Okla. Admin. Code § 252:656-19-2

Current through Vol. 41, No. 19, June 17, 2024

Section 252:656-19-2 - Anaerobic sludge digestion

(a)Tanks.

(1)Multiple units. Provide dual units or alternate methods of sludge processing or emergency storage to maintain continuity of service.

(2)Depth. Provide a sidewater depth of at least 20 feet.

(3)Slope. Slope the tank bottoms towards withdrawal piping. The bottom slope must be at least 1:12 for mechanical removal, or 1:4 for gravity removal.

(4)Manholes. Provide at least two 36-inch diameter manholes in the top of the digester in addition to the gas dome. At least one opening must be large enough for equipment to remove grit and sand. Provide stairways to reach the access manholes.

(b)Sludge inlets and outlets. Provide for sludge recirculation. Provide multiple recirculation withdrawal and return points unless mixing facilities are incorporated within the digester(s). Return flow must discharge above the liquid level near tank center. Discharge raw sludge to the digester through the sludge heater recirculation return piping unless internal mixing facilities are provided.

(c)Tank capacity. Determine total tank(s) capacity by rational calculations based on such factors as volume of sludge added, its percent solids and character, the temperature to be maintained in the digesters, type of mixing provided, the degree of volatile solids reduction and pathogen reduction requirements. Submit all calculations and design assumptions for review. For design purposes, use the following assumptions:

(1) The raw sludge is derived from ordinary domestic wastewater.

(2) The sludge shall be heated, through the controlled biological decomposition of organic material, and maintained at a temperature between 35 deg. C to 55 deg. C (95 deg F to 131 deg F) for 15 days or at 20 deg. C (68 deg F) for 60 days.

(3) That 40 to 50% volatile matter will be maintained in the digested sludge.

(A)Completely-mixed systems. Provide sufficient mixing to prevent stratification and to assure homogeneity of digester content. Active digestion units may be loaded with volatile solids at a rate up to 80 lb/1,000 ft³ tank volume/day.

(B)Moderately-mixed systems. For systems where mixing is accomplished only by circulating sludge through an external heat exchanger, the system may be loaded at a rate up to 40 lb/1,000 ft³ tank volume/day. Where actual data are not available, the following unit capacities may be used for plants treating domestic sewage:

(i) Primary facility - 3 ft³/PE heated or 4 ft³/PE unheated

(ii) Primary and standard rate filter facility - 4 ft³/PE heated or 5 ft³/PE unheated

(iii) Primary and high rate filter facility - 4 ft³/PE heated or 5.5 ft³/PE unheated

(d)Gas collection, piping and appurtenances.

(1)Gas collection and containment. Design all portions of the gas system, including the space above the digester liquor to operate under pressure. Mechanically ventilate all areas where gas leakage might occur and separate from areas where extraneous sparks or fire might occur.

(2)Safety equipment. Where gas is produced, provide pressure and vacuum relief valves and flame traps, together with automatic safety shut-off valves. Water seal equipment shall not be installed. House gas safety equipment and gas compressors in a separate room with an exterior entrance.

(3)Gas piping and condensate. Gas piping must be at least 2 inches in diameter and shall slope to condensation traps at low points. Float-controlled condensate traps are not permitted.

(4)Gas utilization equipment. All gas burning boilers and engines must be located at or above ground level and in well ventilated rooms. Gas lines to these units must have suitable flame traps.

(5)Waste gas. Waste gas burners must have automatic ignition and be located at least 50 feet away from all digesters and suitably isolated from any other plant structure.

(6)Meter. Provide a gas meter with bypass to measure total gas production.

(e)Supernatant withdrawal.

(1)Piping size. Supernatant piping shall be at least 6 inches in diameter.

(2)Withdrawal levels. Arrange withdrawal piping to allow for at least three levels of sludge withdrawal. Provide a positive unvalved emergency overflow, designed to return the flow back to the headworks.

(3)Supernatant withdrawal. Provide at least one draw-off point that is located in the supernatant zone of the tank. On fixed-cover digesters, provide means to adjust the supernatant withdrawal level.

(4)Sampling. Provide a means to sample each supernatant draw-off level. Sampling pipes must be at least 1.5 inches in diameter with a quick-acting valve.

(f)Temperature.

(1) The sludge shall be heated, through the controlled biological decomposition of organic material, and maintained at a temperature between 35 deg. C to 55 deg. C (95 deg F to 131 deg F) for 15 days or at 20 deg. C (68 deg F) for 60 days.

(2) Provide a temperature probe and recording device to continuously record digester temperature.

Okla. Admin. Code § 252:656-19-2

Added at 17 Ok Reg 1140, eff 6-1-00; Amended at 23 Ok Reg 937, eff 6-15-06; Added at 28 Ok Reg 1282, eff 7-1-11