Section NR 110.18 - Settling tanks(1) DESIGN CONSIDERATIONS. (a)Multiple settling tanks.1. Multiple settling tanks shall be provided at all sewage treatment plants with an average design flow which exceeds 1,890 cubic meters per day (0.5 million gallons per day).2. Control appurtenances such as valves, gates, splitter boxes, and flow measuring devices, shall be provided to divide inflow equally to each settling tank.(b)Servicing.1. All settling tanks shall be provided with easy access for maintenance.2. Each settling tank shall be capable of being independently dewatered and isolated for servicing. Provisions shall be made to protect empty settling tanks from the hydrostatic uplift due to high groundwater.(c)Safety. Operator safety shall be considered in the design of settling tanks. Safety features such as machinery covers, life lines, stairways, walkways, handrails and slip-resistant surfaces shall be provided if appropriate.(d)Electrical controls. Electrical fixtures and controls in enclosed settling tanks shall meet the requirements of the national electrical code for class 1, group D, division 1 locations. The fixtures and controls shall be located to provide convenient and safe access for operation and maintenance.(e)Imhoff tanks. Imhoff tanks will not be approved by the department.(2) DESIGN REQUIREMENTS. (a)Inlets.1. Settling tank inlets shall be designed to dissipate the inlet velocity, to distribute the flow equally and to prevent short-circuiting. Either channel shall be designed to maintain a velocity of at least 30 centimeters per second (one foot per second) at one-half of average design flow or some other means of preventing solid deposition shall be used. Corner pockets and dead ends shall be eliminated and corner fillets or channeling used where necessary. Elimination or removal of floating materials in inlet structures having submerged ports shall be required.2. Inlet velocities in rectangular settling tanks may not exceed 15 centimeters per second (0.5 foot per second).3. Inlet velocities in center feed circular settling tanks may not exceed 91 centimeters per second (3 feet per second).(b)Tank features.1. The side water depth of mechanically cleaned settling tanks shall be as shallow as practical but not less than 2.1 meters (7 feet). Final clarifiers for activated sludge may not be less than 3 meters (10 feet) in depth. Final clarifier for fixed film treatment systems may not be less than 2.1 meters (7 feet).2. The tops of troughs, beams, and similar construction features which are submerged shall have a minimum slope of 1.4 vertical to one horizontal. A slope of one to one shall be provided on the underside of such features to prevent the accumulation of scum and solids.3. Effluent weirs shall be adjustable.(c)Sludge and scum removal.1. Scum baffles shall be provided ahead of outlet weirs on all primary and final settling tanks.2. Sludge hoppers shall have a minimum side wall slope of 1.7 vertical to one horizontal. Hopper wall surfaces should be made smooth with rounded corners to aid in sludge removal. The department will not approve increasing the depth of sludge hoppers for the purpose of sludge thickening in settling tanks.3. Each sludge hopper shall have an individually valved sludge withdrawal line at least 15 centimeters (6 inches) in diameter. Head available for withdrawal of sludge shall be at least 76 centimeters (30 inches), or greater as necessary, to maintain a velocity of 91 centimeters per second (3 feet per second) in the withdrawal pipe.4. A sludge well or other appropriate equipment shall be provided for viewing and sampling sludge.5. Provisions for cleaning sludge piping shall be made.6. The department may approve the use of glass lined pipe for sludge. Glass lined pipe may not be less than 10 centimeters (4 inches) in diameter.7. Suitable mechanical sludge and scum collection equipment shall be provided in all settling tanks. Provisions for separate scum washing shall be made for treatment facilities which do not have primary settling facilities.(d)Design parameters.1. Operating design parameters for settling facilities may not exceed the values given in Table 3. The surface settling rate for primary settling tanks shall be calculated with all flows received at the settling tank. The surface settling rate for final settling tanks shall be based on influent flow. The maximum hour solids loading shall be computed based on the maximum day design flow plus the maximum design return sludge rate requirement and the design mixed liquor suspended solids (MLSS) under aeration. Table 3
SURFACE SETTLING RATE
Location | Flow Basis | -m3/m2.d (gal/ft2.d) | Solids Loading -kg/m2.h- (lb/ft2.h) | Weir Overflow Rate -m/m.d- (gal/ft. |
Primary | Average design | 40 (1000) | - | 125 (10,000) |
Maximum Hourly | 60 (1500) | - | - |
Intermediate | Maximum Hourly | 60 (1500) | - | - |
Final (following) -Trickling filter/RBC | Average design | - | 5.0 (1.0) | 125 (10,000) |
Maximum Hourly | 49 (1200) | 8.0 (1.6) | - |
-Activated Sludge | Average design | - | 7.0 (1.4) | 125 (10,000) |
Maximum Hourly Average design | 49 (1200) | 10.0 (2.0) | - |
-Separate Stage Nitrification |
Average design | - | 7.0 (1.4) | 125 (10,000) |
Maximum Hourly | 33 (800) | 10.0 (2.0) |
-Extended Aeration | Average design | - | 6.0 (1.2) | 125 |
Maximum Hourly | 40 (1000) | 10.0 (2.0) | (10,000) |
2. For treatment plants with an average design flow greater than 3,785 cubic meters per day (one million gallons per day), the department may approve an overflow rate of 188 cubic meters per meter per day (15,000 gallons per foot per day).3. The design parameters shown in Table 3 may be waived by the department if the settling tank design is based on settling tests of wastes currently received at the existing treatment facility, or if the effluent from the sewage treatment facility is to be disposed on land.Wis. Admin. Code Department of Natural Resources NR 110.18
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. (2) (d) Register July 2010 No. 655, eff. 8-1-10.