The Altenstadt combined heat and power plant is operated with two water circuits: the primary water-steam circuit and the secondary cooling water circuit. During normal operation, there is just little loss in the closed water-steam circuit. In contrast to this, the recooling circuit is open. Heat is extracted from the water to be cooled by means of evaporation and this makes the air wet. The “used” water is returned to the original hydrologic cycle without any harm to the environment. The raw water required for the two circuits is taken from two own deep wells.