Autotrophic ammonium nitrogen removal process mediated by manganese oxides: Bioreactors performance optimization and potential mechanisms.

Manganese(IV) (Mn(IV)) reduction coupled with ammonium (NH-N) oxidation (Mnammox) has been found to play a significant role in the nitrogen (N) cycle within natural ecosystems. However, research and application of the autotrophic NH-N removal process mediated by manganese oxides (MnOx) in wastewater treatment are currently limited. This study established autotrophic NH-N removal sludge reactors mediated by various MnOx types, including δ-MnO (δ-MSR), β-MnO (β-MSR), α-MnO (α-MSR), and natural Mn ore (MOSR), investigating their NH-N removal performances and mechanisms under different initial N loading and pH conditions. During the 330 d operation, the reactors exhibited NH-N removal efficiencies in the order of δ-MSR > α-MSR > β-MSR > MOSR. Notably, metal-reducing bacteria (Candidatus Brocadia, Dechloromonas, and Rhodocyclaceae) and Mn(II) oxidizing bacteria (Pseudomonas and Zoogloea) were enriched in the reactors, especially in the δ-MSR. The presence of these microorganisms facilitated the reduction of Mn(IV) and utilized the generated Mn(II) to drive autotrophic denitrification (MnOAD), thereby completing the Mn(IV)/Mn(II) cycle and enhancing N removal in the system. An active Mn cycle displayed in δ-MSR, which could be demonstrated by the formation of petal-shaped biogenic MnOx and the increased abundance of Mn cycling genes (MtrCDE, MtrA, MtrB, and CotA, etc.). Meanwhile, genes involved in N metabolism were enriched, particularly functional genes associated with nitrification and denitrification. In this study, the coupling of Mnammox and MnOAD was realized via the Mn cycle, providing a new perspective on the application of autotrophic N removal technologies in wastewater treatment.