Slow Sand Filter Essentially, a concrete basin covered with graded gravel and about 3 feet of sand used to pass raw water through at low velocity, resulting in substantial removal of chemical and biological contaminants. Cleaning is accomplished by removing the surface layers of sand at periodic intervals. These are then washed and stored for reuse. The slow sand filtration process is simple, reliable and relatively inexpensive. The slow sand filter is an appropriate technique for removing suspended organic and inorganic matter and also may remove pathogenic organisms. The filter reduces bacteria, cloudiness, and organic levels, thereby reducing the need for disinfection and, consequently, the presence of disinfection byproducts in the finished water. There are limitations, however. Slow sand filters require a large land area, large quantities of filter media, and manual labor for cleaning. Water with high turbidity levels can quickly clog the fine sand used in these filters. Also, slow sand filters are less effective at removing microorganisms from cold water because as temperatures decrease, the biological activity within the filter bed declines. In operation, a sticky mat of biological matter, called a Schmutzdecke, forms on the sand surface, where particles are trapped and organic matter is biologically degraded. Slow sand filters rely on this cake filtration at the surface of the filter for particulate straining. As the surface cake develops during the filtration cycle, the cake assumes the dominant role in filtration rather than the granular media.
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Author of the Water Words Dictionary source of text: Gary A. Horton
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