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

9VAC25-790-550. Anaerobic digestion.

A. The design of anaerobic digesters should provide an optimum environment for effective microbial degradation of the organic matter in sewage sludge. The digester system design shall address separation and removal of liquid or supernatant. The production of methane gas (CH4) should be optimized. Digester gas should be utilized as a fuel whenever practical.

B. Design. A minimum of two anaerobic digesters, or enclosed reactors, shall be provided, so that each digester may be used as a first stage or primary reactor for treating primary and secondary sludge flows generated at a treatment works with a design flow exceeding 0.5 mgd. Additional digesters are provided to treat the total flow of primary and secondary sludge generated at treatment works with sewage design flows exceeding one MGD.

1. Where multiple digesters are not provided, a storage facility or adequate available sludge processing system shall be provided for emergency use so that the digester may be taken out of service without unduly interrupting treatment works operation.

2. Each digester should have the means for transferring a portion of its contents to other digesters. Multiple digester facilities should have means of returning supernatant from the settling digester unit to appropriate points for treatment.

3. Provisions for side-stream treatment of supernatant shall be included when the supernatant load is not included in the treatment works design.

4. Multiple sludge inlets and draw-offs and multiple recirculation section and discharge points (minimum of three) to facilitate flexible operation and effective mixing of the digester contents to optimize treatment for pathogen control and vector attraction reduction shall be provided. One inlet shall discharge above the liquid level and be located at approximately the center of the digester to assist in scum breakup. Raw sludge inlets should be so located as to minimize short circuiting between the inlets and either the supernatant draw-off, or sludge withdrawal points.

5. The proportion of depth to diameter should provide for a minimum of six feet storage depth for supernatant liquor, or the proportion of total volume allocated for supernatant should be 10% or more.

6. The digester bottom shall slope to drain toward the withdrawal pipe. At least one access manholes shall be provided in the top of the digester in addition to the gas dome. One opening shall be large enough to permit the use of mechanical equipment to remove grit and sand. A separate side wall manhole shall be provided at the basin floor level.

a. To facilitate emptying, cleaning, and maintenance, the digester design shall provide for access and safety features.

b. In accordance with VOSH requirements and these regulations, the operation and maintenance manual should specify: nonsparking tools, rubber soled shoes, safety harness, gas detectors for inflammable and toxic gases, and at least one self contained breathing apparatus.

C. Loadings. Where the composition of the sewage has been established, digester capacity shall be computed from the volume and character of sludge to be digested. The total digestion volume shall be determined by rational calculations based upon such factors as volume of sludge added, its percent solids and character, the temperature to be maintained in the digesters, the degree or extent of mixing to be obtained, expected formation of inactive deposits, and the size of the installation with appropriate allowance for sludge and supernatant storage. These detailed calculations shall be submitted to justify the basis of design.

1. The design average detention time for sludge undergoing digestion for stabilization shall be a minimum of 15 days within the primary digester, but longer periods may be required to achieve the levels of pathogen control and vector attraction reduction necessary for the method used for sludge management.

2. The digester shall be capable of maintaining a minimum average sludge digestion temperature of 35°C (95°F) with the capability of maintaining temperature control within a 4°(+/-)C range.

3. If unheated digesters are utilized, they shall have the capacity to provide a minimum detention time of 60 days within the digestion volume in which sludge is maintained at a temperature of at least 20°C (68°F).

4. For digestion systems where mixing is accomplished only by circulating sludge through an external heat exchanger, the system shall be loaded at less than 40 pounds of volatile solids per 1,000 cubic feet of volume per day or at a volumetric rate that provides not less than a 30 day detention time in the active digestion volume. The design volatile solids loading should be established in accordance with the degree of mixing provided.

5. Where mixing is accomplished by other methods, loading rates shall be determined on the basis of information furnished by the design consultant.

D. Completely mixed systems. For digesters providing for intimate and effective mixing of the digestion volume contents, the systems shall be designed for an average feed loading rate of less than 200 pounds of volatile solids per 1,000 cubic feet of volume per day or at a volumetric loading that provides 15 days or more detention time in the active digestion volume.

1. Confined mixing systems include (i) arrangements where gas or sludge flows are directed through vertical channels; and (ii) mechanical stirring, or pumping systems. Both confined mixing and unconfined continuously discharging gas mixing systems shall be designed to ensure complete turnover of digestion volume every 30 minutes. For tanks over 60 feet in diameter, multiple mixing devices shall be used.

2. Unconfined, sequentially discharging gas mixing systems shall be designed using the number of discharge points and gas flow rates shown for the various tank diameters as listed in this section, unless sufficient operating data is submitted and approved to verify the performance reliability of a alternative designs.

3. Gas discharge lines (lances) mounted on a floating cover or top designed to accumulate gas emissions shall extend to the base of the vertical side wall while the cover is resting on its landing brackets. For floor mounted diffuser boxes or lances mounted to a fixed cover, gas discharge shall extend to the base of the vertical side wall.


Maximum Diameter (Ft.)

Tank Diameter










(Minimum Number of Points)

Discharge Points










Minimum Gas Flow

Gas Flow (CFM)










4. The minimum gas flow supplied for complete mixing shall be 15 cubic feet/min./1000 cubic feet of digestion volume. Flow measuring devices and throttling valves shall be used to provide the minimum gas flow.

