AimsThis study gives a systematic profile of ammonium overconsumption in anammox consortia beyond the stoichiometry ratio, with aim to reveal another ammonium removal process by oxidants OH without nitrite under anaerobic conditions. Methods and ResultsPercoll centrifugation, flow cytometry, cloning and chemical analysis demonstrated that this ammonium removal was related to low Planctomycetes and high biomass decay. Armatimonadetes, Acidobacteria, Verrucomicrobia and Nitrosomonas initially existed in the consortia with the overconsumption of ammonium, and their levels then declined with decreasing overconsumption. The inevitable release of oxidants, such as OH, during bacterial inhibition or decay was found to be a potential source of electron acceptors capable of oxidizing ammonium. ConclusionsAnaerobic ammonium removal without nitrite could occur in anammox consortia. However, it was unsteady and was not sustained over a long period. Under suitable conditions, anammox bacteria competed with others and became the dominant micro-organism responsible for the simultaneous removal of ammonium and nitrite. Significance and Impacts of the StudyThis investigation will have importance to better understand the anammox process operation. The overconsumption of ammonium could be used as an indicator for fluctuations in anammox cultivation.