Many pool professionals consider D.E. swimming pool filter to be the finest, because it is capable of removing smaller particles than either sand or cartridge. Diatomaceous earth (D.E.) is a porous powder with microscopic openings, that look like tiny sponges. Clear water can pass through these openings, but particles, as small as one to three microns, are trapped during the first pass through.The D.E. swimming pool filters have internal elements that become coated with D.E. It is this 'filter cake' that strains dirt, dust, algae and bacteria from the water. Similiar to the sand type, when a D.E. swimming pool filter becomes dirty, it is cleaned either by backwashing, or regenerating and draining.