Flow Equalization Jae K. (Jim) Park Dept. of Civil and Environmental Engineering University of Wisconsin-Madison

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

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

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

Volume Requirement Flowrate Pattern A Flowrate Pattern B Inflow mass diagram Inflow mass diagram Average daily flowrate Average daily flowrate Cumulative inflow volume, ft3 Required equalization volume Required equalization volume M N M N M Time of day Design volume = 1.1~1.2  Theoretical volume

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

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

Snapfinger Plant (GA) Equalization Tank Capacity: 20 million gallons Cost: $6.9 million

Equalization Tank

Mixing

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

Deep Tunnel Milwaukee, WI