Flow Equalization 1

 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 2

l Enhanced biological treatment because of reduced shock loadings, dilution of inhibiting substances, and stable pH l Improved effluent quality and thickening performance of secondary sedimentation tanks due to constant solids loading l Reduced effluent-filtration surface-area requirements, improved filter performance, and uniform filter- backwash cycle l Improved chemical feed control and process reliability by damping mass loading l Attractive option for upgrading the performance of overloaded treatment plants 3

l Location of equalization facilities l Type of equalization flow sheet l Required basin volume 4 Equalization basin Primary treatment Secondary treatment Final effluent Controlled-flow pumping station Flow meter and control devices Grit removal Grit removal Equalization basin Primary treatment Secondary treatment Final effluent Controlled-flow pumping station Off-line equalization In-line equalization

Volume Requirement Cumulative inflow volume, ft 3 Average daily flowrate Inflow mass diagram Required equalization volume Flowrate Pattern A Required equalization volume Average daily flowrate Inflow mass diagram Time of day M M MN N Design volume = 1.1~1.2  Theoretical volume 5 Flowrate Pattern B

l Construction materials: earthen, concrete, or steel l Basin geometry: a complete-mix reactor l 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 6

 Mixing: aeration or mechanical equipment along with corner fillets and hopper bottoms  Aeration: DO - min. 1 mg/L; air supply rate - min cfm/1000 gal of storage capacity (0.16 L/m 3 ∙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 7

Capacity: 20 million gallons Cost: $6.9 million 8

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l To minimize mixing requirement, install grit chamber ahead of equalization basins where possible. l Mixing requirements for medium-strength municipal wastewater with SS of 220 mg/L: 0.02~0.04 hp/10 3 gal of storage (0.004~ kW/m 3 ) l Air supply: 1.25~2 ft 3 /10 3 gal·min (0.01~0.015 m 3 /m 3 ·min) l If equalization tanks are located after primary sedimentation tanks, aeration may not be required. l For mixing and aeration, use mechanical aerators. l Min. operating levels for floating aerators: > 5 ft. l Pumping facilities are required. l Provide a flow-measuring device on the outlet 11

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