Vital layer in slow sand filter is seen:: (A) On top of the water, (B) On the sand bed, (C) Just below the supernatant water, (D) None of the above
Answer is B. When a sand filter is newly laid, it merely acts as a mechanical strainer. Soon the surface of sand bed is covered by a slimy growth known as vital layer consisting of algae, planktons, bacteria, diatoms etc. This vital layer holds back bacteria and oxidises ammoniacal nitrogen to nitrates and yields a bacteria free water. Ref: Park 21st edition page: 659.