out of the sponge the sponge will gradually become discolored and when the color is a light tan color it should be discarded and a new sponge utilized to sterilize the water therefore the color of the iodine itself acts as an indicator of when the sponge has become inactive since only 5 parts iodine in a million are liberated into the water only 5 micrograms of iodine are utilized by one quart of water a sponge containing only a quarter of a gram of iodine would be capable of sterilizing 50 000 quarts of water obviously a properly treated sponge in a small container would last almost indefinitely such a sponge placed in canteens would prevent infection in military troops travelers and users of swimming pools and would protect the public from disease as a public health measure it is also possible to sterilize a swimming pool by repeatedly pumping the water through an open cell polyurethane iodine filter apparatus 20 with swimming pools the sponge 27 would be utilized as a filter through which the swimming pool water would circulate thus water would be pumped through inlet conduit 22 into a clear plastic canister 24 holding an iodine polyurethane sponge 27 which fills the interior.