Abstract: Method and apparatus for utilizing the heat content in waste water which has passed through a biological purification plant, the last step of which comprises subjecting the waste water to sedimentation in a settlement tank. A heat pump is provided having an evaporator section which is heated by means of the waste water which has been purified in the settlement tank which is directed to it through a first conduit. During periods in which the flow rate of the waste water through the purification plant is less than the average flow rate which corresponds to the capacity of the heat pump, at least a portion of the waste water which flows through the evaporator of the heat pump is recirculated to the settlement tank, the latter being utilized as a heat reservoir according to the present invention.