Mixotrophic partial denitrification based on ferrous iron and sodium acetate: performance, microbial community, and potential mechanisms.

Partial denitrification (PD) was configured as a nitrite supplier for anaerobic ammonium oxidation (ANAMMOX). The mixotrophic PD (MPD) system using ferrous iron and sodium acetate as co-electron donors was operated at autotrophic-to-heterotrophic (A/H) ratios of 0.07 (Phase I), 0.21 (Phase II), and 0.85 (Phase III). Stable nitrite accumulation rates (NAR) of 78.0 %, 70.8 %, and 59.6 % were achieved in Phases I-III, respectively, while the autotrophic contribution increased from 21.8 % to 76.1 % with increasing A/H. Thauera, Longilinea and Thermomonas were the predominant denitrifiers, whereas Anaeromyxobacter and Petrimonas mitigated Fe(II) encrustation. Compared with purely heterotrophic PD, the MPD configuration reduced operating costs over by 22.8 % and maintained robust process performance. These findings indicate that Fe(II)- and acetate-driven MPD is a cost-effective and operationally stable strategy for generating nitrite for downstream anammox-based nitrogen removal.