Wastewater, particularly black water and/or gray water, is used, for example in an aircraft, for processing fuels used in a high temperature fuel cell. For this purpose a liquid hydrocarbon fuel is used for producing an emulsion for use as fuel in the high temperature fuel cell. The liquid hydrocarbon fuel is preferably kerosene. The liquid hydrocarbon fuel is first emulsified with the wastewater which is first passed through a treatment such as filtering, reverse osmosis, or any other cleaning treatment that makes the wastewater suitable for emulsifying the liquid hydrocarbon fuel. The emulsion is then supplied as fuel to the high temperature fuel cell. Thus, smaller and hence lighter storage facilities for the wastewater can be used.