Vital layer in slow sand filter is seen in -

Elements of slow sand filter: supernatant or raw water (1-1.5 metre) a bed of graded sand an under-drainage system a system of filter control valves The surface of the sand bed gets covered with a slimy growth known as "Schmutzdecke", vital layer, zoological or biological layer. The formation of the vital layer is called ripening. It is the hea of the slow sand filter Park's textbook of preventive and social medicine.K Park. Edition 25. page no:772