A waste water heat recovery device, comprising - at least one 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 arrangement 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 arrangement to transfer heat from waste water in the waste water line to process water in the process water line; characterized in that the heat pump arrangement comprises a low-temperature heat buffer storage, a high-temperature heat buffer storage, and a heat pump, where - the low-temperature heat buffer storage has a lower working temperature than the high-temperature heat buffer storage and is configured to extract heat from waste water in the waste water line and to store the extracted heat, - the heat pump is configured to transfer the stored heat from the low-temperature heat buffer storage to the high-temperature heat buffer storage, and - the high-temperature heat buffer storage is configured to store the heat transferred by the heat pump and to provide said heat to process water in the process water line, where - the heat exchanger arrangement of the first waste water heat recovery unit comprises a heat buffer storage configured to buffer heat that is transferred from waste water to process water in the first waste water heat recovery unit, and the working temperature of the heat buffer storage of the first waste water heat recovery unit is higher than the working temperature of the low-temperature heat buffer storage of the second waste water heat recovery unit, - the waste water heat recovery device comprises at least one additional first waste water heat recovery unit with respective heat buffer storage, where the additional first waste water heat recovery units are arranged serially between the first waste water heat recovery unit and the second waste water heat recovery unit such that - each subsequent waste water heat recovery unit (2, 3) is configured to be connected to the waste water line downstream of the precedent waste water heat recovery unit (2, 3), and to the process water line downstream of the precedent waste water heat recovery unit (2, 3), where - the heat buffer storage of each subsequent first waste water heat recovery unit has a lower working temperature than the heat buffer storage of the preceding first waste water heat recovery unit, where the heat buffer storages are devices with the purpose of storing heat with a defined capacity. - at least one 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 arrangement 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 arrangement to transfer heat from waste water in the waste water line to process water in the process water line; - the low-temperature heat buffer storage has a lower working temperature than the high-temperature heat buffer storage and is configured to extract heat from waste water in the waste water line and to store the extracted heat, - the heat pump is configured to transfer the stored heat from the low-temperature heat buffer storage to the high-temperature heat buffer storage, and - the high-temperature heat buffer storage is configured to store the heat transferred by the heat pump and to provide said heat to process water in the process water line, - the heat exchanger arrangement of the first waste water heat recovery unit comprises a heat buffer storage configured to buffer heat that is transferred from waste water to process water in the first waste water heat recovery unit, and the working temperature of the heat buffer storage of the first waste water heat recovery unit is higher than the working temperature of the low-temperature heat buffer storage of the second waste water heat recovery unit, - the waste water heat recovery device comprises at least one additional first waste water heat recovery unit with respective heat buffer storage, where the additional first waste water heat recovery units are arranged serially between the first waste water heat recovery unit and the second waste water heat recovery unit such that - each subsequent waste water heat recovery unit (2, 3) is configured to be connected to the waste water line downstream of the precedent waste water heat recovery unit (2, 3), and to the process water line downstream of the precedent waste water heat recovery unit (2, 3), where - the heat buffer storage of each subsequent first waste water heat recovery unit has a lower working temperature than the heat buffer storage of the preceding first waste water heat recovery unit, where