Key Facts to Filtration and Backwashing

By Richard Lamburn, STA Pool Plant Technical Support Officer

Swimming pool water contains various pollutants introduced by bathers, the pool operator and the source water supply. In order to purify and clean the pool water we pass it through a filtration system.

What happens in the filter?

Pool water is passed vertically downward through a bed of sand (or other media) where the sand has regular grain sizes. There are thus fairly evenly sized gaps between the sand grains. By regulating the flow rate so that very small particles are allowed to become trapped between the angled faces of the sand grains; a typical pool filter will trap particles as small as 10 microns. As some of the gaps become narrowed and as they become partially blocked by fine debris it is possible to reduce the particle size entrapment to 5 microns. In order to trap particles of even smaller size, flocculation or coagulation of these very small particles is necessary.

Current industry regulations recommend a filtration velocity between 10 and 25m3/hr (m/hr). This should filter out particles as small as 5 microns, when mature, though is rather less effective immediately after backwashing. This is known to the industry as Medium rate filtration.

filtration mode

As the pool water runs through the filtration mode particles become entrapped on the top of the sand bed, this results in a build up of pressure on the inlet side of the filter and a decrease in outlet pressure. If more resistance occurs during the filtration mode turnover times will be effected and this gives us an indication that the filter needs to be cleaned.

How do we backwash?

Backwashing involves reversing the flow of water through the filter in order to fluidise the bed and lift the particle build up and sending it to drain.


  • Back wash according to pressure differential and/or time.
  • Until sight glass runs clear.
  • When it’s safe to do so.

ineffective backwashing

Ensure that when backwashing manufactures guidelines are followed on water velocity through the filter.  At least 30m3/m2 per hr to ensure the bed is lifted and pollution is sent to drain.

Pool Plant

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