Sand filters use a specially graded sand as the filter media. The water enters the tank through the diffuser.  As the water goes down through the bed of sand, the dirt and debris is trapped between the grains of sand.  When the water reaches the bottom of the filter, it enters the laterals and is returned to the pool.

Sand filters filter out debris down to about 40 microns in size.  Anything smaller than 40 microns will probably not be filtered out.

DE filters use diatomaceous earth as a filter media.  The DE filter has plastic grids covered with a plastic type of fabric.  A layer of filter powder called Diatomaceous Earth covers the grids and does the filtering.

As the water passes through the filter powder, any debris down to 5-8 microns is filtered out.  Because the DE is much finer that sand, it is able to filter much more finely than a sand filter.

CARTRIDGE FILTERS (also sold as Modular Media)

Cartridge filters use a paper-type cartridge as the filter media.  They do not filter as finely as DE, and in our experience produce about the same water quality as sand filtration.

Cartridge filters used to have a bad reputation as a nuisance to maintain, but manufacturers have come up with newer filters with enough surface area (300-500 square feet) to need cleaning only once or twice each year.  This makes the maintenance issue a plus with these filters.