Design Guide for Community Water Systems - The Water, Sanitation ...

j. During air scour, the backwash water rate must be variable and shouldnot exceed eight gallons per minute per square foot unless operatingexperience shows that higher rate is necessary to remove scouredparticles from the filter media surfaces;k. The filter underdrain shall be designed to accommodate air scourpiping when the piping is installed in the underdrain; andl. Subparagraph 4.2.1.10 must be followed when backwashing filters.4.2.1.9. Appurtenances.a. The following shall be provided for every filter:1. Influent and effluent sampling points;2. An indicating, and preferably recording, loss of head gauge ortransmitter;3. An indicating rate-of-flow meter. A rate controller that limits therate of filtration to a maximum rate through each filter may beused. However, equipment that simply maintains a constantwater level on the filters is not acceptable, unless the rate of flowonto the filter is properly controlled. A pump or a control valveor a flow meter in each filter effluent line may be used as thelimiting device for the rate of filtration only after consultationwith the department;4. Influent or effluent controls that distribute plant flows evenlybetween the filters and limit the flow rate to the maximum allowablerate; and5. Provisions for filtering to waste, especially when used in surfacewater, with appropriate measures for backflow prevention.b. It is recommended the following be provided for every filter:1. A continuous turbidity monitoring and recording device forsurface water treatment and lime softening plants to monitor filterto waste and filter effluent turbidity,2. Wall sleeves providing access to the filter interior at severallocations for sampling or pressure sensing.3. A minimum 1½ inch pressure hose equipped with shut-offnozzles or valve and storage rack at the operating floor forwashing filter walls; and4. Turbidity and/or particle monitoring equipment as a means toenhance treatment operation when treating surface waters.c. When the top of the filter walls are below the pretreatment basinoverflow, the filter basins must be provided with overflow piping.4.2.1.10. Backwash.Provisions shall be made for washing filters as follows:a. A minimum backwashing rate of 15 gallons per minute per squarefoot, consistent with water temperatures and specific gravity of thefilter media shall be provided. A rate of 20 gallons per minute perAug. 29, 2003Page 60