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Six of the possible eight outside filters (Figure 10) have been fitted <br />and are in use at present. These filters are of the same size and capacity. <br />For some reason, these exterior units are subject to shorter filter runs, <br />Although not positively determined, this is probably due to the installation <br />of a filter sand of a small effective grain size. <br />There are no controls on any of the filters. The only arrangement which <br />could be construed as a control is the installation of inverted influent pipes <br />which guarantees an equal application of water to each filter until the filter <br />slows down sufficiently to submerge the influent pipe. <br />Figure 10 Figure 11 <br />The filters are backwashed with finished water from a 25,000 gallon tank <br />(Figure 11) located on a high ridge above the filter building. This tank has <br />sufficient capacity to backwash one filter and is refilled by two pumps hav- <br />ing suction into the clear -well. <br />The filter beds appear to be in very good condition and the plant opera- <br />tor has told us that he has never experienced any problems with "mud - balls" <br />or filter channeling. <br />- 34 - <br />