A tank assembly with a tank for storing liquid hydrocarbon, the tank having a floor for supporting a weight of the liquid hydrocarbon. A filter is fitted to the floor of the tank. The filter is arranged to allow liquid water in the tank to drain out of the tank through the filter but substantially prevent the liquid hydrocarbon in the tank from doing so. The filter has a permeation member, such as a membrane, which is formed from a material such as graphene oxide which allows liquid water in the tank to drain out of the tank by permeating through the permeation member but substantially prevent the liquid hydrocarbon in the tank from doing so.