<prompt> The vital layer of slow sand filter isSand bedUnder drainageZoological layerSupernatant  <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, 25th edition, Page No. 772 