Vital layer in slow sand filter is seen:

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.