A waste water heat recovery device, comprising - a first waste water heat recovery unit configured to be connected to a waste water line and to a process water line, where the first waste water heat recovery unit comprises a heat exchanger to transfer heat from waste water in the waste water line to process water in the process water line; - a second waste water heat recovery unit configured to be connected to the waste water line, downstream of the first waste water heat recovery unit, and to the process water line, downstream of the first waste water heat recovery unit, where the second waste water heat recovery unit comprises a heat pump to transfer heat from waste water in the waste water line to process water in the process water line; characterized in that the first waste water heat recovery unit comprises an auxiliary pump to pump process water through the heat exchanger, where the first waste water heat recovery unit is configured to, in a passive first mode of operation, transfer heat from waste water in the waste water line to process water in the process water line with deactivated auxiliary pump, and, in an active second mode of operation, transfer heat from waste water in the waste water line to process water in the process water line with activated auxiliary pump, wherein the first waste water heat recovery unit is configured to automatically switch from the passive mode of operation to the active mode of operation if a preset or presettable criterion is met, the preset or presettable criterion comprising that - a temperature of waste water downstream of the heat exchanger is higher than a preset temperature; or that - waste water is flowing down the waste water line through the heat exchanger for longer than a preset time; or that - the waste water is flowing down the waste water line from a specific source. - a first waste water heat recovery unit configured to be connected to a waste water line and to a process water line, where the first waste water heat recovery unit comprises a heat exchanger to transfer heat from waste water in the waste water line to process water in the process water line; - a second waste water heat recovery unit configured to be connected to the waste water line, downstream of the first waste water heat recovery unit, and to the process water line, downstream of the first waste water heat recovery unit, where the second waste water heat recovery unit comprises a heat pump to transfer heat from waste water in the waste water line to process water in the process water line; characterized in that the first waste water heat recovery unit comprises an auxiliary pump to pump process water through the heat exchanger, where the first waste water heat recovery unit is configured to, in a passive first mode of operation, transfer heat from waste water in the waste water line to process water in the process water line with deactivated auxiliary pump, and, in an active second mode of operation, transfer heat from waste water in the waste water line to process water in the process water line with activated auxiliary pump, wherein the first waste water heat recovery unit is configured to automatically switch from the passive mode of operation to the active mode of operation if a preset or presettable criterion is met, the preset or presettable criterion comprising that - a temperature of waste water downstream of the heat exchanger is higher than a preset temperature; or that - waste water is flowing down the waste water line through the heat exchanger for longer than a preset time; or that - the waste water is flowing down the waste water line from a specific source. - a temperature of waste water downstream of the heat exchanger is higher than a preset temperature; or that - waste water is flowing down the waste water line through the heat exchanger for longer than a preset time; or that - the waste water is flowing down the waste water line from a specific source.