Cleaning is done by (i) Scraping and removal in filters slow sand (ii) Back washing in slow sand filters (iii) Scraping and removal in filters rapid sand (iv) Back washing in rapid sand filters The correct answer is A. (i) and (ii) B. (ii) and (iii) C. (i) and (iv) D. (ii) and (iv)

The correct answer is (ii) and (iv).

Slow sand filters are cleaned by scraping and removal of the top layer of sand, which is about 25 mm thick. This layer becomes clogged with dirt and bacteria over time, and needs to be removed to maintain the filter’s effectiveness. Rapid sand filters are cleaned by backwashing, which is a process of reversing the flow of water through the filter. This causes the sand to be agitated and the dirt and bacteria to be washed out of the filter.

Option (i) is incorrect because slow sand filters are not cleaned by backwashing. Option (iii) is incorrect because rapid sand filters are not cleaned by scraping and removal of the top layer of sand. Option (iv) is correct because rapid sand filters are cleaned by backwashing.