NR 110.26(4)(b)4.4. Air relief valves shall be provided at high points in pressure sludge lines.
NR 110.26(4)(b)5.5. Special consideration shall be given to the corrosion resistance and continuing stability of pipes and supports located inside the digestion tank.
NR 110.26(5)(5)Anaerobic digestion.
NR 110.26(5)(a)(a) General.
NR 110.26(5)(a)1.1. Anaerobic digesters which will also serve as supernatant separation tanks shall have a minimum side water depth of 6 meters (20 feet).
NR 110.26(5)(a)2.2. Multiple sludge inlets and draw-offs shall be provided. Multiple recirculation suction and discharge points to facilitate flexible operations and effective mixing of the digester contents shall be provided unless adequate mixing facilities are provided within the digester. One sludge inlet shall discharge above the liquid level and be located at approximately the center of the tank to assist in scum breakup. Raw sludge inlet discharge points shall be so located as to minimize short circuiting to the supernatant draw-off. Sludge withdrawal for disposal shall be from the bottom of the tank. The pipe shall be interconnected with the recirculation piping to increase flexibility in mixing tank contents.
NR 110.26(5)(b)(b) Tank capacity.
NR 110.26(5)(b)1.1. The total digestion tank capacity shall be calculated based upon the factors indicated in sub. (1). If such calculations are not done, the following minimum requirements shall be met:
NR 110.26(5)(b)1.a.a. A minimum detention time of 15 days at design flows shall be provided;
NR 110.26(5)(b)1.b.b. Completely mixed digestion systems shall provide for intimate and effective mixing to prevent stratification and to assure homogeneity of digester content. The maximum system loading shall be 1.28 kilograms per cubic meter per day (80 pounds of volatile solids per 1,000 cubic feet of volume per day) in the digester;
NR 110.26(5)(b)1.c.c. The maximum system loading for moderately mixed digestion systems in which mixing is accomplished only by circulating sludge through an external heat exchanger shall be 0.64 kilograms per cubic meter per day (40 pounds of volatile solids per 1,000 cubic feet of volume per day) in the digester; and
NR 110.26(5)(b)1.d.d. The loading rates indicated in subd. 1. a., b., and c. assume that the raw sludge is derived from ordinary domestic wastewater. The loading may be modified upward or downward depending upon the degree of mixing provided. Where mixing is accomplished by other methods, loading rates may be approved on the basis of information submitted justifying the design.
NR 110.26(5)(c)(c) Temperature. Heating equipment shall have the ability to maintain digestion temperature in the range of 33° to 38°C (90° to 100°F).
NR 110.26(5)(d)(d) Gas collection, piping and appurtenances.
NR 110.26(5)(d)1.1. All portions of the gas system, including the space above the tank liquor, storage facilities and piping shall be so designed that under normal operating conditions, including sludge withdrawal, the gas will be maintained under positive pressure. All enclosed areas where any gas leakage might occur shall be ventilated.
NR 110.26(5)(d)2.2. Safety facilities shall be included where gas is produced. Pressure and vacuum relief valves and flame traps, together with automatic safety shut off valves, shall be provided. Waterseal equipment may not be installed. Gas compressors with gas safety equipment should be housed in a separate room with an exterior entrance.
NR 110.26(5)(d)3.3. The diameter of gas piping shall be based on the volume of gas which will be generated. Gas piping shall slope to condensate traps. The use of float-controlled condensate traps is prohibited.
NR 110.26(5)(d)4.4. Gas burning boilers, engines and other units using gas as a fuel shall be located in ventilated rooms, preferably at ground level and shall be isolated in accordance with the provisions of chs. SPS 361 to 365. Gas lines to these units shall be provided with suitable flame traps.
NR 110.26(5)(d)5.5. Electrical fixtures and controls in enclosed places where gas may accumulate shall comply with the national electrical code requirements for class 1, group D, division 2 locations.
NR 110.26(5)(d)6.6. Waste gas burners shall be readily accessible and shall be located at least 7.6 meters (25 feet) away from any plant structure if placed at ground level. Waste gas burners may be located on the roof of the control building if sufficiently removed from the tank. All waste gas burners shall be equipped with automatic ignition, such as a pilot light or a device using a photoelectric cell sensor. Consideration should be given to the use of natural or propane gas to insure reliability of the pilot light. If the waste gas burner is in a remote location, the department may approve the discharge of gas to the atmosphere through a return-bend screened vent terminating at least 3 meters (10 feet) above the walking surface, provided the assembly incorporates a flame trap.
NR 110.26(5)(d)7.7. A gas meter with by-pass shall be provided to meter total gas production. Additional gas meters may be required to measure gas usage.
NR 110.26(5)(e)(e) Digestion tank heating.
NR 110.26(5)(e)1.1. Whenever possible, digestion tanks shall be constructed above groundwater level. Digestion tanks shall be insulated to minimize heat loss.
NR 110.26(5)(e)2.2. Piping shall be designed to provide for the heating of feed sludge before introduction to the digesters. Heat exchanger sludge piping shall be sized for heat transfer requirements.
NR 110.26(5)(e)3.3. Sufficient heating capacity shall be provided to consistently maintain the design sludge temperature. Where digestion tank gas is used for other purposes, an auxiliary fuel supply shall be provided.
NR 110.26(5)(e)4.4. Consideration should be given to equipping hot water internal heating controls with an automatic mixing valve to temper the boiler water with return water so that the inlet water to the heat jacket can be maintained between 49° to 55°C (120° to 130°F) to prevent excessive caking or encrustation of sludge on the heat jacket. Manual controls shall also be provided.
NR 110.26(5)(e)5.5. The boiler shall be provided with automatic controls to maintain the boiler temperature at approximately 82°C (180°F). Automatic controls shall also be provided to shut off the main gas supply in the event of pilot burner or electrical failure, low boiler water level or excessive temperature.
NR 110.26(5)(e)6.6. Thermometers shall be provided to show temperatures of the sludge, hot water feed, hot water return and boiler water.
NR 110.26(5)(f)(f) Supernatant withdrawal.