Filtration is an essential part of swimming pool water treatment and its importance has been emphasised in recent years due to several outbreaks of cryptosporidiosis, because this organism is not killed by the disinfectant in the pool water and therefore must be removed in order to prevent bathers becoming infected by it.
Chemical by-products of the disinfection process will also remain in the swimming pool water until they are filtered out, not to mention physical pollution such as dirt and sand etc.
Filtration is a fairly simple process; water leaves the pool via the deep end outlets and the surface water draw-off system (deck-level, skimmer baskets, overflow channels etc.). It is piped to the plant room and gets directed into the top of the filter (or several filters in large pools), passes through the filter media (usually sand) where all the contaminants and pollution are trapped and the pool water comes out of the bottom and continues through the remaining components of the pool plant system.
There are 3 main processes happening during filtration:
Involves dirty water passing through the filter media and particles of pollution becoming trapped in the small gaps (pores) between the grains of sand because they are too large to pass through.
This is where fine particulate matter settles on the upward-facing surfaces of the sand grains. The process of sedimentation can remove finer particles of pollution than straining. As the amount of sediment increases, the amount of space in between sand grains (pores) decreases. This will cause the velocity of water through the filter to increase. Further sedimentation can then no longer occur and, due to the higher velocity, some sediment could get pushed further down into the filter bed.
This is where particles of pollution adhere to the sand grains. It is not to be confused with absorption. With adsorption, very small particles of pollution adhere to the surface of the sand grains. This process is promoted by electrostatic charges within the particles (similar to a balloon 'sticking' to a wall). Once particles begin to adhere to the sand grains, a sticky coating builds up, which promotes further adherence of particles onto the filter media.