<prompt> Vital layer in slow sand filter is seen in -Top of waterOn the sand bedNear filterNone  <response>: 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 Parks textbook of preventive and social medicine.K Park. Edition 25. page no:772 