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

square foot or a rate necessary to provide for a 50 percent expansion ofthe filter bed is recommended. A reduced rate of 10 gallons perminute per square foot may be acceptable for full depth anthracite orgranular activated carbon filter media;b. Filtered water provided at the required rate by washwater tanks, awashwater pump, from the high service main, or a combination ofthese;c. Washwater pumps in duplicate unless an alternate means of obtainingwashwater is available and if air scouring is provided, duplicate aircompressors/blowers except where water flow is adequate to backwashthe filters at the required rates with water alone;d. Not less than 15 minutes wash of one filter at the design rate of wash;e. Timer to record total backwash time;f. A washwater regulator or valve on the main washwater line to obtainthe desired rate of filter wash with the washwater valves on theindividual filters open wide;g. A totaling rate-of-flow on the main washwater line, with an indicatorso that it can be easily read by the operator during the washingprocess; andh. Equipment designed to prevent rapid changes in backwash water flow.4.2.2. Rapid rate pressure filters.4.2.2.1. General.a. The normal use of these filters is for iron and manganese removal.Pressure filters shall not be used in the filtration of surface water orgroundwater under the direct influence of surface water or following alime-soda softening process.b. The minimum requirements regarding number, rate of filtration,structural details and hydraulics, filter media, etc., for rapid rategravity filters (subsection 4.2.1.) also applies to pressure filters, whereappropriate.4.2.2.2. Rate of filtration.The rate of filtration shall not exceed three gallons per minute per squarefoot of the filter area. Higher rates may be considered based on satisfactoryresults of pilot or full-scale testing. The filter piping must be arranged assimple as possible to provide for filtration, backwashing and filtering towaste of each filter individually.4.2.2.3. Details of design.The filters shall be designed to provide for:a. Loss of head gauges on the inlet and outlet pipes of each filter;Aug. 29, 2003Page 61