Correct option is B
Slow sand filter consists of fine sand as the filter media. After cleaning the filter, it is not used for next 24 to 36 hours since we need to wait for the formation of Schmudzdecke layer, which plays a crucial role in the working of slow sand filter.
The cleaning of slow sand filters is not done by back washing as is done for rapid gravity filters at least 30 min, but is done by scrapping and removing the 1.5 to 3 cm of top sand layer.
