The Pre-Filter for the Gard'N Gro Dechlorinator is used to remove sediment from the water prior to entering the dechlorinator proper. This filter features a 15 micron double screen that can filter out sediment up to 5 times smaller than the diameter of a human hair. To extend the life of the pre-filter and screen it can be reversed each month to dislodge particulate that may have accumulated.