the water purification cartridge treats water with a halogen liberated from halogenated hydantoinyl resins and polymers , resulting in the safe , complete , and economical purification of non - potable water at low flow rates typically found in gravity feed filtration systems and at water pressures of less than 1 psig . the cartridge of the invention purifies non - potable water of bacteria , viruses , and some protozons without the need for additional power sources or risk to the consumer for exposure to chemicals that have been identified to have harmful , long - term effects . the cartridge described herein enables a gravity fed water purification system by combining the cartridge with conventional filters , such as porous diatomaceous earthen ceramic water filters or activated carbon filters packed either with granular activated carbon or block .