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)
Further reading
It’s hard to imagine now, but there was a time when people disagreed over the best word to use when answering the phone. Alexander Graham Bell suggested answering with ahoy! but Thomas Edison was partial to hello! A fascinating new book about...
The edge of the Grand Canyon. A remote mountaintop. A medieval cathedral. Some places are so mystical you feel like you’re close to another dimension of space and time. There’s a term for such locales: thin places. And: did you ever go...