A review on new ammonium oxidation alternatives for effective nitrogen removal from wastewater

As a highly oxidizing ion, ammonium (NH4+) is easily reduced by other reducing substances. Recently, NH4+ oxidation coupled with Fe3+, SO42-, and Mn4+ reduction (i.e., Feammox, Sulfammox, and Mnammox) has been confirmed and their collaborative roles in removing NH4+ have been revealed in several studies. However, only limited knowledge could be obtained on observed interactions with several variable parameters due to the exiguous research in this field, which makes it challenging to draw basic and mechanistic conclusions. This review focuses on these new NH4+ oxidation alternatives and provides a systematic and comprehensive overview of recent advances in these processes. Specifically, based on the summaries of their reaction conditions, nine influencing factors (including microorganisms, concentrations of reactants, pH, redox potential, temperature, organic carbon, oxygen concentration, salinity, and inhibitory substances) are discussed in detail. Then, the drawbacks of the three processes are pointed out. The results show that, at present, the practical engineering applications for NH4+ removal are not suitable due to their harsh reaction conditions and low removal efficiency. Finally, suggestions for further research on these processes are presented to inspire more interest from researchers in related fields and to promote and realize their large-scale engineering application in the near future. (c) 2022 Society of Chemical Industry (SCI).