Filtration treatment systems can be an attractive growth medium for biological organisms such as bacteria, mold and algae. In some cases, these growths can build up in the filters. In most cases, however, the concern is that these organisms will contaminate the water leaving the treatment system and entering the drinking water supply. Therefore, potable water supplies generally contain chlorine, which is added as a disinfectant. More information on resin sterilization can be found online.