NR 811.49(1)(f)6.6. ‘Supporting media.’ Torpedo sand and gravel shall be provided as supporting media except when proprietary filter bottoms are used. In that case, the department, on the basis of substantiating information provided by the owner, may allow elimination of certain layers of supporting media or a reduction in the depth of the layers. Otherwise, the following apply: NR 811.49(1)(f)6.a.a. A 3-inch layer of torpedo sand shall be used as a supporting media for the filter sand. The torpedo sand shall have an effective size of 0.8 mm to 2.0 mm, and a uniformity coefficient not greater than 1.7. NR 811.49(1)(f)6.b.b. Gravel, when used as the supporting media, shall consist of hard, rounded silica particles and may not include flat or elongated particles. The coarsest gravel shall be 2.5 inches in size when the gravel rests directly on the strainer system, and shall extend above the top of the perforated laterals or strainer nozzles. Not less than 3 layers of gravel, in addition to the layer of torpedo sand, shall be provided in accordance with the following size and depth distribution when used with perforated laterals or strainer nozzles. Reduction of gravel depths may be considered upon justification to the department when proprietary filter bottoms are specified. NR 811.49(1)(g)(g) Filter bottoms and strainer systems. Departures from these standards by using proprietary bottoms may be approved by the department on a case-by-case basis if the effectiveness of the method is demonstrated. Porous plate bottoms may not be used where iron or manganese may clog them or with waters softened by lime. The design of manifold type collection systems shall: NR 811.49(1)(g)2.2. Assure even distribution of washwater and even rate of filtration over the entire area of the filter. NR 811.49(1)(g)3.3. Provide a ratio of the area of the final openings of the strainer systems to the area of the filter of about 0.003. NR 811.49(1)(g)4.4. Provide a total cross-sectional area of the laterals about twice the total area of the final openings of the strainer system. NR 811.49(1)(g)5.5. Provide a cross-sectional area of the manifold at 1.5 to 2 times the total cross-sectional area of the laterals. NR 811.49(1)(g)6.6. Lateral perforations without strainers shall be directed upwards. NR 811.49(1)(h)(h) Surface wash. Surface wash facilities consisting of either fixed nozzles or a revolving mechanism are required unless air scour equipment is provided. All surface wash devices shall be designed with: NR 811.49(1)(h)2.2. Volume of flow of 2.0 gallons per minute per square foot of filter area with fixed nozzles and 0.5 gallons per minute per square foot with revolving arms. NR 811.49(1)(h)3.3. A vacuum breaker installed above the high water elevation in the filter or other approved device to prevent back siphonage. NR 811.49(1)(i)(i) Air scouring. Air scouring may be provided in place of surface wash. The following requirements apply: NR 811.49(1)(i)1.1. Air flow for air scouring the filter shall be 2 to 5 standard cubic feet per minute per square foot of filter area when the air is introduced in the underdrain. Air scour distribution systems placed above the underdrains shall use the lower end of the range. NR 811.49(1)(i)2.2. A method for avoiding excessive loss of the filter media during backwashing shall be provided. NR 811.49(1)(i)3.3. Air scouring shall be followed by a fluidization wash sufficient to restratify the media. NR 811.49(1)(i)5.5. Air scour distribution systems shall normally be placed below the media and supporting bed interface; if placed at the interface the air scour nozzles shall be designed to prevent media from clogging the nozzles or entering the air distribution system. NR 811.49(1)(i)6.6. Piping for the air distribution system may not be flexible hose which will collapse when not under air pressure and may not be a relatively soft material which may erode at the orifice opening with the passage of air at high velocity. NR 811.49(1)(i)7.7. Air delivery piping may not pass down through the filter media nor may there be any arrangement in the filter design which would allow short circuiting between the applied unfiltered water and the filtered water except if all of the following criteria are met: NR 811.49(1)(i)7.a.a. The vertical piping is double wall, welded at top and bottom, schedule 40 stainless steel for the internal pipe and schedule 5 stainless steel for the external pipe. NR 811.49(1)(i)7.c.c. An air connection to the double-wall annulus shall be provided including piping with a pressure gauge, regulator, flow switch and ball valve along with an air reservoir and compressor. NR 811.49(1)(i)7.d.d. The flow switch shall alarm and trigger filter shutdown if a pressure drop of over 10 psi is detected. NR 811.49(1)(i)8.8. The backwash delivery system shall be capable of 15 gallons per minute per square foot of filter surface area; however, when air scour is provided, the backwash rate shall be variable and may not exceed 8 gallons per minute per square foot unless operating experience shows that a higher rate is necessary to remove scoured particles from filter surfaces. NR 811.49(1)(i)9.9. The filter underdrains shall be designed to accommodate air scour piping when the piping is installed in the underdrain.
