For the energy supply for the waste water treatment plant (rwzi-Bath) of the Hoogheemraadschap West-Brabant three energy sources are used: biogas of the digesters, natural gas and electricity delivered by the PZEM. For a good balance between heat/power demand and production a heat/power plant is installed. By using this system a high efficiency for the use of energy will be obtained. To save energy the oxygen concentration in the aerationtanks is automatically controlled by means of regulating the position of the air supply control valves and the capacity and number of the turbocompressors. For the oxygen controlsystem a Siemens PLC is used.