schmutzdecke
 n.— «This system, introduced into the U.S. in 1872, uses low water flow rates (0.05 gpm/sq ft) and relies on the formation of a “schmutzdecke” (a fine, mud-like layer) in the top surface of the fine sand (specific gravity of 2.6|+ or -~) for filtering. However, it may take several weeks for the filtration layer to form (during which the “filtered” water is lost) and this type filter is ineffective for high turbidity waters. Cleaning is by scraping off the schmutzdecke and top layer of sand, and placing a layer of new sand.» —“Filtration” Public Works Apr. 15, 1994. (source: Double-Tongued Dictionary)

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