Abstract: Heat can be recovered by biological generation of heat upon aeration of refuse, such as garbage or sludge, in an aeration vessel (11) by introducing oxygen-containing gas, such as air, in a closed cycle (11, 33, 31, 35) to thereby enrich gas withdrawn from the vessel with the oxygen, typically by reintroduction of gas withdrawn from an upper gas portion (43) of the vessel, after introduction of additional oxygen, for example controlled by a valve (65) into a lower portion of the contents of the aeration chamber. Control can be effected automatically, by a control unit (55, 55') through a valve (51, 51') or manually; automatic control can be effected, for example, by sensing oxygen or carbon dioxide concentration by suitable sensors (57, 59) within the vessel.