Development of fermentation and respiration bioprocesses for efficient nitrogen removal through microbial catabolism

Conventional activated sludge processes are primarily designed for nitrogen and phosphorus removal, with carbon transformation regarded as a concomitant process that supports downstream denitrification rather than a proactively regulated process. Inspired by the metabolic division of labor in gut ecosystems, we proposed and validated a metabolism-guided strategy for an experimental membrane bioreactor (MBR-E) with a prefermentation unit that couples upstream fermentation with downstream denitrification to restructure carbon flux toward more bioavailable electron donors for nitrogen removal. Long-term operation showed that MBR-E achieved significantly lower effluent total nitrogen (7.9 ± 2.4 mg/L) compared with the control MBR system (MBR-C, 12.3 ± 3.5 mg/L), which was attributed to its elevated specific denitrification rate. Influent organics were efficiently converted into volatile fatty acids (VFAs) via fermentation and shortening hydraulic retention time from 0.67 h to 0.5 h shifted VFA composition from propionate/butyrate dominance to acetate enrichment. Further, 16S rRNA gene sequencing demonstrated that functional denitrifiers and nitrifiers were selectively enriched in MBR-E. Co-occurrence network analysis revealed strengthened cooperative interactions and tighter functional coupling between carbon degradation and nitrogen removal in MBR-E. Overall, this study demonstrates that fermentation-driven carbon reprogramming can effectively regulate downstream respiratory pathways and reshape the microbial community structure, providing a novel approach for efficient nitrogen removal from low-carbon/nitrogen wastewater.