NR 110.17(3)(a)1.1. Inlet turbulence into grit chambers shall be minimized.
NR 110.17(3)(a)2.2. Drains or other means for dewatering each grit unit must be provided.
NR 110.17(3)(a)3.3. An adequate supply of water under pressure shall be provided for cleaning grit equipment.
NR 110.17(3)(b)(b) Velocity controlled grit chambers. Positive hydraulic control shall be provided to maintain a channel velocity of 30 centimeters per second (one foot per second) through the expected flow range. Positive hydraulic control shall be provided by one of the following:
NR 110.17(3)(b)1.1. A flow channel with a parabolic cross-section;
NR 110.17(3)(b)2.2. A proportionate weir; or
NR 110.17(3)(b)3.3. A Parshall flume.
NR 110.17(3)(c)(c) Aerated grit chambers.
NR 110.17(3)(c)1.1. Air rates should be in the range of 4.6 to 12.4 liters per second per meter (3 to 8 cubic feet per minute per foot) of tank length.
NR 110.17(3)(c)2.2. The detention time at the maximum design flow rate should not exceed 3 minutes.
NR 110.17(3)(c)3.3. Inlets and outlets shall be designed to prevent short circuiting.
NR 110.17(3)(c)4.4. The design of the grit chamber shall be such to avoid producing dead spaces.
NR 110.17(3)(d)(d) Short-term sedimentation tanks.
NR 110.17(3)(d)1.1. Inlets shall be designed to distribute flow evenly across the tank.
NR 110.17(3)(d)2.2. Tanks shall be deep enough to prevent turbulent flow.
NR 110.17(3)(d)3.3. An additional depth of 15 to 25 centimeters (6 to 10 inches) shall be provided for raking mechanisms.
NR 110.17(3)(d)4.4. Surface area of the sedimentation tank shall be designed not to exceed a surface settling rate of 1,900 cubic meters per day per square meter (46,000 gallons per day per square foot).
NR 110.17 HistoryHistory: Cr. Register, November, 1974, No. 227, eff. 12-1-74; r. and recr. Register, February, 1983, No. 326, eff. 3-1-83.
NR 110.18NR 110.18Settling tanks.
NR 110.18(1)(1)Design considerations.
NR 110.18(1)(a)(a) Multiple settling tanks.
NR 110.18(1)(a)1.1. 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).
NR 110.18(1)(a)2.2. Control appurtenances such as valves, gates, splitter boxes, and flow measuring devices, shall be provided to divide inflow equally to each settling tank.
NR 110.18(1)(b)(b) Servicing.
NR 110.18(1)(b)1.1. All settling tanks shall be provided with easy access for maintenance.
NR 110.18(1)(b)2.2. 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.
NR 110.18(1)(c)(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.
NR 110.18(1)(d)(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.
NR 110.18(1)(e)(e) Imhoff tanks. Imhoff tanks will not be approved by the department.