Exploring mixotrophic denitrification in a continuous double-chamber bioelectrochemical system treating nitrate-contaminated wastewater

Nitrate (NO₃^-) pollution in aquatic environments, mainly due to the excessive use of nitrogen-based fertilizers and chemicals, poses significant risks to water quality and human health. Bioelectrochemical systems (BESs) have emerged as innovative solutions to complement traditional wastewater treatment methods for effective NO₃^- removal, addressing the need for sustainable approaches to wastewater management at different scales. In this study, the NO₃^- removal potential of a denitrifying double-chamber flow-through BES applied as post-treatment of an aerobic granular sludge system used for combined carbon and nitrogen removal was evaluated for 63 days under different feed (anodic and cathodic) and hydraulic retention time (HRT) (2–10 h) conditions. After 6 days of biomass acclimatization and at an HRT of 2 h, complete removal of N-NO₃⁻ and N-NO₂⁻ was achieved primarily through heterotrophic denitrification due to the presence of acetate. Nevertheless, using only the electrode as the electron source resulted in NO_x (NO₃⁻+ NO₂⁻) removal efficiencies of up to 65 ± 16 %, with NO_x concentrations remaining below the Italian standard for industrial effluent discharge into sewers (30 mg N-NO₃⁻·L⁻¹ and 0.6 mg N-NO₂⁻·L⁻¹), demonstrating the good performance and applicability of the system even in the absence of organic carbon in the feed. The combined autotrophic and heterotrophic denitrification resulted in lower specific energy consumption compared to the use of autotrophic denitrification alone, ranging from 2.3·10⁻² to 9.6·10⁻⁵ kWh·g NO_{x, removed}⁻¹ at an HRT of 2 h.