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

conveniently on the filter plant operating floor and equipped with suitablelengths of 1-inch to 1½-inch pressure hose and nozzle. A suitable rackshould be available for storing the hose. Auxiliary surface or subsurfacewash may be accomplished by a system of fixed nozzles or a revolving typeapparatus. All devices shall be designed with:a. Provisions for water pressures of at least 45 psi;b. A properly installed backflow prevention assembly to prevent backsiphonage if connected to the treated water system; andc. Rate of flow of 2.0 gallons per minute per square foot of filter areawith fixed nozzles or 0.5 gallons per minute per square foot withrevolving arms.4.2.1.8. Air Scouring.Air scouring may be considered in place of surface wash based on thefollowing standards:a. Air scouring controls must allow the operator to control the air andwater flow rates and duration. Rate of flow indicators for air andwater shall be provided. Provide manual over-ride to the automatedbackwash controls for backwashing the filters including air scour.Automated backwash controls shall not automatically start filterbackwash. Filter backwashing must be initiated manually;b. Air flow for air scouring the filter must be three to five cubic feet perminute per square foot of filter area when the air is introduced in theunderdrain; a lower air rate shall be used when the air scourdistribution system is placed above the underdrains;c. A method for avoiding excessive loss of the filter media duringbackwashing must be provided;d. Air scouring should be followed by a fluidization wash to restratify themedia;e. Air must be free from contamination;f. Air scour distribution system should be placed at or below the mediaand supporting bed interface; if placed at the interface, the air scournozzles shall be designed to prevent media from clogging the nozzlesor entering the air distribution system;g. Piping for the air distribution system shall not be flexible hose whichmay collapse when not under pressure and shall not be relatively softmaterial which may erode at the orifice opening with the passage of airat high velocity;h. Air delivery piping shall not pass down through the filter media unlessa minimum of two anti-seepage collars, six inches apart are providedin each pipe. The anti-seepage collars shall extend three inches outfrom the pipe and be continuous around the entire circumference of thepipe. No arrangement in the filter design shall allow short circuitingbetween the applied unfiltered water and the filtered water;i. Consideration should be given to maintenance and replacement of airline;Aug. 29, 2003Page 59