Hydrogenotrophic denitrification for treatment of nitrate-contaminated groundwater: Using a microbubble diffuser to optimize performance

Nitrate nitrogen (NO₃ ⁻-N) contamination of groundwater as a drinking water resource has been detected in many countries and poses a serious threat to human health. This study optimized the performance of a hydrogenotrophic denitrification (HD) reactor for nitrate-contaminated groundwater treatment by introducing a microbubble diffuser (MD), and evaluated its performance stability. The HD reactor continuously maintained effluent NO₃ ⁻-N and NO₂ ⁻-N concentrations below the drinking water standard for 200 days under optimized conditions (8 h hydraulic retention time, 10 mL min⁻¹ H₂ flow rate, and 50 mg L⁻¹ NO₃ ⁻-N). The nitrogen removal rate and nitrogen removal efficiency were 142.0 gN m⁻³ d⁻¹ and 98 %, respectively. Additionally, the HD reactor required 0.02 m³ of H₂ for 1 g of nitrogen removal, which indicated that approximately 24 % of the supplied H₂ was utilized for HD. The MD produced finer gas bubbles and showed approximately 2.3-times higher gas-dissolving efficiency than an air stone diffuser producing ordinary bubbles. The efficiency is attributed to reactor performance optimization. In addition, Azoarcus spp. was the predominant genus in the HD reactor and accounted for 42.3 % of the total bacteria, suggesting that Azoarcus spp. might be related to HD in the reactor. Functional genes for denitrification (narG and nirS) were continuously detected. The experimental results lead to the development of an effective HD reactor for nitrate-contaminated groundwater treatment—in terms of elimination of H₂ supply for denitrification under safe operation—and can facilitate the improvement of water security.