schmutzdecke
n.— «This type of filter works because of a biological microbe named schmutzdecke. It grows on top of the sand and its job is to eat organisms in the water. It can take up to two weeks to form so any water before that is not fit to drink. Also, to keep ol’ Schmutzy alive you need to keep the water level above the sand at all times.» —“Sand Filter” by James M. Dakin Bison Survival Blog Jan. 20, 2007. (source: Double-Tongued Dictionary)
Further reading
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