Achieving stable nitrogen removal through mainstream partial nitrification, anammox and denitrification (SNAD) with a hybrid biofilm-granular reactor.

Simultaneous partial nitrification, anammox, and denitrification (SNAD) process offers a promising method for the effective removal of carbon and nitrogen from wastewater. However, ensuring stability is a challenge. This study investigated operational parameters such as hydraulic retention time (HRT) and biomass retention to stabilize SNAD operation, transitioning from synthetic to anaerobically pre-treated municipal wastewater (APMW) in an upflow hybrid biofilm-granular reactor (UHR). The incorporation of hybrid biomass in the form of biofilms and granules resulted in a significant improvement in ammonium oxidation, increasing the efficiency from 45% to 60%. This outcome underscores the significance of biomass retention as a crucial parameter in achieving optimal performance. Furthermore, extending the HRT resulted in a significant improved nitrogen removal, increasing it from 40% (8h) to 70% (12h), which was attributed to the enhanced specific activities of ammonium-oxidizing bacteria (AOB) and anammox bacteria (AnAOB). Microbial characterization unveiled the emergence of partial denitrifiers (Thauera genus) and the suppression of nitrite-oxidizing bacteria (NOB) (Nitrospira genus) at low aeration rates (0.35 L min.L; estimated 0.5 mgDO.L). Notably, stable operation persisted throughout the experimental period, primarily due to the consistent nitrite supply from partial nitrification/denitrification. Our findings highlight the potential of innovative hybrid reactor configuration, for achieving stable and efficient SNAD performance in mainstream wastewater treatment.