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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 790. Sewage Collection and Treatment Regulations

9VAC25-790-830. Flow equalization.

Article 8
Advanced Treatment

A. Flow equalization is a unit process whereby the variability of wastewater flows, in terms of volume and strength, is lessened. Where flow equalization is utilized within a sewerage system or treatment works to reduce the peak flow conveyed to, or processed by, the treatment works, the performance of the treatment process should be improved in relation to the estimated conventional effluent values. The ability of a treatment works that is provided with flow equalization to meet permit or certificate effluent limitations shall be evaluated on a case-by-case basis.

B. Usage. Flow equalization shall be provided in the flow scheme ahead of advanced chemical-physical processes, unless engineering analysis shows that absence of flow equalization is more cost effective while maintaining the same degree of reliability and operational control.

1. Flow equalization should be provided upstream of biological treatment works designed to process a mean daily flow of 0.1 mgd or less, and receiving hourly peak flows in excess of twice the design flow, if such peak flows will occur daily in excess of 50 times annually.

2. Flow equalization shall be provided upstream of biological treatment works designed to process a mean daily flow of 0.1 mgd or less that are permitted with effluent limitations less than 20 mg/l of BOD5 or TSS, or a TKN of less than 5 mg/l, or a total phosphorus of less than 2 mg/l, unless approved downstream unit operations are also provided.

C. Design. The design of an equalization basin shall incorporate the evaluation and selections of a number of features as follows:

a. On-line versus off-line basins.

b. Basin volume providing for a total storage detention of one-third or more of the daily design flow.

c. Degree of compartmentalization relative to dry weather and wet weather peak flows.

d. Type of construction: earthen, concrete or steel.

e. Aeration and mixing equipment.

f. Pumping and control in order to uniformly introduce flow into the treatment process at approximately the daily design flow rate during peak flow events.

g. Location in treatment system to provide uniform loadings on downstream unit operations.

The design decisions shall be based on the nature and extent of the treatment processes used, the benefits desired and local site conditions and constraints.

1. The minimum mixing requirements for equalization basins receiving raw or untreated domestic wastewaters or sewage containing an average suspended solids concentration exceeding 45 mg/l, shall equal or exceed 0.02 hp/1,000 gallons at a depth providing at least one-third of the maximum storage volume. Oxygen shall be supplied at a rate of 15 pounds per hour per gallon. Multiple mixing and aeration units shall be provided for continuous operability.

2. Flow equalization basins receiving treated wastewater or sewage with an average suspended solids concentration of 45 mg/l or less shall be provided with a means of sludge removal or mixing equipment that shall have a minimum power input of 0.01 hp/1,000 gallons of maximum storage volume. Aerobic conditions shall be maintained. Multiple mixing and aeration units shall be provided for continuous operability.

3. Sufficient storage shall be provided to allow subsequent downstream unit operations that follow equalization to operate at or less than their ted design capacity.

a. Storage capacity shall be determined from flow data when available. Basin volume for equalization shall at a minimum be determined from an inflow mass hydrograph of the hourly fluctuations for a typical daily wastewater flow, where typical daily wastewater flow is defined as the desired flow rate out of the equalization basin. Additional equalization basin volume shall be provided to accommodate:

(1) Continuous operation of aeration and mixing equipment.

(2) Anticipated concentrated treatment works recycle flows.

(3) Unforeseen changes in diurnal flow.

b. An evaluation of infiltration/inflow shall be conducted where influent flow data are not available. The minimum detention time shall be eight hours of the estimated daily maximum flow as determined by the study.

4. Flow equalization basins with a storage capacity exceeding 20,000 gallons should be constructed as compartmentalized or as multiple basins. Single basin installation with a bypass to downstream treatment units may be used for treatment works with capacities less than 200,000 gpd that are not located in critical water areas. The storage basins shall be provided with the means to be dewatered.

5. Basins designed for a combination of storage of wet weather flows and equalization shall be compartmentalized to allow for utilization of a portion of the basins for dry weather flow equalization. Floating surface aerators shall have provisions to protect the units from damage when the tank is dewatered.

6. Multiple pumping units shall be provided that are capable of delivering flow to an overflow device so that the desired flow rate can be maintained from the equalization basin with the largest pumping unit out of service, unless a suitable gravity flow system is provided. Gravity discharge from equalization shall be regulated by an automatically controlled flow-regulating device. If a flow-measuring device is provided downstream of the basin to monitor and control the equalization discharge, then a raw sewage influent flow meter will not be required in accordance with this chapter.

7. Equalization shall be preceded with screening and should be preceded by grit removal. Facilities shall be provided to flush solids and grease accumulations from the basin walls. A high-water-level takeoff shall be provided for withdrawing floating material and foam.

8. An overflow shall be provided for equalization basins so that such basins are not flooded, and these overflows are transmitted to downstream treatment units prior to the disinfection unit operation.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-890 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-830, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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