Chapter NR 110

NR 110.16(1)(d)3. 3. Where multiple screening units are installed the channels shall be equipped with the necessary gates to direct flow from any one screening unit. Methods for dewatering each channel shall be provided.

NR 110.16(1)(d)4. 4. Entrance channels shall be designed to distribute flow uniformly to the screening units.

NR 110.16(1)(e) (e) Handling screenings. Adequate facilities must be provided for removal, handling and storage of screenings in a sanitary manner. Hand-cleaned screening facilities must include an accessible platform from which the operator may rake screenings. Suitable drainage facilities must be provided for both the platform and the storage areas.

NR 110.16(2) (2) Design requirements for screening devices.

NR 110.16(2)(a) (a) Bar screens.

NR 110.16(2)(a)1. 1. Clear spacing between bars may not be less than 2.5 centimeters (1 inch), nor more than 5 centimeters (2 inches).

NR 110.16(2)(a)2. 2. Bar screens must be placed on a slope of 30 to 45 degrees with the horizontal with the exception of those installed for emergency use.

NR 110.16(2)(a)3. 3. Approach velocities may be no less than 38 centimeters per second (1.25 feet per second) at design average flow conditions to prevent settling, and no greater than 91 centimeters per second (3 feet per second) at maximum design daily flow to prevent forcing material through the openings.

NR 110.16(2)(b) (b) Mechanical screens.

NR 110.16(2)(b)1. 1. Maximum clean spacing between bars may not exceed 5 centimeters (2 inches).

NR 110.16(2)(b)2. 2. Approach velocities may be no less than 38 centimeters per second (1.25 feet per second) at average design flow conditions to prevent settling, and no greater than 91 centimeters per second (3 feet per second) at maximum daily flow to prevent forcing material through the openings.

NR 110.16(2)(b)3. 3. All mechanical units which are operated by timing devices shall be provided with auxiliary controls which will set the cleaning mechanism in operation at a preset high water elevation.

NR 110.16(2)(b)4. 4. Automatic controls shall be supplemented by a manual override. Manual overrides shall be located in view of the equipment.

NR 110.16(2)(b)5. 5. Electrical fixtures and controls in screening areas where hazardous gases may accumulate shall meet the requirements of the national electrical code for class 1, group D, division 1 locations.

NR 110.16(2)(c) (c) Comminutors.

NR 110.16(2)(c)1. 1. Comminutors shall be designed to comminute the maximum design flow rate.

NR 110.16(2)(c)2. 2. Comminutor channels shall be designed to contain the upstream water depth associated with the head loss which occurs at maximum design flow without surcharging the incoming sewer or other treatment processes. The expected head loss shall take into account the effects of clogging during operation.

NR 110.16(2)(c)3. 3. Comminutor channels shall be equipped with drains.

NR 110.16(2)(c)4. 4. A screened bypass channel shall be provided so that the comminutor may be removed from service for maintenance.

NR 110.16(2)(c)5. 5. Bypass channels will not be required where 2 comminutors are installed. Each comminutor shall be capable of comminuting the maximum design flow.

NR 110.16(2)(c)6. 6. Control switches or a disconnecting device for the comminutor shall be located in view of the comminutor.

NR 110.16(2)(c)7. 7. Electrical fixtures and controls in comminutors areas where hazardous gases may accumulate shall meet the requirements of the national electrical code for class 1, group D, division 1 locations.

NR 110.16 History History: Cr. Register, November, 1974, No. 227, eff. 12-1-74; r. and recr. Register, February, 1983, No. 326, eff. 3-1-83.

NR 110.17 NR 110.17 Grit removal facilities.

NR 110.17(1) (1) Applicability. Grit removal facilities are recommended for all sewage plants and are required for plants receiving sewage from combined sewers or from sewer systems receiving substantial amounts of grit.

NR 110.17(2) (2) Design considerations.

NR 110.17(2)(a) (a) Location. Where practical, grit chambers should precede influent pumps. When installed, grit chambers shall precede all major treatment units.

NR 110.17(2)(b) (b) Protection. All grit chambers shall be preceded by a bar rack, coarse screen or comminutor.

NR 110.17(2)(c) (c) Housed facilities.

NR 110.17(2)(c)1. 1. Enclosed grit removal areas shall be ventilated. Fresh air shall be introduced continuously at a rate of 12 air changes per hour, or intermittently at a rate of 30 air changes per hour.

NR 110.17(2)(c)2. 2. All electrical work in enclosed grit removal areas where hazardous gases may accumulate shall meet the requirements of the national electrical code for class 1, group D, division 1 locations.

NR 110.17(2)(d) (d) Number of units. Grit removal facilities shall have at least 2 hand-cleaned units, or a mechanically cleaned unit with a bypass.

NR 110.17(2)(e) (e) Grit handling.

NR 110.17(2)(e)1. 1. All facilities not provided with positive velocity control shall include means for grit washing to further separate organic and inorganic materials.

NR 110.17(2)(e)2. 2. Grit removal facilities located in deep pits shall be provided with mechanical equipment for hoisting or transporting grit to ground level. Impervious, nonslip, working surfaces with adequate drainage shall be provided for grit handling areas. Storage areas for wasted grit shall be provided.

NR 110.17(2)(f) (f) Basis of design. Design of grit chambers shall be based on the size and specific gravity of the grit particle to be removed. If this information is not obtained from actual field measurements, then the design shall assume removal of all particles retained on a 65 mesh (0.21 mm) sieve and having a minimum specific gravity of 2.65. The design requirements of sub. (3) are based on these assumptions.

NR 110.17(3) (3) Design requirements.

NR 110.17(3)(a) (a) General requirements.

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 History History: Cr. Register, November, 1974, No. 227, eff. 12-1-74; r. and recr. Register, February, 1983, No. 326, eff. 3-1-83.

NR 110.18 NR 110.18 Settling 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.

NR 110.18(2) (2) Design requirements.

NR 110.18(2)(a) (a) Inlets.

NR 110.18(2)(a)1. 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.

NR 110.18(2)(a)2. 2. Inlet velocities in rectangular settling tanks may not exceed 15 centimeters per second (0.5 foot per second).

NR 110.18(2)(a)3. 3. Inlet velocities in center feed circular settling tanks may not exceed 91 centimeters per second (3 feet per second).

NR 110.18(2)(b) (b) Tank features.

NR 110.18(2)(b)1. 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).

NR 110.18(2)(b)2. 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.

NR 110.18(2)(b)3. 3. Effluent weirs shall be adjustable.

NR 110.18(2)(c) (c) Sludge and scum removal.

NR 110.18(2)(c)1. 1. Scum baffles shall be provided ahead of outlet weirs on all primary and final settling tanks.

NR 110.18(2)(c)2. 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.

NR 110.18(2)(c)3. 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.

NR 110.18(2)(c)4. 4. A sludge well or other appropriate equipment shall be provided for viewing and sampling sludge.

NR 110.18(2)(c)5. 5. Provisions for cleaning sludge piping shall be made.

NR 110.18(2)(c)6. 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.

NR 110.18(2)(c)7. 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.

NR 110.18(2)(d) (d) Design parameters.

NR 110.18(2)(d)1. 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

NR 110.18(2)(d)2. 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).

NR 110.18(2)(d)3. 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.