Diatomaceous earth (D.E.) is mined and is made up of the fossilized exoskeletons of tiny diatoms. They are used to coat "grids" in the filter housing and act as tiny sieves which help in the removal of debris as small as 5 microns. Upon rising pressure, the filter is backwashed (ran in reverse) just like a sand filter and then they are "recharged" with more D.E. powder. D.E. filters trap debris as small as 2 - 5 microns.