Abstract: A drainwater/sewage treatment installation for producing electrical power and for regulating waterflow in a drainage network is provided. The installation comprises at least a first reservoir, at least a second reservoir below the first reservoir and conduits connecting the reservoirs. The invention combines a drainage network having a pumped energy storage hydroelectric station and at least a two reservoir system wherein the second reservoir is connected to the drainage network in order to be able to serve as a temporary storage facility for the purpose of spreading out peaks in the flow rate of rain water in the drainage network.