Anaerobic
ammonium oxidation (anammox) represents an energy-efficient process for
biological nitrogen removal from
ammonium-rich
wastewater. However, there are mechanistic issues unsolved regarding the low microbial electron transfer and undesired accumulation of
nitrate in treated water, limiting its widespread engineering applications. We found that the addition of
pyrite (1 g L-1 reactor), an earth-abundant
iron-bearing
sulfide mineral, to the anammox system significantly improved the
nitrogen removal rate by 52% in long-term operation at a high substrate
shock loading (3.86 kg N m-3 d-1). Two lines of evidence were presented to unravel the underlying mechanisms of the
pyrite-induced enhancement. Physiochemical evidence indicated that an increase of
cytochromes c and Fe-S
protein was responsible for the accelerated electron transfer among metabolic
enzymes. Multi-omics evidence showed that the depletion of
nitrate was attributed to the Fe-N-S cycle driven by
nitrate-dependent Fe(II) oxidation and S-based denitrification. This study deepens our understanding of the roles of electron transfer and the Fe-N-S cycle in anammox systems, providing a fundamental basis for the development of mediators in the anammox process for practical implications.