Schmutzdecke (German, "grime or filth cover", sometimes incorrectly spelt Schmutzedecke) is a complex biological layer formed on the surface of slow sand filter beds. The Schmutzdecke is the layer that provides the effective purification in potable water treatment, the underlying sand providing the support medium for this biological treatment layer.

The composition of any particular Schmutzdecke varies, but will typically consist of a gelatinous matrix of bacteria, fungi, protozoa, rotifera and a range of aquatic insect larvae. As a Schmutzdecke ages, more algae tend to develop, and larger aquatic organisms may be present including some endoprocta , snails and annelid worms.