Recommended Design Criteria for Flow Equalization...
Introduction...
1. Objectives :

a. The primary objective of flow equalization basins is to dampen the diurnal flow variation, as well as variations caused by inflow/infiltration, and thus achieve a nearly constant flow rate through the downstream treatment processes.
b. The secondary objective is to dampen the strength of wastewater constituents by blending the wastewater in the equalization basin to maintain a degree of reliability and operational control.

2. General :

Flow equalization should be provided in mechanical plants where large diurnal variations are expected to cause mechanical, hydraulic, or biological process upsets. For treatment plants with capacities of less than 0.5 mgd, employment of this process is recommended.

3. Location :

Equalization basins should be located downstream of pretreatment facilities such as bar screens, comminutors, grit chambers, and where possible, primary clarifiers.

4. Type :

Equalization basins may be designed as either in- line or side-line units. Separate basins or unused on-line treatment units, such as primary clarifiers or aeration tanks, may be utilized for flow equalization during the early period of the plants design life. If no other unit is available and it is a single basin, then a by-pass pipe shall be provided around the basin to the downstream portion of the treatment facility. The by-pass pipe shall be valved.

5. Size of Basins :

With a diurnal flow pattern, the volume required to achieve the desired degree of equalization can be determined from a cumulative flow volume hydrograph over a representative 24-hour period. The volume required for equalization of flows will generally vary from approximately 20 to 40 percent of the 24-hour flow for smaller plants and from approximately 10 to 20 percent of the average daily dry weather flow for larger plants. It is recommended that the equalization basin be divided into two or more separate cells.

6. Basin Construction :

Equalization basins may be constructed of earth, concrete, or steel. Earthen basins should be constructed according to aerated pond criteria. At least 1 foot of freeboard shall be provided above the highest possible water level in the basin. Corner fillets and hopper bottoms with draw-offs shall be provided to prevent accumulation of sludge and grit. The basins shall be capable of being isolated and drained. Basins shall have provisions for cleaning of basin walls and for scum control with suitable access provided to facilitate the cleaning and the maintenance of equipment.

7. Air and Mixing Requirements :

Mixing requirements for normal domestic wastewater shall range from 0.02 to 0.04 hp/1,000 gallons of storage. Multiple aeration and/or mixing units shall be provided to maintain adequate mixing and for continuous operation. The air should be supplied at a rate of 1.25 to 2.0 cfm/1,000 gallons of storage. Aeration shall be sufficient to maintain a minimum of 1.0 mg/l of dissolved oxygen in the basin at all times.

8. Controls :

All inlets and outlets of each basin compartment shall be provided with accessible flow control devices. Flow-measuring devices or facilities shall be provided downstream of the basins to monitor liquid levels and equalized flow rates. Suitable protection from the weather and elements shall be provided for all electrical works.
Deviations From Design Criteria...
The Department may consider and allow deviations where adequate documentation is provided to prove the need for such deviation.