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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 790. Sewage Collection and Treatment Regulations
11/21/2024

9VAC25-790-560. Aerobic sludge digestion.

A. Aerobic sludge digestion reactors shall be designed for effective mixing and aeration. If diffusers are used, they shall be of the type that minimizes clogging and they should be designed to permit removal for inspection, maintenance and replacement without dewatering the tanks.

B. Design. Multiple aerobic digesters are required at all treatment works having a design flow capacity of more than 0.5 mgd. The size and number of aerobic sludge digesters shall be determined by rational calculations based upon such factors as (i) volume of sludge added; (ii) type and percent solids; (iii) the required volatile solids reduction for stabilization; (iv) allowance for sludge and supernatant storage; and (v) the minimum design temperature of the digester contents.

1. Calculations shall be submitted to justify the design and shall include design digester temperature based on the type of mixing equipment and other factors.

2. Digester volume shall be a minimum of 20% of the average design flow of the treatment works. The design digester volume should be increased up to 25% of the average design flow if the wastewater temperature will remain below 10°C (50°F) for an extensive period of time (60 days/year).

3. A reduction in requirements for hydraulic detention time may be given for treatment works designed to be operated in the extended aeration mode, or coupled with additional stabilization processes, or operated at elevated temperatures.

4. Facilities shall be provided for effective separation and withdrawal, or decanting of supernatant.

C. Loadings. The volatile solids loading should be in the range of one- to two-tenths (0.1 to 0.2) pounds of volatile solids per cubic foot per day.

1. Dissolved oxygen concentration maintained in the liquid shall be in the range of one to two milligrams per liter.

2. Energy input requirements for mixing should be in the range of 0.5 to 1.5 horsepower per 1,000 cubic feet, where mechanical aerators are utilized, and 20 to 30 standard cubic feet per minute per 1,000 cubic feet of aeration tank, where air mixing is utilized.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-620 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-560, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

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