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

2. 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

3. 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

4. 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

5. 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

6. 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

7. Operation
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/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

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

9. Equalization Tank

10. Mixing

11. 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~ 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

12. Deep Tunnel Milwaukee, WI