mle conversion process
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MLE Conversion Process
July 14, 2016
Ryan Hendrix
Wastewater Operations Superintendent
Town of Christiansburg
Doug Urquhart P.E.
Project Engineer
CHA Consulting
Overview
Background
Facility Description
MLE Design
Operations
Lessons Learned
Conclusions
Facility Background
Current WWTF
• 6 MGD Design Capacity
• 2.5 MGD Average Flow
Wastewater Sources
• 9,600 Connections
• 21,500 Population
Facility Background
1980• 2 MGD WWTP; 1 MGD Average Flow
• Discharged to Crab Creek
• Aerobic Sludge Digestion
1989 Upgrade to 3 MGD• Added Additional Primary Clarifier
• Added Secondary Clarifier
• Changed to Anaerobic Digestion
1999 Upgrade to 4 MGD• Upgraded Headworks
• Aeration Basins Constructed
• Equalization Basin Constructed
• Discharge Relocated to New River
Description
Liquid Stream Treatment• Headworks/Grit Collection• Primary Clarifiers• Activated Sludge (MLR)• Secondary Clarifiers• UV Disinfection• Effluent Pump Station
Solids Treatment• Anaerobic Digestion• Combined Heat Power System • On-Site Storage• Land Application of Liquid Class B Biosolids
Aeration Basins
1.4 million gallons of aeration volume
Two 80-foot secondary clarifiers
Set up for tapered aeration
More diffusers in first cell (now anoxic)
Design per SCAT Regulations resulted in oversizing blowers (now use BioWin)
RAS influent flow-paced
Designed for future conversion to 5-Stage Bardenpho or MLE with common partition walls
Aeration Basin Design
ZONE 5 ZONE 4
ZONE 2ZONE 1
BASIN 2
ZONE 1
ZONE 4ZONE 5
INFLUENT CHANNEL
ZONE 2
ZON
E 3ZO
NE 3
BASIN 1
Primary Effluent
Q
RASQ
RASQ
Lime Feed
- Installed in 1996
- Used equipment
- Gravity feed
Design Goals
Influent Ammonia (NH3-N)
• 27 mg/L (average)
• 49 mg/L (maximum)
Effluent Toxicity
Reduce Operating Costs
• Alkalinity requirements 500 lb per day
Pending Freshwater Ammonia Criteria
Public Health Concerns
Conventional Biological Nitrification
Nitrification - Two step oxidation of ammonia (autotrophic bacteria)
Step 1: Oxidation of ammonia to nitrite ammonia oxidizing bacteria (AOB)
NH4+ + 1.5O2 NO2
- + 2H+ + H2O
Step 2: Oxidation of nitrite to nitrate by nitrite oxidizing bacteria (NOB)
NO2- + 0.5O2 NO3
- + H2O
Nitrite intermediate is short term in most wastewater, NOB growth faster than AOB
Autotrophic bacteria, Nitrosomonas (AOBs) and Nitrobacter (NOBs), are ubiquitous to wastewater (i.e. do not need to buy or dose bacteria)
Use HCO3- as carbon source for growth (alkalinity)
Per 1 lb of NH4-N oxidized need 4.57 lb oxygen and 7.14 lb of alkalinity as CaCO3
Heterotrophic Denitrification - Reduction of nitrate to nitrogen
NO3- + electron donor (COD) 0.5 N2 + OH-
Per 1 lb NO3-N reduced produce 3.57 lb of alkalinity as CaCO3
Nitrification Factors - Solids Residence Time (SRT)
SRTactual = MLVSS × Volume of Aeration Tank
QEFF× 𝑋EFF+QWAS× 𝑋WAS
Need to maintain minimum SRT for healthy nitrifying population
SRTmin =1
µ𝑚𝑎𝑥𝑁 −𝑏𝑁
If actual SRT > minimum SRT then complete nitrification can occur
SRT is temperature dependent:
10-16˚C SRTmin of 12-20 days
16-20˚C SRTmin of 8-10 days
At Christiansburg WWTF, started 15-20 days, now 8-12 days
Nitrification Factors - Inhibition
Nitrification by AOBs is limiting factor in ammonia removal
NH4+ + 1.5O2 NO2
- + 2H+ + H2O
Primary requirements to maintain healthy nitrification process
• Oxygen > 2 mg/L
• pH pH = 6.8 to 8.2
Other Factors• Ammonia < 100 mg/L (pH dependent)
• Sulfate < 500 mg/L
• Chromium (VI) < 0.25 mg/L
• Nickel < 0.25 mg/L
• Zinc < 0.50 mg/L
• Copper < 0.10 mg/L
Nitrifying bacteria are very dependent on pH
