Backwashing

As water passes through a filter, oxidized minerals (such as iron and manganese) and foreign matter collect in the top portion of the sand.  Continued build-up of these particles tends to clog the filter.  The clogged filter requires more force or pressure to pass water through and filter at the same rate as an unclogged filter.  A head loss gauge measures this greater pressure and shows when a filter needs to be cleaned.  

Both pressure filters and rapid sand filters can be cleaned by backwashing.  The diagrams below show how backwashing works in a pressure filter.  

Diagram of a pressure filter


Water passing through a pressure filter in the normal direction.

To backwash a pressure filter, the influent valve is closed and a waste line is opened.  Treated water from the system is pumped upward through the filter bed.  The water pumped upward has the velocity and volume to agitate the sand and carry away the foreign matter that has collected there. 

Backwashing a pressure filter


Water passing through a pressure filter in the reverse direction.

Backwashing normally takes about 10 minutes, though the time varies depending on the length of the filter run and the quantity of material to be removed.  Filters should be backwashed until the backwash water is clean.