NR 811.49(1)(a)(a) Pretreatment. Rapid rate gravity filters may only be utilized after coagulation, flocculation and sedimentation. NR 811.49(1)(b)(b) Number. At least 2 filter units or cells shall be provided. Provisions shall be made to meet the plant design capacity at the approved filtration rate with one filter out of service. If only 2 units or cells are provided, each shall be capable of meeting the plant design capacity, normally the projected maximum daily demand. NR 811.49(1)(c)(c) Rate of filtration. The permissible rate of filtration shall be determined after consideration of factors such as raw water quality, degree of pretreatment provided, filter media, water quality control parameters, competency of operating personnel, and other factors required by the department. If effective coagulation, flocculation, sedimentation, and filtration processes are to be utilized with relatively clean water sources, the following filtration rates may be approved: In all cases, the filtration rate shall be proposed and justified by the design engineer and shall be approved by the department prior to the preparation of final plans and specifications. The department may approve higher rates than indicated in this paragraph with sufficient justification by the design engineer or through pilot testing under s. NR 811.44. NR 811.49(1)(d)(d) Structural details and hydraulics. The filter structure shall be designed to provide: NR 811.49(1)(d)2.2. No protrusion of the filter walls or other structures into the filter media or the area between the top of the media and the high water line during backwashing. NR 811.49(1)(d)7.7. Trapped effluent pipe to prevent backflow of air to the bottom of the filters. NR 811.49(1)(d)8.8. Prevention of floor drainage to the filter with a minimum 4-inch curb around the filters. NR 811.49(1)(d)9.9. Prevention of flooding by providing an overflow if this is not provided in a pretreatment unit. NR 811.49(1)(d)10.10. Maximum velocity of treated water in the pipe and conduits to the filter of 2 feet per second. NR 811.49(1)(d)11.11. Cleanouts and straight alignment for influent pipes or conduits where solids loading is heavy or following lime-soda softening. NR 811.49(1)(d)14.14. Safety handrails or walls around the filter areas adjacent to walkways. NR 811.49(1)(d)15.15. Construction to prevent cross connections and common walls between potable and nonpotable water. NR 811.49(1)(e)(e) Washwater troughs. Washwater troughs shall be designed to provide: NR 811.49(1)(e)1.1. A bottom elevation above the maximum level of expanded media during washing. NR 811.49(1)(e)4.4. Spacing so that each trough serves the same number of square feet of filter area. NR 811.49(1)(e)5.5. A maximum horizontal travel of suspended particles not exceeding 3 feet in reaching the trough. NR 811.49(1)(f)(f) Filter material. The media shall be clean silica sand or other natural or synthetic media approved by the department and shall meet the following general requirements: a depth of not less than 24 inches; an effective size of the smallest material no greater than 0.45 mm to 0.55 mm, depending upon the quality of the raw water; a uniformity coefficient of the smallest material not greater than 1.65; a minimum of 12 inches of media with an effective size range no greater than 0.45 mm to 0.55 mm; and a specific gravity greater than other filtering materials within the filter. The following specific requirements shall be met: NR 811.49(1)(f)1.1. ‘Sand.’ Sand shall have an effective size of 0.45 mm to 0.55 mm, a uniformity coefficient of not greater than 1.65, specific gravity greater than 2.5 and an acid solubility less than 5 percent.