Nitrogen removal and microbial community diversity in single-chamber electroactive biofilm reactors with different ratios of the cathode surface area to reactor volume

Highlights: • Single-chamber BERs with different AV ratios (EBR-500, EBR-250 and EBR-125) were operated at different HRTs. • The higher the AV ratio, the greater is the increase in nitrification. • EBR-250 showed the highest TN removal (62.0%) with well-balanced nitrification and denitrification at an HRT of 6 h. • Various nitrogen cycle bacteria were observed in the biofilms (anode and cathode) Removal of nitrogen compounds is particularly important domestic wastewater treatment. Our recent study reported the successful removal of nitrogen in single-chamber electroactive biofilm reactors (EBRs) under aeration-free conditions. We hypothesized that the oxygen diffused from the air-cathode is a key factor in the removal of nitrogen in the EBR. If so, the effect of the penetrated oxygen would vary according to the ratio of the air-cathode surface area to the reactor volume (AV ratio) and the hydraulic retention time (HRT). In this study, single-chamber EBRs with three different AV ratios: 125 m²/m³ (EBR-125), 250 m²/m³ (EBR-250), and 500 m²/m³ (EBR-500) were evaluated for the removal of nitrogen under different HRTs of 0.5–6 h. The higher the AV ratio, the greater the increase in nitrification. The total nitrogen (TN) removal efficiency of EBR-125 and EBR-250 decreased as the HRT decreased, while that of EBR-500 increased. EBR-250 showed the highest TN removal (62.0%) with well-balanced nitrification (83.9%) and denitrification (75.1%) at an HRT of 6 h. However, EBR-500 appeared to be superior for practical application because it showed a comparable TN removal (59%) at a substantially short HRT of 1 h. The microbial communities that were involved in the nitrogen cycle varied according to whether the biofilms were located on the anodes, separators, and cathodes but were similar among EBRs with different AV ratios. Nitrifying bacteria were detected in the biofilms that were presented on the cathodes (approximately 7.8% of the total phylotypes), while denitrifying bacteria were mainly found in biofilm that were located on the anodes (approximately 23.3%). Anammox bacteria were also detected on the anode (approximately 3.7%) and in the separator biofilms (approximately 1.9%) of all the EBRs. These results suggest that both the A/V ratio and the HRT could affect the counter diffusion of substrates (NH₄⁺ and organic compounds) and oxygen in the biofilms and allow interactions between a diversity of microorganisms for the successful removal of nitrogen in EBRs. These findings are expected to aid in the development of new applications using EBR for energy-saving wastewater treatment.