Method for operating a water system, wherein the water system comprises a fresh water subsystem being configured to provide fresh water to water consumers (13, 14, 15, 16, 17, 18, 19), wherein first water consumers (13, 14) generate black water as waste water, wherein second water consumers (15, 16, 17, 18, 19) generate grey water or black water as waste water, a waste water subsystem being configured to receive the waste water from the water consumers (13, 14, 15, 16, 17, 18, 19), wherein the waste water subsystem comprises a waste water diverter configured to provide in a black water status of the waste water diverter waste water to a drain thereby treating waste water as black water, and provide in a grey water status of the waste water diverter waste water to a grey water tank thereby treating waste water as grey water, wherein the method comprises the following steps: detect by a fresh water controller being installed in the fresh water subsystem a fresh water consumption in the fresh water subsystem and provide a respective first output signal to a grey water controller, detect by a common flow sensor being installed in the waste water subsystem a waste water flow in the waste water subsystem and provide a respective second output signal to the grey water controller, determine by the grey water controller on basis of the first output signal provided by the fresh water controller and on basis of the second output signal provided by the common flow sensor a control signal for the waste water diverter to switch the same either into the black water status or the grey water status, wherein the waste water diverter is switched into the grey water status with a first time-delay (Δt1) after detecting by the fresh water controller that there is fresh water consumption, wherein the waste water diverter is switched into the black water status with a second time-delay (Δt2) after detecting by the fresh water controller that there is no fresh water consumption. a fresh water subsystem being configured to provide fresh water to water consumers (13, 14, 15, 16, 17, 18, 19), wherein first water consumers (13, 14) generate black water as waste water, wherein second water consumers (15, 16, 17, 18, 19) generate grey water or black water as waste water, wherein first water consumers (13, 14) generate black water as waste water, wherein second water consumers (15, 16, 17, 18, 19) generate grey water or black water as waste water, a waste water subsystem being configured to receive the waste water from the water consumers (13, 14, 15, 16, 17, 18, 19), wherein the waste water subsystem comprises a waste water diverter configured to provide in a black water status of the waste water diverter waste water to a drain thereby treating waste water as black water, and provide in a grey water status of the waste water diverter waste water to a grey water tank thereby treating waste water as grey water, provide in a black water status of the waste water diverter waste water to a drain thereby treating waste water as black water, and provide in a grey water status of the waste water diverter waste water to a grey water tank thereby treating waste water as grey water, detect by a fresh water controller being installed in the fresh water subsystem a fresh water consumption in the fresh water subsystem and provide a respective first output signal to a grey water controller, detect by a common flow sensor being installed in the waste water subsystem a waste water flow in the waste water subsystem and provide a respective second output signal to the grey water controller, determine by the grey water controller on basis of the first output signal provided by the fresh water controller and on basis of the second output signal provided by the common flow sensor a control signal for the waste water diverter to switch the same either into the black water status or the grey water status, wherein the waste water diverter is switched into the grey water status with a first time-delay (Δt1) after detecting by the fresh water controller that there is fresh water consumption, wherein the waste water diverter is switched into the black water status with a second time-delay (Δt2) after detecting by the fresh water controller that there is no fresh water consumption.