KDF is an alloy containing zinc and copper that can be incorporated into carbon filters to make them bacteriostatic. That means that bacteria cannot grow inside the filter. This is particularly good for people who use their RV intermittently, and do not want to empty their canisters of water as KDF will protect the filter while the RV is not being used. A word of warning, Solid Carbon block filters with KDF should be kept submerged in water for their life after installing into the canister. If they are removed and dried, they may seize up and not pass water on the next use. There are several other chemicals that manufacturers use to make their filters bacteriostatic, but many of them affect you as well as the bacteria! KDF has a very good record as being the safest agent yet found for keeping bacteria from growing in filters, although silver nitrate is common, also.