A system for purifying water includes a perforated member disposed within a container confining water therein, the perforated member being spaced above the bottom of the container. A pump transfers water between an upper portion of the container above the perforated member and the bottom of the container below the perforated member to recirculate the water, and the recirculated water is aerated. A bacteria culture bed overlies the perforated member and includes a mixture of gravel, clay and other material or materials, such as skeletal residue material and readily decomposable vegetal material. The pump includes a sump box containing water therein and a pumping device mounted therein. The pumping device includes an impeller mounted within a housing disposed within the water in the box. A shaft extends through an opening in the housing, and the impeller is fixed to the lower end portion of the shaft. A motor is connected drivingly to the upper end portion of the shaft, and a non-metallic sleeve surrounds the shaft and engages sealingly at its lower end the impeller so that the sleeve extends through the opening in the housing between the impeller and the space above the water in the box, whereby all of the wetted surfaces are non-corrosive.