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

THANK YOU

Eric Hartman, Wastewater Operator

8360 Huron Street

Dexter, MI 48130