The filter medium is the part of the system that actually catches and removes foreign bodies from the pool water. The better the performance of the filter, the less chemicals will be needed to maintain the water quality. In traditional filters, this filter medium is sand. The sand is washed and graded, when the water passes through the filter, the particles of dirt etc are caught in the spaces between the grains of sand.

There are alternatives to sand.

Sand works very well, but there are some alternatives to it, such as Zeolite and Crushed glass.

Zeolite - The advantage of this filter medium is that it filters out particles down to 2 or 3 microns, where as sand can only go as low as 15 microns. The more debris that can be filtered out the better, this will in turn reduce the amount of chemicals that are needed, making the pool a more pleasant place to be.

Zeolite based mediums are also far longer lasting
than sand, reducing maintenance costs, not only
through less sand changes, but because the
equipment is not subjected to as much
punishment from the corrosive chemicals in the
water.There are certain draw backs with Zeolite
based mediums - Zeolite is made from volcanic
rock and it is not completly inert and may react
with hard water.

Crushed Glass - Crushed glass has been used as a filter medium since the 1800´s, but only started to be used in swimming pools in the mid 1990´s. The glass is crushed down to a the appearance of sugar. Its almost totally inert and will filter down to 2 microns, being inert means it will not react in any way with swimming pool chemicals. The other advantage of the crushed glass filtration is that the whole of the filter is operational, rather than just the top few inches, as in a traditional filter. This means less backwashing of the swimming pool filter, less wastage of chemicals and lower water consumption.

The problem with these alternative filter medias, is the high initial cost of replacing the cheap sand, however, in the longer term maintenance costs are greatly reduced and the swimmers have a far higher standard of water quality.