Abstract: Apparatus is described for sanitizing a flow of water recirculated from a body of water such as a swimming pool by a pump (16) having an inlet conduit (10) and an outlet conduit (18) both in fluid communication with the body of water. The apparatus comprises a container (22) which is adapted to hold sources of silver and copper metal and halogen reagent which, under the action of oxidising agents forms colloidal dissolved metals. Metering means (12) is adapted to deliver hydrogen peroxide reagent into the inlet conduit (10). Water is pumped from outlet conduit (18) through feed conduit (20) which is connected to the container (22) and then through return conduit (24) which connects to either inlet conduit (10) or the outlet conduit (18) to a position upstream of the connection of the feed conduit (10) to the outlet conduit (18). The amount of hydrogen peroxide and halogen added are predetermined by the volume of water to be sanitized.