The vital layer is layer of slimy gelatinous material which covers the surface of the sand bed after a few days. It consists of thread like algae, plankton, diatoms and bacteria. When fully formed it extends for 2 - 3 cm into the top poion of the sand bed. It removes organic material, holds back bacteria and oxidises ammoniacal nitrogen into nitrates and helps in yielding a bacteria free water. Ref: Park, Edition 21, Page - 654