GB1520439A - Method for recovering backwash liquid - Google Patents

Abstract

1520439 Filtering with filter aid ECODYNE CORP 2 Dec 1975 [23 Dec 1974] 49476/75 Heading BID One or more filters using a filter aid layer, particularly of mixed anion and cation resins of 60-400 U.S. mesh, are cleaned using water which is subsequently separated from the filter aid and stored for use in the next cleaning cycle. In the cleaning cycle the water is first passed forwards through the filter to purify the water and then recycled through the filter in the reverse direction to clean the filter. Filters 10, 12 are cleaned alternately so that one is always on service flow, e.g. in the boiler water circuit or treating fuel pool water of a nuclear reactor. Each filter housing contains nylon wound tubular filter elements and service flow is from 14 to 18. When the pressure drop across a filter or the effluent conductivity indicate that the filter aid has clogged with solid impurities or become saturated with dissolved impurities it is taken out of service for cleaning. Tank 50 contains fairly pure water recovered from the last backwashing cycle and this is purified, either by adding resins to tank 50 and pumping the water round circuit 50, 66, 80, 70 or by adding a fresh layer of resin from mixing tank 30 to the filter, on top of the contaminated layer, before pump 80 conveys the water from tank 50 forwards through the filter to tank 48, via valves 86, 24, 28, 56. Valves 86, 56 are closed, make-up water may be added at 64, valves 60, 82, 78 are opened and pump 80 delivers the purified water from tank 48 back through the filter, flushing the resins and water back to tank 50. The backwashing may be accompanied by airscouring. While a fresh filter aid layer is being applied to the clean filter from tank 30, the contaminated water is pumped from tank 50 via valves 98, 104 to settling tank 52. After a settling period, water is pumped via valves 92, 102 from tank 52 to tank 50, leaving the solids to be discharged at 106. If a filter has clogged with solids, then during forward passage of water therethrough from tank 50 to tank 48 a lower flow rate than normal service flow is employed to minimise pressure drop. Typical flow rates are exemplified. Filters may have individual pumps (not shown) to maintain forward flow round a closed circuit, holding filter aid layers in place when the filters are otherwise isolated.