Ammonia in aqueous environments leads to eutrophication and toxicity of the receiving water body (Metcalf & Eddy, 2003). In order to prevent environmental pollution, ammonia needs to be removed from residual water streams, before the water can be discharged to the aqueous environment (Song et al., 2012). Waste water treatment plants (WWTPs) reduce the ammonia-nitrogen concentration by applying biological treatment technologies. However, the oxidation of ammonia by bacteria requires high amounts of oxygen and consequently has a high energy consumption. The nitrification and denitrification of wastewater requires 15.83 kWh per kg-N (Magrí et al., 2013). Moreover, the reject water from the digested sludge contains high amounts of nitrogen, which is fed back to the biological treatment, and can contribute 15-20% of the nitrogen load (Fux et al., 2002). Concentrating ammonium fluxes lead to the production of a concentrated ammonium stream, used as potential energy source in the N2kWh project, and a sweet stream.