Dosing with pyrite significantly increases anammox performance: Its role in the electron transfer enhancement and the functions of the Fe-N-S cycle

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 en-gineering 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 in-crease 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 develop-ment of mediators in the anammox process for practical implications.