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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 790. Sewage Collection and Treatment Regulations
12/30/2024

9VAC25-790-710. Sequencing batch reactors (SBR).

A. Usage. In accordance with the requirements of this chapter and standards contained in this chapter, batch operation modifications of the activated sludge process will be considered as conventional secondary treatment processes. Adequate performance data and information from a full scale treatment works of similar design, including the levels of influent wastewater characteristics that produce hydraulic and organic loading rates within 25% of values used in the proposed design, shall be provided if an SBR design is to accomplish an effluent quality more advanced than a secondary level.

B. Design. The design shall meet the applicable loading requirements. The operating cycle normally consists of FILL, REACT, SETTLE, DRAW, and IDLE sequences with alternating sequences of mixing and aeration on and off.

1. A minimum of two basins shall be provided for design flows in excess of 0.1 mgd. The minimum total basin volume shall be equal to the design daily influent flow volume and either upstream in-line or off-line storage is necessary to minimize influent flow during settling and decanting. Effective scum collection and removal equipment shall be provided.

2. The design basis for meeting oxygen requirements shall consider the variation in liquid level depth and the aeration sequence time.

3. The basin depth shall be sufficient to provide optimum separation of the settled biomass and the point of effluent withdrawal.

4. Adequate mixing shall be provided to resuspend settled solids at the start of the FILL sequence and maintain solids in suspension over the design liquid volume range.

5. A high liquid level overflow shall be provided between basins. The overflow shall be located as far as possible from the outlet device and in no case be closer than 10 feet from the outlet device.

6. Inlets to each basin shall be located as far as possible from the outlet and in no case be closer than 10 feet from the outlet.

7. Scum baffles or other suitable arrangements shall be provided to prevent scum from being withdrawn with the effluent.

8. Outlet facilities shall be designed to prevent resuspension of the settled solids in the basins. An adjustable flow rate control device shall be provided on each basin outlet.

9. Waste sludge control facilities shall have a rate per day equal to 50% of the total basin volume.

10. The FILL and DRAW sequences for an individual basin shall not overlap.

C. Features. Automatic control valves and switches shall be provided for controlling the operating sequences of each basin. Automatic control valves shall be capable of manual operation. Control sequences shall be adjustable to allow flexibility in operating time periods for each sequence. The control system shall provide automatic operation of the inlet valve and outlet valve to each basin, the air supply valve to each basin and the blowers or mechanical aerators.

1. The control system shall provide for automatic operation of downstream units or equipment as necessary. The control system should also provide automatic control of the waste sludge removal system. A spare automatic control unit shall be provided.

2. A monitoring system shall be provided that will indicate control system status and actual valve position of each automatically operated valve. Also, the control system status and the actual operational status of both the air supply system (blowers or mechanical aerators) and sludge removal system shall be monitored, when equipped for automatic operation. The monitoring system shall include an alarm to indicate a malfunction of the control system.

3. Dual hydraulic pumps and air compressors shall be provided when such facilities are utilized in conjunction with valve operators. Electrically operated valves shall be designed so as to fail only in the open position.

4. A high liquid level alarm for each basin shall be provided to signal an alarm condition prior to reaching the overflow pipe or port.

5. Influent flow and effluent flow measurement for the treatment facility shall be provided. The extent of the flow measurement equipment shall be in accordance with this chapter.

6. Disinfection and other downstream treatment units shall be sized based on the maximum design DRAW sequence flow rate.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-770 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-710, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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