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CITY OF DEXTER WWTP
Eric Hartman | City of Dexter | November 10, 2016 | MWEA Process Seminar
AERATION SUCCESS STORY
The Problem
▪ 2 of 3 blowers are needed to meet DO demand
▪ 1 blower failed; no longer operational
▪ Diffusers experiencing high discharge pressure ▪ Potentially caused blower failure and could cause others to fail
▪ Blowers located in basement
– prone to flooding
▪ Microbrewery discharging higher
than anticipated loads
Overview of Dexter WWTP
Equalization Tanks
Primary Clarifiers
Biosolids Storage Digesters
Aeration Tanks & Secondary Clarifiers
Sand Filters & Admin Bldg
Key Issues
▪ Rapid response to address blower and diffusers
▪ Keep plant on-line & meeting permit during rehab
▪ Energy (cost) savings for new aeration system
▪ Allow for future automated DO control
▪ Consider future increased loadings
The Solution
▪ Design/Build project delivery for accelerated response
▪ Diffusers replaced within 2-4 weeks after contract signed ▪ Ceramic diffusers replaced in-kind
▪ After replacement of diffusers in three tanks, only one blower req’d for typical day
▪ Added one new, variable speed blower capable of handling entire aeration load
Diffuser Cleaning Tests
▪ Existing diffusers significantly fouled
▪ Pressures of >50 inches (DWP) measured at Aeration Tanks
▪ Higher than normal airflows were required to maintain DO
▪ 2 acid cleaning methods were tested
▪ “Spray & soak” restored 40-50% of capacity
▪ “Fumigation” restored 70% of capacity
Clean Dirty Spray &
Soak Fumigate Cleaning
2 scfm 7.3 20.1 15.2 11.5
3 scfm 8.0 26.7 19.4 14.7
Note reaction between acid and
mineral scale
Lab Test Results by Xylem
Diffuser Cleaning Tests - Continued
▪ Original diffusers also cleaned by City staff
▪ Cleaning involves scrubbing disc and soaking in muriatic acid for 3 days
▪ Cleaning cost is about $1.5 / disc
Inspection by Xylem Engineer “The clean diffusers have like new DWP values. If these were ever in operation, I would like to know the
cleaning method.”
New Blower Capabilities/Benefits
▪ Type: Rotary screw compressor (Aerzen Delta Hybrid)
▪ 32% more efficient than old blowers
▪ Variable frequency drive provides 560 to 1,200 scfm 40% turndown
▪ HMI/control capabilities
▪ Rebate of $20,529 from DTE
Reduction in Power Costs
▪ Reduction in power: 32% measured per blower
▪ Overall reduction (with new blower and diffusers) = 64%
0
20000
40000
60000
Aug 2015 Aug 2016
KW
H
Monthly Electricity Usage
23% KWH Reduction for Plant $500 per month
Process Control – Dissolved Oxygen
▪ Target DO at 4-5 mg/L
▪ In recent past, variable organic loadings due to microbrewery
▪ Ensure DO does not drop at bottom of clarifier and cause denitrification (rising sludge)
Process Control – Phosphorus
▪ Current effluent TP limit is 1.0 mg/L
▪ Ferric chloride added before sand filtration
▪ More stringent future limits
▪ Will evaluate biological P removal before new limits are required
Future Effluent TP Limits, mg/L
October – March 0.6
April 0.62
May, June & September 0.186
July & August 0.165