Exploring the potential of anaerobic simultaneous nitritation and denitritation for low-strength nitrogen wastewater: Role of electric field and activated carbon

This study explores electric field-enhanced simultaneous nitritation and denitritation (eSND) for nitrogen removal from low-strength, carbon-limited wastewater under anaerobic conditions. Sequencing batch reactors were operated without external organic carbon, using alkalinity as the sole inorganic carbon source. Under control conditions, autotrophic SND occurred but showed limited removal rates of 0.98 mg NH₄⁺-N/L·d and 0.70 mg NO₂⁻-N/L·d. Applying a 1 V/cm electric field or conductive granular activated carbon (GAC) alone moderately improved performance, while their combined use achieved significantly higher rates of 7.10 mg NH₄⁺-N/L·d and 3.92 mg NO₂⁻-N/L·d. Microbial analysis revealed enrichment of Geofilum rhodophaeum and Nitrospira moscoviensis, associated with redox activity and nitrite metabolism. GAC-assisted systems favored DIET-capable, biofilm-forming species, including Aquicella siphonis, Racemicystis persica, and Croceimicrobium hydrocarbonivorans. Predicted functional profiling based on 16S rRNA sequencing indicated a higher abundance of functional genes in the BeSBR w/AC compared to the control. Notably, genes related to nitrogen metabolism such as nirB, nirD, and nirA were predicted to be upregulated by 2.8–7.1 fold, while narB showed a substantial 22-fold increase. The expression of nirK was also estimated to rise by 3.3-fold. Additionally, quorum sensing genes rpfF and lasI increased by 5.5-fold and 4.4-fold, respectively, suggesting enhanced microbial communication. The EET-related gene pilA exhibited a 19.8-fold predicted increase, supporting greater electron transfer potential under BeSBR w/AC conditions. These results demonstrate that the combined application of electric field and GAC enhances the activity of electroactive microbial communities and functional gene expression, enabling efficient autotrophic nitrogen removal under anaerobic, carbon-limited conditions.