water supply in aircraft for on - board use in conjunction with water outlets in lavatories and a galley , includes a waste water tank connected to at least some of the waste discharge outlets on board of the aircraft ; a water purifying plant connected to and receiving water from the waste water tank ; and a fresh water tank having outlet means , connected at least to some of the water outlets , and having an inlet connected to said water purification plant , for receiving therefrom purified waste water ; an additional tank for drinking water may be provided as well as a separate supply line therefrom leading to drinking water outlets on board of the aircraft .