<prompt> In a slow sand filter ,the element responsible for yielding bacteria freewater is theValveVital layerSupernatant waterUnder drainage system  <response>: SLOW SAND OR BIOLOGICAL FILTERS Slow sand filters were first used for water treatment in 1804 in Scotland and subsequently in London. During the 19th century their use spread throughout the world. Even today, they are generally accepted as the standard method of water purification. Elements of a slow sand filter Essentially these consists of : 1 supernatant raw water 2 a bed of graded sand 3 an under-drainage system; and 4 a system of filter control valves. Vital layer : When the filter is newly laid, it acts merely as a mechanical strainer, and cannot truly be considered as biological. But very soon, the surface of the sand bed gets covered with a slimy growth known as Schmutzdecke, vital layer, zoogleal layer or biological layer. This layer is slimy and gelatinous and consists of threadlike algae and numerous forms of life including plankton, diatoms and bacteria. The formation of vital layer is known as ripening of the filter. It may take several days for the vital layer to form fully, and when fully formed it extends for 2 to 3 cm into the top poion of the sand bed. The vital layer is the hea of the slow sand filter. It removes organic matter, holds back bacteria and oxidizes ammoniacal nitrogen into nitrates and helps in yielding a bacteria-free water. Until the vital layer is fully formed, the first few days filtrate is usually run to waste. Ref: Park 25th edition Pgno : 751 