<prompt> Vital layer in slow sand filter is seen inTop of waterOn the sand bedNear filterNone  <response>: Ans. is b i.e., On the sand bed Vital layer is seen into the top poion of sand bed not on the top of water. Slow sand filter Also known as biological filter It has following layers from above down words : - 1. Supernatant raw water to be purified. 2. Sand bed 3. Gravel suppo 4. Filter bottom o Vital layer, when full formed, extends for 2 to 3 cm into the top poion of sand bag. 