Long-term nitrogen removal performance and microbial mechanisms in a single-stage composite-carrier MBBR for rural wastewater treatment

To evaluate the nitrogen removal performance of a single-stage reactor system for decentralized rural wastewater treatment and to elucidate the underlying microbial mechanisms, this study developed an AO (anaerobic–oxic) moving bed biofilm reactor (MBBR). During 129 days of continuous operation, the overall nitrogen removal performance of the composite-carrier MBBR system was evaluated, together with carrier-associated microbial community structures and functional characteristics of the biofilms developed on different carriers. The results showed that, under stable AO operation, the maximum removal efficiency of COD reached 80.7%, whereas the average removal efficiency of ammonium reached 98.9%. AOB were identified as the dominant ammonia-oxidizing microorganisms, accounting for 74.3% of ammonia oxidation, whereas AOA and Comammox contributed 11.8% and 13.9%, respectively. Carrier-specific batch tests demonstrated differences in nitrogen transformation activity among the tested carriers. Among the four carriers, the Biochip carrier exhibited relatively higher mass transfer potential, specific surface area-based nitrogen loading capacity, and simultaneous nitrification and denitrification activity. Microbial community analysis revealed pronounced enrichment of dominant functional phyla, including Bacteroidota and Nitrospirota . On day 114, the relative abundances of key functional genera, such as Thauera and Nitrospira , reached 15.2% and 11.9%, respectively. PICRUSt-based functional prediction further suggested high predicted abundances of genes associated with nitrification, including amoA and hao , as well as denitrification, including nirK and nosZ . This study provides a theoretical basis and a process optimization strategy for the application of single-stage MBBR systems in decentralized rural wastewater treatment.