A waste-watermill for generating electrical energy from a flow of waste water running through an underground waste-water conduit. The flow of waste water defines a free surface and a flow direction inside the waste water conduit. The waste-watermill comprises 1) a water wheel for being driven by the flow of waste water in an undershot configuration, 2) an axle for defining an rotational axis for the water wheel, where the flow of waste water passes below and transverse to the rotational axis in the undershot configuration, 3) a wheel support for rotationally supporting the water wheel in the undershot configuration and in a stationary position relative to the underground waste-water conduit, and 4) an electric generator operatively connected with the water wheel and the axle for being driven by the water wheel to produce electrical energy.