Swimming Pool Codes - City of Fargo

water) to the filter in an amount sufficient to cover the sand. Soak the sandseveral hours, re-circulating through the make-up water tank when possible.Drain to waste and thoroughly backwash.The filter piping should have a re-circulation line to permit re-circulation of the water fromthe effluent back to the influent for chemical cleaning of the filter media. This line should beabout one-half the size of the main return piping.Sand contaminated with oil and grease had been traditionally cleaned with a lye solution.This procedure should only be used if other materials are not effective. Lower the waterlevel in the filter to within 2 inches of the top of the sand and apply the lye at a rate of 1pound per square foot of the sand surface. Allow the upper portion of the sand to soakseveral hours and slowly empty the filter to waste. Follow with a longer than usualbackwash. This treatment will prevent replacement of costly filter sand. Provide adequatewash water flow rates to prevent further accumulation of such conditions. Use extremecaution when handling lye as it can cause severe burns. Lye is particularly destructive to thetissue of the eye and may cause loss of sight.5.4 HIGH RATE SAND FILTERSHigh rate sand filters may be operated at filter flow rates of up to 10 gallons per minute persquare foot of surface area (as compared to 2 to 3 for conventional sand filters). Filtration isaccomplished in-depth rather than on the surface, thereby, increasing the dirt holdingcapacity of the filter.Operation and cleaning of high rate sand filters should be compliant with the instructionssupplied by the manufacturer. Usually the filter is back-washed when the differentialpressure reaches 25 psi. The design of the under drain system causes strong agitation,rubbing together the sand grains to release suspended materials. Trade literature indicatesthat the collected solids are generally discharged during the first minute or less of backwashwater source within about 2 minutes. Recommended backwash periods vary, but for somefilters it is approximately 2 ½ minutes.High rate sand filters have had erratic success in North Dakota. Operation of the filter at aconsiderably reduced flow rate after back washing may prove beneficial. The reduced flowwill allow the suspended materials to collect and settle on the sand, rather than being drivencompletely through the clean filter media. Once a coating of suspended material has beenbuilt up on the filter surface, the flow may be increased to the normal re-circulation rate.5.5 DIATOMACEOUS EARTH FILTERSThis type of filter has several advantages over the sand filter; greater removal of turbidity,smaller size and the elimination of chemical coagulants such as alum. They also require verylittle water for back washing.20