Characteristics Effects on Chlorine
Ammonia Forms chloramines when combined with chlorine; 10:1 free chlorine demand factor
Biochemical Oxygen Demand
Chlorine oxidizes BOD and becomes ineffective; 0.1:1 free chlorine demand factor TOC to chlorine
Hardness, Iron, Nitrate Minor effect
Nitrite Nitrite oxidized to nitrate; 5:1 free chlorine demand factor
PhAffects distribution between hypochlorous acid and hypochlorite ions and among the various chloramine species; Lower pH forms more hypochlorous acid than hypochlorite ion
Total Suspended Solids Shielding of embedded bacteria and chlorine demand
Temperature Disinfection rate increases with higher temperature
Organic and Inorganic Materials
Chlorine oxidizes the organic and inorganic materials and becomes ineffective; 1.0:1 ratio of organic-N to chlorine
Operator Techniques for Disinfection Reliability
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Operator Concerns with Chlorine Disinfection
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Chlorine Residual Sampling (Hach CL17)
Not real time – one sample every 3 minutes
Slow response to changes in demand
Difficult to adjust sample points to optimize system
Sample pumps require accessible location
Only 5 fecal excursions allowed in current permit
Operational Adjustments to Improve Disinfection Reliability
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Addition of ORP probes
Fine tuning ORP and chlorine residual chemical feed control
(hypochlorite and bisulfite)
Grab samples of nitrite in MBBR effluent –
confirm stress
Adjustment of dewatering
timing
DISINFECTION RELIABILITY
ORP Probes
Initial startup
• Measure potential of solution to oxidize or reduce• Oxidative = positive mV (Cl2); • Reductive = negative mV
• Initial ORP Targets: 380mV hypo; 180mV bisulfite
After 6 months of operation
• Moved probe closer to feed pt• Adjusted hypo probe ORP target:
500mV, then 400mV• Alarms: Low @ hypo @300
High @ bisulfite @270
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High ORP @ hypo feed indicates high oxidation potential, chlorine residual
Low ORP @ hypo feed indicates low chlorine residual, warning of MBBR stress
Frequent replacement of salt bridge; recalibration
Solids Handling Batch Cycles
Draw / Fill SST’sfor Thickening and Dewatering
6AM to 4PM
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Diurnal Influent Loading + Volute Dewatering Press Recycle
Ammonia rise during dewatering
Impact of Dewatering Timing
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Summary
Many Causes of Biological Stress
Variable influent loading
Intermittent plant recycles
Wet weather peak flows / I/I
Change of seasons / process temperature
Operational Strategies for Reliable Disinfection
ORP probes in combination with chlorine residual analyzers
Optimized chemical feed control strategies
Secondary effluent nutrient sampling program
Experience with the process is a clear path to reliability
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Lessons Learned: MBBR Internal Recycle Slipstream
Primary effluent MBBR bypass
Accidental during loss of power
No check valves – limited space
Added check on FM
Interlocked pumps on fail
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Disinfection Optimization Goals
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Evaluate existing system and identify limitations
Eliminate low flow spots or short-circuiting
Create highly turbulent injection point for 1 second complete mix
30 min plug flow contact time at peak flow
Dosage high enough to overcome chlorine demand. Dosage is typically 5 to 20 mg/L.
Breakpoint Chlorination
Medora Corporation Informational Bulletin http://www.medoraco.com
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MBBR Biofilm by Zone
AirMixBOD - Heterotrophs (H)Denitrifiers – Heterotrophs-DNNitrifiers – Autotrophs (A)
The Wrong Mix or Type of Biofilm by Zone?
Process Performance by Stage
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“Planned”
H A A
H? H A H, A?
High Load, or Low Air
H? H? A?H A, DN?
Intermittent / Insufficient Air
Phase 1
“Planned”
Sw A A H-DN
H? H, A A H,
A?
High Load, Low Air, or no Auto
H? H? A?H A, DN?
Intermittent / Insufficient Air
Phase 2
H-DN
H-DN
H-DN?