Nitrification reduces alkalinity by 7.14 lb per pound of NH4+ oxidized
Phosphorus removal chemicals also reduce alkalinity
• 5.3 - 13.5 lbs Alkalinity as CaCO3 per lb Fe added
• 6.0 - 9.0 lbs Alkalinity as CaCO3 per lb Al added
Nitrification Factors – pH and Alkalinity
Reference: Gradyand Lim, 1980
Modified Ludzack-Ettinger (MLE)
Effective BNR process, pre-anoxic and adaptable for existing activated sludge facilities
Internal recycle (2-4 times influent flow) provides BOD for denitrification in pre-anoxic reactor
Anoxic AerobicInfluent Effluent
SecondaryClarifier
Sludge
Return Activated Sludge
Anoxic AerobicInfluent Effluent
SecondaryClarifier
Sludge
Return Activated Sludge
Internal Recycle
WWTP Simulation(Existing)
Aeration 1.1
Aeration 2.1
Secondary Clarifier Effleunt
Aeration 1.2 Aeration 1.3 Aeration 1.4 Aeration 1.5
Aeration 2.2 Aeration 2.3 Aeration 2.4 Aeration 2.5
Primary Clarifier Effluent
WAS
WWTP Simulation(Proposed MLE)
Aeration 1.1
Aeration 2.1
Secondary Clarifier Effleunt
Aeration 1.2 Aeration 1.3 Aeration 1.4 Aeration 1.5
Aeration 2.2 Aeration 2.3 Aeration 2.4 Aeration 2.5
Primary Clarifier Effluent
WAS
WWTP Simulation
Mixed Liquor Return (MLR) from Zone 5 to Zone 1
Simulated varying recycle rates
• 100% Recycle Rate:
TN: 21.6 mg/L
NH3-N: 1.1 mg/L
• 200% Recycle Rate:
TN: 22.2 mg/L
NH3-N: 1.4 mg/L
• 300% Recycle Rate:
TN: 22.6 mg/L
NH3-N: 1.6 mg/L
Influent Alkalinity = 250 mg/L
Effluent Alkalinity = 100 mg/L
No lime addition required with simulation
MLE Conversion
ZONE 5 ZONE 4
ZONE 2ZONE 1 (pre-anoxic)
BASIN 2
ZONE 1 (pre-anoxic)
ZONE 4ZONE 5
INFLUENT CHANNEL
ZONE 2
ZON
E 3ZO
NE 3
BASIN 1
WINDOWPUMP
WINDOWPUMP MIXER
MIXER
Primary Effluent
Q
RASQ
RASQ
MLE Conversion Design
MLE Conversion Design
MLE Conversion Installation
Anoxic Mixers (2011)
• Two 2.5 HP submersible mixers
• $28,000 (total installation cost)
MLE Conversion Installation
Window Pumps (2014)
• Two 2.5 HP Window pumps
• 3600 gpm at 1.1’ TDH
• Variable speed
• $43,000 (total project)
MLE Conversion Installation
MLE Conversion Installation
MLE Conversion Installation
MLE Conversion Installation
MLE Conversion Installation
MLE Conversion Installation
MLE Conversion Installation
MLE Conversion Installation
Results
Effluent Nitrate (as N)
• 12 mg/L (before MLE Conversion)
• 6 mg/L (after MLE Conversion)
Lime feed turned off July 2014
Annual savings of ~$24,000 (4.0 year ROI)
Results - Nitrate
0
2
4
6
8
10
12
14
16
3/31/2014 06/03/14 7/30/2014 10/29/2014 2/9/2015 6/18/2015 1/11/2016
Nit
rate
(as
N)
mg
/L
NRS OFF 7/17/16
WINDOW PUMPSINSTALLED (7/1/14)
LIME FEEDSHUT OFF (7/22/14)
Results - MLSS
0
50
100
150
200
250
300
350
400
450
500
0
500
1000
1500
2000
2500
3000
3500
4000
4500
10/18/2012 5/6/2013 11/22/2013 6/10/2014 12/27/2014 7/15/2015 1/31/2016 8/18/2016 3/6/2017
SRT
and
SV
I
MLS
S (m
g/L)
MLSS SVI SRT
Optimization
Pumps flow paced
• 4.4 to 5.0 MGD Recycle Flow
• 250% to 300% Recycle to Influent Ratio
• Changed from influent flow-paced to effluent flow-paced
Controlling Recycle Flows
Minimizing D.O. carryover in MLR
Increase use of equalization basins
Lessons Learned
Blowers are even more oversized now
Low cost lime system saved money but difficult to operate
SRT took some fine tuning
Accommodating Wet Weather
• Effluent flow pacing over influent flow pacing
Need for additional Process Control Testing
• ORP, Nitrate & Ammonia Analyzers, etc.
Future Projects
Turbo Blowers
DO and Ammonia Control
Cell 5 Mixers
• No aeration, use as “swing zone”
Improved Diurnal Flow Control
Questions and Comments
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