A ballast water treatment system for treating ballast water for a vessel has a pathway for navigable raw water to the tank and a discharge for discharging water back to the body of navigable raw water. A back washable filter in the pathway receives the raw water and screens sediment from the raw water to produce initially processed water which contains at least some marine organisms. A UV or other biocidal device is connected between the filter and the ballast tank, for disabling or killing the marine organisms in the initially processed water to produce treated ballast water for the main outlet to the ballast tank.