In a gray water heat recovery system (1) comprising a cleaning system (2) and a heat transfer device (3), wherein the cleaning system (2) is adapted to clean gray water from a gray water line (4), wherein an output of the cleaning system (2) with the heat transfer device (3 ), wherein the heat transfer device (3) is adapted to transfer heat energy from the purified gray water to a fresh water in a fresh water line (5), it is proposed that the cleaning system (2) comprises an activation tank (6) with microorganisms, wherein the microorganisms decompose organic constituents of the gray water, and that in the aeration tank (6) a filter membrane (7) is arranged.
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