Filtration is simply the process of removing debris, by pushing water through a filter media of some kind. In a pool, this is a plumbing system set in motion by an electrical pump that circulates the water from the pool through the filter and back into the pool. The filter traps debris before the water is pushed back into the pool.
In most filtering systems, water is pulled from the surface via skimmers, using the natural surface tension of water. Large pieces of debris, like leaves, are left in the skimmer baskets. The water then travels to the filter, where smaller debris is trapped, before the water is chemically treated and then pushed back into the pool itself through the return inlets. This is all done in a continual flow that goes unnoticed by swimmers.
There are three basic kinds of filtering systems: cartridge, sand and DE (diatomaceous earth). A cartridge filter is much like coffee filter. Made of corrugated paper or polyester cloth, a cartridge filter can be removed and cleaned with a hose. Cartridge filters must be replaced after several years, as they lose their efficiency.
In pools with sand filters, the debris is trapped in tanks with a layer of sand. Gravity pulls the water down through the sand, where fine debris is trapped. The filtered water is then pushed through the outlet pipe and back into the pool through the return inlets.
DE filters depend on diatomaceous earth, which is the ground fossilized remains of sea organisms called diatoms. In this system, grids are covered with a fabric septum and then coated with DE. The water flows through the DE and through the septum on the the grids, where debris is trapped. DE is particularly good at trapping very fine debris.
Pressure gauges indicate when the filters need to be cleaned or replaced or when the pool should be backwashed. Backwashing is the process of reversing the flow of water and closing the pool outlet pipe. The water is pushed backwards through the filtering system and then output through a waste pipe or a collection tank depending on the type of filter system.
Clearly, a properly functioning filtering system is important for a clean pool. All of the components — skimmers, pump, motor, plumbing and filters — should be monitored carefully to be sure that they are in top working condition.