NR 110.26(5)(f)2.2. Piping shall be arranged so that withdrawal can be made from 3 or more levels in the tank. A positive unvalved vented overflow shall be provided. NR 110.26(5)(f)3.3. If a supernatant selector is provided, provisions shall be made for at least one other draw-off level located in the supernatant zone of the tank in addition to a positive unvalved vented overflow pipe. High pressure backwash facilities shall be provided. NR 110.26(6)(a)1.1. Aerobic digesters shall be designed to provide effective air mixing, reduction of organic matter, supernatant separation and sludge concentration. NR 110.26(6)(a)2.2. The digester detention time may be calculated based upon the factors indicated in subd. 1. If such calculations are not done, the following minimum requirements shall be met: NR 110.26(6)(a)2.a.a. A minimum of 15 days detention time shall be provided for waste activated sludge and 20 days for primary sludge or a combination of primary and waste activated sludge. Where sludge temperature is lower than 10°C (50°F), additional time shall be provided; and NR 110.26(6)(a)2.b.b. The volatile suspended solids loading may not exceed 1.60 kilograms per cubic meter per day (100 pounds per 1,000 cubic feet per day) in the digestion unit. Lower loading rates may be necessary depending on temperature, type of sludge and other factors. NR 110.26(6)(a)3.3. The aeration system for aerobic digesters shall be capable of meeting the oxygen requirements of par. (b), or the mixing requirements of par. (c), whichever is greater. NR 110.26(6)(b)1.1. Aeration systems shall be capable of maintaining a minimum digester dissolved oxygen concentration of one milligram per liter. NR 110.26(6)(b)2.2. In the absence of experimentally determined values, the design oxygen requirements for aerobic digesters shall be 2.0 kilograms oxygen per kilogram (2.0 pounds oxygen per pound) anticipated volatile suspended solids reduction. An additional 1.8 kilograms oxygen per kilogram (1.8 pounds oxygen per pound) of BOD5 applied to the digester by primary sludge shall be supplied. NR 110.26(6)(c)2.2. Diffused aeration systems shall be capable of delivering a minimum air flow rate of 30 cubic meters per minute per 1,000 cubic meters (30 cubic feet per minute per 1,000 cubic feet) of digester volume. NR 110.26(6)(c)3.3. Mechanical aerators shall deliver 26.3 kilowatts per 1000 cubic meters (1.0 horsepower per 1,000 cubic feet) of digester volume. NR 110.26(6)(e)(e) Supernatant withdrawal. Aerobic digesters shall be equipped for supernatant draw-off. It is recommended that multi-level draw-off be provided. NR 110.26(7)(a)(a) Lime stabilization. Sufficient lime shall be added to produce a pH of 12 after 2 hours of contact. NR 110.26(7)(b)(b) Composting. Static aerated pile, within vessel, or windrow composting methods shall be maintained at a minimum operating temperature of 40°C (104°F) for at least 5 days. For 4 hours during this period the temperature must exceed 55°C (131°F). Composting design, siting and operation shall be done in accordance with chs. NR 204 and 500 to 538. NR 110.26(7)(c)(c) Other methods. Other methods or operating conditions may be acceptable for sludge stabilization if pathogens and volatile solids are reduced to an extent equivalent to anaerobic digestion. NR 110.26(8)(a)1.1. The drying bed area shall be calculated based upon such factors as climatic conditions, character and volume of sludge to be dewatered, the method and character of sludge removal and other methods of sludge disposal. At least 0.19 square meters (2 square feet) of drying bed area per capita population equivalent shall be provided when the drying bed is the primary method of dewatering, and 0.09 square meters (1 square foot) per capita population equivalent if it is to be used as a back-up dewatering unit. The bed area shall be increased by 25% if the beds are paved. NR 110.26(8)(a)3.3. Percolation type drying beds shall meet the following conditions: NR 110.26(8)(a)3.a.a. The lower course of gravel around the underdrains shall be properly graded and shall be at least 30 centimeters (one foot) in depth, extending at least 15 centimeters (6 inches) above the top of the underdrains. It is desirable to place this in 2 or more layers. At least 8 centimeters (3 inches) of the top layer shall consist of gravel 3 to 6 millimeters (1/8 to 1/4 inches) in size.