NR 110.21(4)(d)6.6. Channels and pipes carrying liquids with suspended solids shall be designed to maintain self-cleansing velocities or shall be agitated to keep the solids in suspension at all rates of flow within the design limits. NR 110.21(4)(d)7.7. All aeration tanks shall have a freeboard of not less than 46 centimeters (18 inches). NR 110.21(5)(a)(a) General. The aeration system shall be capable of meeting the oxygen requirements of the activated sludge system, or of maintaining adequate mixing of the mixed liquor suspended solids, whichever is greater. NR 110.21(5)(b)1.1. Aeration equipment shall be capable of maintaining a minimum mixed liquor dissolved oxygen concentration of 2 milligrams per liter. NR 110.21(5)(b)2.2. In the absence of experimentally determined values, the design oxygen requirements for all activated sludge processes shall be 1.1 kilograms oxygen per kilogram peak hour BOD5 (1.1 pounds oxygen per pound peak hour BOD5) removed in the aeration tanks, with the exception of the extended aeration process, for which the value shall be 1.5 kilograms oxygen per kilogram peak hour BOD5 (1.5 pounds oxygen per pound peak hour BOD5) to include endogenous respiration requirements. NR 110.21(5)(b)3.3. To provide nitrification, the oxygen requirement for oxidizing ammonia shall be added to the requirement in subd. 2. for carbonaceous BOD5 removal and endogenous respiration requirements. In the absence of experimentally determined values, the nitrogen oxygen demand (NOD) shall be 4.6 kilograms of oxygen per kilogram removed peak hour total Kjeldahl nitrogen (TKN) (4.6 pounds oxygen per pound removed peak hour TKN). NR 110.21(5)(c)1.1. The design of the aerator system to provide the oxygen requirements calculated in accordance with par. (b) shall be done using standard design equations for diffused and mechanical aeration systems. Calculations shall incorporate such factors as tank depth, alpha factor of the waste, beta factor of the waste, certified aerator oxygen transfer efficiency, minimum aeration tank dissolved oxygen concentration, critical wastewater temperature and altitude of the wastewater treatment facility. NR 110.21(5)(c)2.2. In the absence of specific design information, the air requirements for diffused aerators shall be calculated using an oxygen transfer efficiency of 7% in clean water under standard test conditions. The air requirements for mechanical aerators shall be based on a transfer rate of 1.2 kilograms oxygen per kilowatt-hour (2 pounds oxygen per horsepower-hour) in clean water under standard test conditions. NR 110.21(5)(d)(d) Mixing requirements. The following minimum requirements shall be met to insure adequate mixing of mixed liquor suspended solids. NR 110.21(5)(d)1.1. Diffused aeration systems shall be capable of delivering a minimum air flow rate of 20 cubic meters per minute per 1,000 cubic meters (20 cubic feet per minute per 1,000 cubic feet) of aeration volume. NR 110.21(5)(d)2.2. Mechanical aerators shall deliver a minimum of 15 kilowatts per 1,000 cubic meters (0.6 horsepower per 1,000 cubic feet) of aeration volume. NR 110.21(5)(e)(e) Other air-use demands. The aeration system shall also be capable of providing the air required for channel aeration, air-lift pumps, aerobic digesters, and any other air-use demand. NR 110.21(6)(a)1.1. Multiple blowers shall be provided. The blowers shall be sized to meet the maximum air demand with the largest blower out of service. The design shall also provide for varying the volume of air delivered in proportion to the air demand of the plant. NR 110.21(6)(a)2.2. Diffusers and air piping shall be capable of supplying the peak hour air demand or 200% of the design average air demand, whichever is larger. NR 110.21(6)(a)3.3. The arrangement of diffusers shall permit their removal for inspection, maintenance and replacement without dewatering aeration tanks or channels and without shutting off the air supply to other diffusers in the treatment system. The department may waive this requirement for systems with multiple aeration tanks provided the treatment efficiency of the system can be maintained with one aeration tank out of service. NR 110.21(6)(b)1.1. Multiple mechanical aeration units shall be designed and located so as to meet the peak hour oxygen demand or 200% of the design average oxygen demand, whichever is larger, with one unit out of service. NR 110.21(6)(b)2.2. Due to high heat loss, the mechanical aerators shall be protected from freezing. NR 110.21(6)(c)(c) Pure oxygen. Where pure oxygen is proposed, supporting data from pilot plant installations or full-scale installations similar to the one proposed shall be submitted to justify the aerator loading rate and the amount and type of aeration capacity and equipment proposed. NR 110.21(7)(a)(a) Return sludge rate. The rate of sludge return expressed as a percentage of the average design flow of sewage shall lie within the limits shown in Table 6: Table 6
NR 110.21(7)(b)1.1. If motor driven return sludge pumps are used, the maximum return sludge capacity shall be met with the largest pump out of service. A positive head shall be provided on pump suctions. Pumps shall also have at least 7.6 centimeter (3-inch) suction and discharge openings.