Microbial electricity-driven anoxic ammonium removal could remove ammonium from wastewater without the presence of oxygen (aeration) using electricity. This study aims at unveiling the potential biologic pathways for the bioelectrochemical oxidation of ammonium to dinitrogen gas in an anaerobic bioelectrochemical system (BES). Known intermediate metabolites of this process (hydroxylamine, nitrite and nitrate) were monitored in two BES replicates. Ammonium was fully oxidized to dinitrogen gas without intermediates accumulation in the anodic chamber. Achromobacter sp. was the most abundant microorganism (up to 60%, according to sequence reads) in the mixed community. Hydroxylamine and nitrite oxidation were electroactive processes, reinforcing the role of the anodic electrode as the electron acceptor for ammonium oxidation. Taking it all together, ammonium can be removed in BES by a combination of different bio/electrochemical processes. A deeper understanding on how the different metabolisms are coupled together is required for increasing the current ammonium removal rates.