Filtration is a process whereby suspended solids are removed from the influent by forcing the water through a filter of porous medium such as sand or sand with anthracite or coal. The purpose of filtration is to reduce the concentration of suspended solids, such as carbon columns, prior to certain treatment processes. The most common filter is a dual-media system with a layer of anthracite over a layer of sand. This filter provides better suspended solids removal with longer filter runs at higher flow rates than the more conventional single-medium filter (Corbitt 1990). The design of filters is based on the flow rate, flow scheme, and the type of medium used in the filter as discussed in Chapter 8. Up to 75% of suspended solids can be removed by dual-media filters operating at flow rates ranging from 2 to 8 gpm/ft (Oakley et al. 1994), bed depths of 24 to 48 inches, sand to anthracite ratios of 1:1 to 4:1, and a filter run of 8 to 148 hours (Corbitt 1990).

Filtration is a reliable and effective means of removing low levels of solids provided that the solid content does not vary greatly. Also, periodic filter backwashing is necessary to remove collected materials from the media. Typical backwash flow rates are 15 to 25 gpm/ft (Oakley et al. 1994) for eight to ten minutes (Corbitt 1990). The spent backwash water can be routed to the plant's headworks or to an intermediate process which provides settling.

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