Anaerobic purification of heated waste water

The invention is directed to a process for the anaerobic purification of waste water in an anaerobic reactor, said process comprising burning the biogas produced in the purification, in a gas-burner, using the hot gases obtained thereby for indirect heating of water in a hot water boiler, which boiler is connected with the reactor by means of a line between the gas head of the boiler and a compartment through which the waste water flows, such as a conditioning tank or the anaerobic reactor, which line ends underneath the water level in the said compartment at such a level that a pressure of between 0.25 and 3 m of water column is maintained in the gas head of the boiler, in which boiler steam is generated which enters the said compartment through the said line and heats the waste water, and wherein the said boiler is further provided with a valve to let off steam in case the amount of biogas is in excess of the amount needed to heat the waste water.