Diatomaceous earth (D.E.) 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 allowing for the removal of debris as small as 5 microns. Once the pressure rises, the filter is backwashed just like a sand filter and then "recharged" with more D.E. powder. D.E. filters trap debris as small as 2 - 5 microns.