Section NR 110.21 - Activated sludge(1) APPLICABILITY. The activated sludge process, and its various modifications, may be used where sewage is amenable to biological treatment.(2) DESIGN REPORT. A design report shall be submitted in accordance with s. NR 110.15(1).(3) DESIGN CONSIDERATIONS. (a)Process selection. The choice of activated sludge process will be influenced by the degree of treatment needed to achieve the required effluent limits, the proposed treatment facility size, and the characteristics of the waste to be treated.(b)Winter protection. Activated sludge processes and aeration equipment which are subject to freezing or icing shall be designed to minimize the degree of freezing and icing.(c)Pretreatment. Where primary settling tanks are not used, effective removal of grit, debris, excessive oil or grease, and comminution or screening of solids shall be provided prior to the activated sludge process.(d)Measuring devices. Devices shall be installed for measuring and displaying flow rates of raw sewage or primary effluent, return sludge, and air to the aeration facilities. It is recommended that these devices totalize and record, as well as indicate, flows if the average design flow for the treatment plant is greater than 5,680 cubic meters per day (1.5 million gallons per day).(e)Equalization. Equalization chambers shall be provided when large daily fluctuations of influent flow or organic loading are expected to occur.(4) AERATION TANKS. (a)Process design. The size of aeration units for any particular adaptation of the activated sludge process shall be determined by pilot plant studies, or calculations based mainly on food to microorganism (F/M) ratio and mixed liquor suspended solids (MLSS) levels. Other factors such as size of treatment plant, diurnal load variations and degree of treatment required shall also be considered. In addition, temperature, pH bicarbonate hardness, and reactor dissolved oxygen shall be considered when designing for nitrification. The calculations used to determine the aeration capacity shall be included in the design report required by s. NR 110.15(1). Designs based on mixed liquor suspended solids levels greater than 5,000 milligrams per liter will not be approved unless adequate data is submitted showing the aeration and settling systems are capable of supporting such levels.(b)Permissible loadings. In lieu of the design calculation requirements of par. (a), the parameters shown in Table 5 may be used to design aeration tank capacities. The volumetric loadings in Table 5 shall be based on the organic load influent to the aeration tank at the average design BOD5 loading rate. Table 5
Process | F/M Ratio mg BOD 5/d/mg MLVSS | Volumetric Loading kg BOD / m3/d (lbs. BOD /1000 ft3/d) | MLSS mg/1 |
Conventional | 0.2 - 0.5 | 0.6 (40) | 1,000 - 3,000 |
Step Aeration | 0.2 - 0.5 | 0.6 (40) | 2,000 - 3,500 |
Complete Mix | 0.2 - 0.6 | 0.8 (50) | 3,000 - 5,000 |
Contact-Stabilization | 0.2 - 0.6 | 0.8a (50) | 1,000 - 3,000b 4,000 - 10,000c |
Extended Aeration | 0.05 - 0.15 | 0.25 (15) | 3,000 - 5,000 |
footnotes: | a) total aeration | capacity |
b) contact tank |
c) reaeration tank |
(c)Number of units. Multiple aeration tanks shall be provided where the average design flow exceeds 1,890 cubic meters (500,000 gallons) per day.(d)Aeration tank design features.1. The dimensions of each aeration tank or return sludge reaeration tank shall be such as to maintain effective mixing and use of air.2. Liquid depths in aeration tanks may not be less than 3 meters (10 feet). The department may allow liquid depths to exceed 5 meters (16 feet) on a case-by-case basis.3. Baffling or the placement of aeration equipment shall provide positive control of hydraulic short-circuiting through aeration tanks.4. Process piping, influent channels and inlet structure shall be arranged to provide operational flexibility.5. Inlets and outlets for each aeration tank unit shall be equipped with valves, gates, stop plates, weirs or other devices to permit controlling the flow to each tank and to maintain a constant liquid level. The hydraulic properties of the system shall permit the peak instantaneous design flow to be carried with any single aeration tank unit out of service.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.7. All aeration tanks shall have a freeboard of not less than 46 centimeters (18 inches).(5) AERATION SYSTEMS. (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.(b)Oxygen demand.1. Aeration equipment shall be capable of maintaining a minimum mixed liquor dissolved oxygen concentration of 2 milligrams per liter.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.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).(c)Air supply to meet oxygen demands.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.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.(d)Mixing requirements. The following minimum requirements shall be met to insure adequate mixing of mixed liquor suspended solids. 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.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.(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.(6) AERATION EQUIPMENT.(a)Diffused aeration systems.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.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.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.(b)Mechanical aerators.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.2. Due to high heat loss, the mechanical aerators shall be protected from freezing.(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.(7) SLUDGE EQUIPMENT. (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
Minimum | Maximum |
Conventional | 15 | 75 |
Step aeration | 20 | 75 |
Contact stabilization | 50 | 150 |
Extended aeration | 50 | 200 |
Complete mix | 20 | 75 |
(b)Return sludge pumps.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.2. If air lifts are used for returning sludge from each settling tank hopper, no standby unit will be required provided the design of the air lifts allows rapid and easy cleaning. Air lift pumps shall be designed to provide positive control of the return sludge rate.(c)Return sludge piping. Suction piping and discharge piping for returning activated sludge shall be at least 10 centimeters (4 inches) in diameter and must be designed to maintain a velocity of not less than 60 centimeters per second (2 feet per second) at normal return sludge rates. Suitable devices for observing, sampling and controlling return activated sludge flow from each settling tank shall be provided.(d)Waste sludge piping. Waste sludge piping shall comply with the requirements of s. NR 110.26(4)(e)Waste sludge pumps. Variable speed or multiple constant speed waste sludge pumps shall be provided. The maximum sludge pumping rate shall be at least 200% of the anticipated volumetric sludge production rate. Devices for measuring waste activated sludge flow rates shall be provided.Wis. Admin. Code Department of Natural Resources NR 110.21
Cr. Register, November, 1974, No. 227, eff. 12-1-74; r. and recr. Register, February, 1983, No. 326, eff. 3-1-83; CR 09-123: am. (4) (b), (d) 5., (5) (b) 2., 3., (c) 2., (6) (a) 2., (b) 1. and Table 5 (title) Register July 2010 No. 655, eff. 8-1-10.