The vital layer in a slow sand filter is?

Sandfliters are two kinds slow sand filtwr and rapid sand filter.Slow sand fliter contains supernatant water,a bed of graded sand,under drainage system,system of filter control valves.When filter is newly laid it act as a mechanical filter later the surface of sandbed gets covered by aslimy growth called Schmutzdecke,vital layer,zoological layer or biolkgical layer.The formation of vital layer is called ripening of filter. The vital layer containing thread like algae, planktons,diatoms and bacteria act as hea of sand filter.It removes organic matter,holds back bacteria and oxidises ammonuacal nitrogen into nitrates and help in yielding bacteria free water Parks textbook of preventive and social medicine.K Park. Edition 23.Pg no:712