backwashing

Backwashing is a cleaning procedure used in filtration systems to remove accumulated solids from the filter media by reversing the flow of water. It is commonly applied in drinking water treatment, wastewater treatment, industrial process filtration, and swimming pool filtration.

In granular media filters such as those using sand or composite beds, forward filtration traps particles between grains. Backwashing reverses flow to fluidize the bed, lift trapped solids to the surface, and carry them out through a waste line. This process helps restore the permeability of the filter and maintain its contaminant removal performance.

Most systems perform backwashing in two stages: backwash, which uses reversed flow to suspend and flush out solids, and rinse, which temporarily returns the flow in the normal direction to recompact the bed and remove remaining fines before returning to service. The exact flow rates, duration, and valve sequencing depend on the filter design and media.

Control is typically automatic, using differential pressure, turbidity, or flow indicators to determine when cleaning is complete. Operators monitor head loss or effluent quality to determine appropriate backwash frequency and duration.

Disadvantages include water and energy consumption and the generation of wastewater that must be managed or treated. Improper backwashing can damage media or cause loss of fines and reduce filter effectiveness. Backwashing is distinct from chemical cleaning or media replacement and is generally used to extend filter life and maintain filtration efficiency.

See also filtration, head loss, and filter media.