Section NR 110.17 - Grit removal facilities(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.(2) DESIGN CONSIDERATIONS. (a)Location. Where practical, grit chambers should precede influent pumps. When installed, grit chambers shall precede all major treatment units.(b)Protection. All grit chambers shall be preceded by a bar rack, coarse screen or comminutor.(c)Housed facilities.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.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.(d)Number of units. Grit removal facilities shall have at least 2 hand-cleaned units, or a mechanically cleaned unit with a bypass.(e)Grit handling.1. All facilities not provided with positive velocity control shall include means for grit washing to further separate organic and inorganic materials.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.(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.(3) DESIGN REQUIREMENTS. (a)General requirements.1. Inlet turbulence into grit chambers shall be minimized.2. Drains or other means for dewatering each grit unit must be provided.3. An adequate supply of water under pressure shall be provided for cleaning grit equipment.(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: 1. A flow channel with a parabolic cross-section;2. A proportionate weir; or(c)Aerated grit chambers.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.2. The detention time at the maximum design flow rate should not exceed 3 minutes.3. Inlets and outlets shall be designed to prevent short circuiting.4. The design of the grit chamber shall be such to avoid producing dead spaces.(d)Short-term sedimentation tanks.1. Inlets shall be designed to distribute flow evenly across the tank.2. Tanks shall be deep enough to prevent turbulent flow.3. An additional depth of 15 to 25 centimeters (6 to 10 inches) shall be provided for raking mechanisms.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).Wis. Admin. Code Department of Natural Resources NR 110.17
Cr. Register, November, 1974, No. 227, eff. 12-1-74; r. and recr. Register, February, 1983, No. 326, eff. 3-1-83.