Sand
Water is pushed through a bed of filter sand and removed through a set of lateral tubes at the bottom. The principle behind this filter is that water is pushed through the filter sand, somewhat like an espresso machine. Dirty water goes in the top and clean water exits out the bottom. As the filter sand becomes plugged with debris from the pool, the pressure increases on the filter and the water flow drops. To keep a sand filter working, depending on the size of the filter you must clean it as often as once a week during swimming season. Maintenance means backwashing where the flow of clean water is reversed back into the filter. The problem with this, however, is that backwashed water is simply wasted. A typical backwashing session can waste a few hundred gallons of water – water that must be replaced in the pool. Also, on a micron-to-micron comparison, sand filters remove the least amount of dirt – particles as small as 20 to 25 microns.

Cartridge
Water passes though the filter material and the debris are captured there. This is just like the water filters used under your sink. Cartridges have much more available area to filter than sand. Most start at 100 sqft and the majority of the cartridge filters sold are larger than 300 sqft. Cartridge filters are designed to run at lower pressure than sand. This puts less back-pressure on the pump and hence you get more flow and turnover for an equivalent pump size. Generally these filters have to be cleaned once or twice a season by simply hosing them off, so they don't require much labor-intensive maintenance. In terms of particle size filtered out, cartridge is somewhere between sand and DE.  The cartridge can filter out anything down to about 5 to 10 microns in size. A grain of table salt is about 90 microns; anything below about 35 microns is invisible to the naked eye.

D.E.
Diatomaceous earth is mined and is the fossilized exoskeletons of tiny diatoms. They are used to coat “grids” in the filter housing and act as tiny sieves to remove debris. They are very small and can filter out particles as small as 5 microns. Diatom filter area are sized between sand and cartridge – around 60-70 sqft are most common. Once the filter pressure rises, the filter is backwashed just like a sand filter and then “recharged” with more DE powder. Typically it is poured in a slurry into the skimmer and it then coats the filter grids.  DE Filters are usually the most expensive type, and they get your pool water cleaner than the other filters. These products can filter out dirt as small as 3 to 5 microns, but the necessary maintenance can be a drawback for some homeowners. 
 

Once you have chosen the right type of filter for your pool, it is important to size it properly. It must be sized to match the pump which in turn must match the circulation needs of the pool. If the filter is undersized, it will fill up quickly and also can be damaged by the force of the pump. If the filter is undersized, it may not backwash well. It is important to keep in mind that the manufacturer's flow ratings for filters are often very optimistic. Our experience suggests that the following guidelines are much more realistic.