Backwash mechanisms

Backwashing is a technique used for cleaning sand filters. This is employed in-case the filter becomes dirty which is indicated by loss of head. In order to complete this operation,

• The raw water supplies and the filtered water supply should be cut-off and the bed has to be drained.
• Treated water from storage is used for the backwash cycle. 
• The washwater along with compressed air is sent back upwards through the filter beds. 
• This forced upward movement of washwater and compressed air agitates the sand particles thereby removing suspended impurities from it. 
• The filter backwash rate should be high enough to expand and agitate the filter media and suspend the floc in water for removal. However, if the backwash rate is too high, it may push the sand out of the filter. 
• The process of washing the filters and removing the dirty water is continued for a period of three to five minutes. 
• The entire process of backwashing the filters and re-maintaining the filtered supplies takes about 15 minutes. 
• The amount of water required for washing a rapid gravity filter varies from 2 to 5 percent of the total amount of water filtered. 
• Rapid gravity filters get clogged very frequently and have to be washed every 24 to 48 hours.

The filter should be backwashed when:

• The head loss is so high that the filter no longer produces water at the required rate
• Floc starts to break through the filter
• Turbidity in the filter effluent increases
• A fillter run  reaches a given hour of operation