TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Method: The method involves crystallizing at least part of the available ammonium from the liquid into an ammonium salt and separating at least part of the ammonium as ammonium salt from the liquid; decomposing at least part of the ammonium salt into ammonia and other decomposition products; and feeding at least part of the ammonia to a fuel cell. In the method, the ammonium containing aqueous liquid is obtainable by providing ammonia containing air from an animal accommodation to a liquid comprises acid such as sulfuric acid and hydrochloric acid, or by providing ammonia containing off-gas from industrial emission to a liquid comprises acid such as sulfuric acid and hydrochloric acid; crystallization is achieved by adding phosphate to the ammonium containing aqueous liquid; crystallization is achieved by adding phosphate and an alkaline earth cation to the ammonium containing aqueous liquid (especially magnesium); crystallizing at least part of the available ammonium from the liquid into magnesium ammonium phosphate (MAP) or potassium ammonium phosphate (KAP); crystallization is induced by controlling the pH and temperature of the ammonium containing aqueous liquid; crystallization is performed in a fluid bed crystallizer, in a slurry crystallizer, or in a crystallization reactor; and separation is performed in a separator, arranged downstream of the crystallization reactor; the separator is a three-way separator; the decomposition is a thermal decomposition or chemical decomposition; before feeding the ammonia comprising gas to the fuel cell the ammonia comprising gas is purified, especially by drying the ammonia comprising gas; other decomposition products is returned to crystallize at least part of the available ammonium; simultaneously or sequentially ammonia and another fuel, or a mixture of these two are fed to the fuel cell, where the other fuel comprises fuels selected from syngas, hydrogen and methane; ammonia and biogas are fed to the fuel cell; the methane or biogas originates from an anaerobic digester or a biological waste water treatment plant; and the methane or biogas originates from a gasification unit for gasifying organic matter.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The ammonium containing aqueous liquid is a waste stream.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The ammonium containing aqueous liquid comprises an effluent of an anaerobic digester, a stripper water from anaerobic sections of biological phosphor-removing waste water treatment plant (wwtp), a concentrate or filtrate stream from a dewatering section of a biological waste water treatment plant (wwtp), a (concentrated) urine containing liquid or filtrate, condensate or other liquid waste streams from digesters, or stripper units. The fuel cell is solid oxide fuel cell (SOFC). The fuel cell further includes an ammonia converter (preferably a cracker). The apparatus further comprises a crystallization unit, arranged to crystallize at least part from the available ammonium from the liquid into an ammonium salt; the crystallization unit is a fluid bed crystallizer or a slurry crystallizer; and the decomposition unit is a thermal decomposition unit. EXAMPLE - No suitable example given.