Of the three types of swimming pool filters (see Swimming Pool Filters: Cartridge and Swimming Pool Filters: Sand), diatomaceous earth filters, also called DE filters, capture the finest particles— as small as 3 microns.

Water passes into the bottom of the filter and is pushed up through a number of grids, each of which is coated with a fine powder called diatomaceous earth, an extremely fine powder actually made of fossilized plankton.

Though it is the most effective pool filter, there are several reasons it is not a popular choice for a residential pool. It is the most expensive and also requires a good deal more maintenance than does a cartridge filter.

To clean the filter, you must reverse the water flow to “backwash” the filter and then drain out the dirty water.

Unfortunately, you may need to use as much as 500 gallons of water to do this, and the washed-out diatomaceous earth is a hazardous material that must be disposed of in accordance with local regulations. Plus, many experts think that backwashing is not good enough; they say the grids should be removed and cleaned individually.