Jae K. (Jim) ParkDept. of Civil and Environmental EngineeringUniversity of Wisconsin-Madison

Flow Equalization

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Flow Equalization

To overcome operational problems caused by flowrate variations

To improve the performance of the downstream processes

To reduce the size and cost of downstream treatment facilities

Principal Applications Dry-weather flows Wet-weather flows from separate sanitary sewers Combined stormwater and sanitary wastewater flows

http://10statesstandards.com/wastewaterstandards.html 2

Principal Benefits

Enhanced biological treatment because of reduced shock loadings, dilution of inhibiting substances, and stable pH

Improved effluent quality and thickening performance of secondary sedimentation tanks due to constant solids loading

Reduced effluent-filtration surface-area requirements, improved filter performance, and uniform filter-backwash cycle

Improved chemical feed control and process reliability by damping mass loading

Attractive option for upgrading the performance of overloaded treatment plants

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Theoretical Analysis Location of equalization facilities Type of equalization flow sheet Required basin volume

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Equalizationbasin

Primarytreatment

Secondarytreatment

Finaleffluent

Controlled-flowpumping station Flow meter and

control devices

Gritremoval

Gritremoval

Equalizationbasin

Primarytreatment

Secondarytreatment

Finaleffluent

Controlled-flowpumping station

Off-line equalization

In-line equalization

Volume Requirement

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infl

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Average daily flowrate

Inflow massdiagram

Required equalization

volume

Flowrate Pattern A

Required equalization

volume

Average daily flowrate

Inflow massdiagram

Time of dayM M MNN

Design volume = 1.1~1.2 Theoretical volume5

Flowrate Pattern B

Basin Construction Construction materials: earthen, concrete, or steel

Basin geometry: a complete-mix reactor Operational appurtenances: facilities for flushing any

solids and grease, emergency overflow in case of pump failure, a high water takeoff for the removal of floating materials and foam, and water sprayer

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Operation Mixing: aeration or mechanical equipment

along with corner fillets and hopper bottoms Aeration: DO - min. 1 mg/L; air supply rate -

min. 1.25 cfm/1000 gal of storage capacity (0.16 L/m3∙sec); air supply isolated from other treatment plant aeration requirements

Controls: Inlets and outlets suitably equipped with accessible external valves, stop plates, weirs, or other devices to permit flow control and the removal of an individual unit from service; devices for liquid level and flow rate measurements

Electrical: National Electrical Code for Class I, Division 1, Group D locations

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Snapfinger Plant (GA) Equalization Tank

Capacity: 20 million gallons

Cost: $6.9 million

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Equalization Tank

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Mixing

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Mixing and Air Requirement To minimize mixing requirement, install grit chamber

ahead of equalization basins where possible. Mixing requirements for medium-strength municipal

wastewater with SS of 220 mg/L: 0.02~0.04 hp/103 gal of storage (0.004~0.0088 kW/m3)

Air supply: 1.25~2 ft3/103 gal·min (0.01~0.015 m3/m3·min)

If equalization tanks are located after primary sedimentation tanks, aeration may not be required.

For mixing and aeration, use mechanical aerators. Min. operating levels for floating aerators: > 5 ft. Pumping facilities are required. Provide a flow-measuring device on the outlet

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Deep TunnelMilwaukee, WI

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