FILTERS AND STRAINERSDOE-HDBK-1018/2-93Miscellaneous Mechanical ComponentsIf too much filter medium is used, the layer that builds up on the septums will bridge the areabetween the septums. When the filter is backwashed, these bridges are usually not removed.Therefore the bridging continues, and the filter runs become progressively shorter. Eventually,the filter must be opened and the filter medium must be removed manually.Precoat filters are much more complicated than cartridge filters, and the equipment required ismuch more expensive to install and maintain. The major advantage of precoat filters is theremote operation, which eliminates the physical handling of highly radioactive filter cartridges.Deep-BedFiltersDeep-bed filters are usually found only in makeup water systems, where they are used to filterwater after it has been treated in a clarifier. They are used to remove organic matter, chlorine,and very fine particulate matter.A deep-bed filter is based on a supportFigure 22 Deep-Bed Filterscreen (decking), which is mounted afew inches above the bottom of thetank. The screen is perforated toallow water to flow through it. Acoarse, aggregate layer of crushed rockor large lumps of charcoal is placedon top of the screen, and the deep beditself (2 to 4 feet of granular anthraciteor charcoal) is placed on top of theaggregate. The filter is sized so thatthere is 1 to 2 feet of "free board"above the deep bed.When the filter is in service, rawwater is pumped in through a pipe thatfeeds a distribution pipe above thedeep bed. The water is filtered as itpercolates down through the granules.(Charcoal granules will filter outorganic matter, chlorine, and fineparticulates, while anthracite granulesremove only the particulates.) Thewater collects in the bottom of thetank, below the support screen, andleaves the filter through a pipe in thebottom of the filter vessel.ME-05Rev. 0Page 44
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