Answer : Option D

Explanation :

A small biological sand filter (slow sand filter or Biosand filter) is maintained or cleaned by actively managing only the top 5 cm of sand. This top layer is either removed and replaced when the flow rate becomes unacceptable (typically 4 to 6 months) with clean sand or "wet harrowed" meaning the sand is gently agitated causing partial break up of the biological surface. This may be slightly different for a very large slow sand filter; more sand and more work.

A rapid sand filter is maintained and cleaned by forcefully injecting water backwards through the entire sand bed every 2 or 3 days. Backwashing a biological sand filter will destroy it and may result in people getting very sick if water from it is being consumed.