Bio-electrochemical denitrification – The impact of cathode potential, inoculum source and nitrate loading rate

Bio-electrochemical systems (BESs) offer a promising strategy for nitrate (NO 3 − ) remediation in groundwater. However, treatment efficiency is often limited in wastewater with low organic carbon to NO 3 − ratios. This study investigates the impact of cathodic potential on microbial biofilm development and long-term operation of bio-electrochemical denitrifying (BED) systems. BED reactors were operated under fed-batch and continuous modes at either −700 mV or −1100 mV (vs Ag/AgCl), to provoke direct electron transfer or hydrogen-mediated pathways, respectively. Reactors poised at −1100 mV achieved up to a 600 % increase in NO 3 − removal compared to −700 mV systems, reaching a maximum rate of 5.41 ± 0.05 g NO 3 − -N m −2 d −1 . Microbial community analysis revealed that −1100 mV BED reactors were enriched with hydrogenotrophic denitrifiers (e.g. Hydrogenophaga spp., Dechloromonas spp., and Simplicispira spp.), whereas the −700 mV BED reactor was dominated by Candidatus Nitrotoga spp., commonly associated with NO 2 − oxidation, suggesting electroactive denitrification. Collectively, results demonstrate that cathodic potential plays a critical role in shaping the microbial biofilm community and NO 3 − removal performance within BED systems. • Cathodic potential influences the rate of nitrate removal in bio-electrochemical denitrifying systems (BEDs). • Cathode potential below −700 mV (vs Ag/AgCl) led to the enrichment of hydrogenotrophic denitrifiers. • Synergistic microbial direct electron transport and electrohydrogenesis enhanced nitrate removal. • Candidatus Nitrotoga spp. is a potential denitrifier capable of direct electron uptake.