5. The design power supplied for mechanical stirring or pumping type complete mixing systems shall be capable of achieving a minimum of 0.5 horsepower per 1,000 cubic feet of digestion volume.

6. Where low speed mechanical mixing devices are specified, more than one device shall be provided unless other mixing devices are also provided.

E. Gas collection. All portions of the gas system, including the space above the liquid surface in the digester, storage facilities and piping shall be so designed that under all normal operating conditions, including sludge withdrawal, the gas will be maintained under positive pressure.

1. All enclosed areas where any gas leakage might occur shall be adequately ventilated.

2. All necessary safety facilities should be included where gas is produced in accordance with VOSH requirements.

3. Pressure and vacuum relief valves and flame traps, together with automatic safety shut-off valves, may be provided.

4. Water seal equipment shall not be installed on gas piping.

5. Gas piping shall be of adequate diameter to provide a velocity less than 12 feet per second at a flow of two times the average rate and shall slope to condensation or drip traps at low points.

6. The use of float controlled condensate traps is not permitted. Condensation traps shall be placed in accessible locations for daily servicing and draining.

7. Electrical fixtures and equipment located in enclosed places where gas may accumulate will be evaluated in accordance with the National Board of Fire Underwriters' specifications for hazardous conditions and other applicable codes and regulations.

8. The electrical equipment provided in sludge-digester pipe galleries containing gas piping should be designed and installed to eliminate potential explosive conditions. The design of electrical equipment located in any location where gas or digested sludge leakage is possible will be evaluated in accordance with applicable codes and regulations.

9. Waste gas burners shall be readily accessible and should be located at least 50 feet away from any structure, if placed at ground level. Gas burners may be located on the roof of the control building if sufficiently removed from the digester and gas storage tank and will comply with all applicable state and federal air pollution control requirements. Waste gas burners shall not be located on top of the digester or gas storage tank.

10. In remote locations it may be permissible to discharge small quantities of digester gas (less than 100 CFH) to the atmosphere through a return bend screened vent terminating at least 10 feet above the walking surface, provided the assembly incorporates a flame trap and is in compliance with all applicable state and federal regulations.

11. A gas meter with bypass shall be provided to meter total gas production. Gas piping lines for anaerobic digesters shall be equipped with closed type indicating gauges. These gauges shall read directly in inches of water. Normally, three gauges will be provided: (i) one to measure the main line pressure; (ii) a second to measure the pressure to gas-utilization equipment; and (iii) the third to measure pressure to waste burners. Gas tight shut-off and vent cocks shall be provided. The vent piping shall be extended outside the building, and the opening shall be screened and arranged to prevent the entrance of rainwater. All piping of the manometer system shall be protected with safety equipment.

12. Any underground enclosures connecting with anaerobic digester tanks or containing sludge or gas piping or equipment shall be provided with forced ventilation in accordance with VOSH requirements and this chapter and standards contained in this chapter. Tightly fitting, self closing doors shall be provided at connecting passageways and tunnels to minimize the spread of gas.

F. Energy control. If practical, digesters should be constructed above the water table and should be suitably insulated to minimize heat loss. The use of digester gas as a heating fuel source is encouraged.

1. Sludge shall be heated by circulating the sludge through external heaters unless effective mixing is provided. Piping shall be designed to provide for the preheating of feed sludge before introduction to the digesters. Provisions shall be made in the layout of the piping and valving to facilitate cleaning of these lines. Heat exchanger sludge piping shall be sized for design heat transfer requirements.

2. Sufficient heating capacity shall be provided to maintain consistently the design temperature required for sludge stabilization. For emergency usage, an alternate source of fuel shall be available and the boiler or other heat source shall be capable of using the alternate fuel.

3. The heating system design shall provide for all controls necessary to ensure effective and safe operation. Facilities for optimizing mixing of the digester contents for effective heating shall be provided.

4. Sludge heating devices with open flames should be located above grade in areas separate from locations of gas production or storage.

G. Supernatant handling. Supernatant withdrawal piping shall not be less than six inches in diameter, and piping shall be arranged so that withdrawal can be made from three or more levels in the tank. A positive, unvalved, vented overflow shall be provided.

1. On fixed cover digesters the supernatant withdrawal level should preferably be selected by means of interchangeable extensions at the discharge end of the piping.

2. If a supernatant selector is provided, provision shall be made for at least two other draw-off levels located in the supernatant zone of the digester in addition to the unvalved emergency supernatant draw-off pipe. High pressure backwash facilities shall be provided.

3. Provisions shall be made for sampling at each supernatant draw-off level. Sampling pipes shall be at least 1-1/2 inches in diameter.

4. Management of digester supernatant must be addressed in the treatment works design. Also, sidestream treatment alternatives for digester supernatant shall be considered in the preliminary engineering design.

Statutory Authority

§ 62.1-44.15 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-610 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-550, Virginia Register Volume 20, Issue 9, eff. February 12, 2004; amended, Virginia Register Volume 24, Issue 6, eff. January 1, 2008.

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