Enhanced-nitrogen removal through Fe(III)-triggered partial dissimilatory nitrate reduction to ammonium coupling with anammox in anammox bioreactor.

[Display omitted] • Anammox sludge drives iron reduction using extracellular polymeric substances. • Fe(II) promotes partial DNRA pathway of anammox with nitrite as end products. • Accumulated nitrite is further eliminated with ammonium via anammox pathway. • Nitrogen removal efficiency of the iron-assisted anammox process reaches 95%. Anammox is recognized as a prospective alternative for future biological nitrogen removal technologies. However, the nitrate by-products produced by anammox bacteria limit its overall nitrogen removal efficiency below 88 %. This study introduced Fe(III) into the anammox bioreactor to enhance the nitrogen removal efficiency to approximately 95 %, surpassing the biochemical limit of 88 % imposed by anammox stoichiometry. Anammox sludge was demonstrated to utilize extracellular polymeric substances to reduce Fe(III) into Fe(II), and this process promoted the dominance of Ca. Brocadia. The iron addition improved the abundance of narGHI genes and facilitated the partial dissimilatory nitrate reduction to ammonium, with nitrite as the end product. The accumulated nitrite was then eliminated through the anammox pathway, along with the excess ammonium (30 mg/L) in the influent. Overall, this study deepens our understanding of the enhanced nitrogen removal triggered by Fe(III) in anammox sludge and offers an effective approach to boost anammox process. [ABSTRACT FROM AUTHOR]