Carbon-based conductive carriers promote coupled Fe(II)-driven autotrophic and heterotrophic denitrification of wastewater with low C/N ratios

Denitrification of wastewater with a low organic carbon to NO₃^--N ratio (C/N ratio) faces challenges due to slow rates and low efficiency. This study reported that carbon-based conductive carriers are able to enhance the removal of nitrogen from wastewater with low C/N ratio by coupling Fe(II)-driven autotrophic and heterotrophic bioelectrochemical denitrification. When Fe(II) was the sole electron donor, the bioreactor using conductive carrier achieved a denitrification rate constant (k_DN) of 0.016 h⁻¹, 1.7 times of that with non-conductive materials. This enhancement was due to the conductive carrier boosting direct electron transfer and supporting the growth of electroactive microorganisms. For wastewater with a low C/N ratio of 0.76, the bioreactor featuring both Fe(II) and the conductive carrier reached a k_DN of 0.095 h⁻¹, five times higher than without Fe(II). The presence of Fe(II) promoted denitrification by enhancing electron transfer and serving as a mediator. Microbial analysis showed that adding Fe(II) enriched electroactive bacteria like Comamonas and denitrifiers such as Chryseobacterium. Our findings suggest a promising strategy to enhance denitrification in wastewater treatment systems with low C/N ratios.