06-I Water Production Study - Laserfiche WebLink

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or automatic effluen[ IIow control valve. At the begirming of the filter run, the filter is clean and has little resistance. If the fiill driving force were applied across the filter, the flow rate would be very high. To maintain a constant flow rate or water level, some of the available driving force is dissipated by a effluent control valve. Variable declining rate filters operate with no individual control rystems except for a flow limiting device (orifice plate). The T'NRCC allows at the beginning of filter run a maximum range for declining rate filters, the maximum filiration rate is 6.5 gpm/sf. The filter dischazge piping is usually designed wich an ori5ce or other permanently installed flow limiting device to ensure that the maximurn filter rate can not be exceeded. In variable declining rate filters, each filter will accept the porcion of the total flow permitted by the common water level above all the filters. The water level above the filters, or head, provides the driving force for the water through the filter media As filtrarion continues, the flow through dirtiest filter will decrease more rapidly than tlvough the other filters, causing the flow to re-distribute itself automatically so that the cleaner filters pick up the capaciry lost by the dirtier filte{s. The water level rises slighNy and the re-distribution of flow to provide the additional head needed by cleaner filters to compensate for the decreased flow through the dirtier filters. The cleanest filter accepts the greatest flow increase and the redistriburion. This method of operation causes a gradually declining rate towazd the end of a filter run. Rate changes throughout the day due to changes in total plant flow, both upward and downwazd, occur gradually and smootkily without any automatic control equipment. The declining rate fillrarion rystems main advantage is the savings in valve and piping arrangements. It is a simpler rystem to operate and allows for greater operating flexibility. Figure 5 illustrates the variable declining rate filter structure. 6) Air assisted backwash capabilities provided at the filters. There are three tradi[ionally used backwash methods: • water only backwazh • water and surface or sub-surface wash media scour • water and air scour ~ Pigure 5: Declining Rate Filter Structure ~ ~