Cleaning period for a rapid sand filter is taken as-

Slow sand filter: They consist of fine sand, supported by gravel. They capture particles near the surface of the bed and are usually cleaned by scraping away the top layer of sand that contains the particles.

Cleaning:

• For cleaning, the top layer of sand is scrapped and 1.5 to 3 cm of sand is removed in each cleaning.
• The frequency of cleaning is 1 to 3 months.
• Rate of filtration is 100-200 l/hr/m2 of filter area.
• It is used for smaller plants in villages.

Rapid sand Filter: They consist of larger sand grains supported by gravel and capture particles throughout the bed. They are cleaned by backwashing water through the bed to 'lift out' the particles.

Cleaning:

1. Cleaning of filter can also be done using surface wash but it should be accompanied by back wash.
2. Washing period = 24 to 48 hrs.