schmutzdecke
n.— «After the filter has been operated for some time a gelatinous layer (Schmutzdecke of Piefke) is formed of such imperviousness that each square meter of surface will no longer furnish as much as 3 cubic meters (800 gallons) of water in twenty-four hours.…Whenever the demand for the filter is not too great it is allowed to rest after cleaning for some time, in the belief that those particles of the dirt deposit (Schmutzdecke) which have penetrated unto the lower layers will be oxidized under prolonged contact with the air.» —Report of the Secretary of War (Washington, D.C.) , 1894. (source: Double-Tongued Dictionary)
Further reading
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