Removal of nitrogen, mainly in form of ammonium (NH4 +), in wastewater treatment plants (WWTPs) is a highly energy demanding process, mainly due to aeration. It causes costs of about half a million Euros per year in an average European WWTP. Alternative, more economical technologies for the removal of nitrogen compounds from wastewater are required. This study proves the complete anoxic conversion of ammonium (NH4 +) to dinitrogen gas (N2) in continuously operated bioelectrochemical systems at the litre-scale. The removal rate is comparable to conventional WWTPs with 35 ± 10 g N m−3 d−1 with low accumulation of NO2 −, NO3 −, N2O. In contrast to classical aerobic nitrification, the energy consumption is considerable lower (1.16 ± 0.21 kWh kg−1 N, being more than 35 times less than for the conventional wastewater treatment). Biotic and abiotic control experiments confirmed that the anoxic nitrification was an electrochemical biological process mainly performed by Nitrosomonas with hydroxylamine as the main substrate (mid-point potential, Eox = +0.67 ± 0.08 V vs. SHE). This article proves the technical feasibility and reduction of costs for ammonium removal from wastewater, investigates the underlying mechanisms and discusses future engineering needs.