Revisiting simultaneous sulfate reduction and ammonium oxidation in wastewater treatment – from inexplicable experimental observations to extended mechanistic hypotheses†

Abstract

Over the last two decades, reference has been made to the ‘sulfammox’ conversion, comprising the anaerobic oxidation of ammonium with sulfate, with nitrogen gas (N₂) and elemental sulfur (S⁰) as the main end products. However, this phenomenon has been associated with inexplicable experiment results in terms of variable end products and unclear reaction stoichiometry, besides the fact that it has been reported to occur under both heterotrophic and autotrophic conditions. This contribution sheds light on the ‘sulfammox’ phenomenon through a comprehensive revisit of experimental observations. The hypothesis for sulfammox-related reaction mechanisms was systematically extended, considering other end products than N₂ and S⁰, and as well as potential syntrophic bioprocesses. This resulted in additional reactions which were more general than the specific sulfammox one and which were denoted by the term – simultaneous sulfate reduction and ammonium oxidation (SRAO). Multiple thermodynamically feasible reaction pathways of SRAO under heterotrophic and autotrophic conditions were identified in a systematic and intelligible way, and compared against previously reported experimental results regarding reactor performance and microbial community analysis.