Rice husk-intensified cathode driving bioelectrochemical reactor for remediating nitrate-contaminated groundwater

To achieve economical and eco-friendly denitrification, rice husk-intensified cathode driving bioelectrochemical reactor (RCBER) was constructed with rice husk as solid-phase carbon source and microbial carrier. Results demonstrated that the application of current improved the utilization of rice husk and enhanced the denitrification, and the quenching of anodic hydroxyl radicals by rice husk also improved the microbial resistance to current. The highest nitrate removal rate as 0.34 mg-N/(L∙d), higher economic benefits, i.e., current efficiency as 31.6% and energy consumption as 2.43 kWh/g NO₃^--N, and the highest environmental benefit, i.e., hydrogenotrophic denitrification contribution as 37.9%, were obtained at 200 mA/m². The best performance at 200 mA/m² was related to its better microenvironment, such as lower accumulation of anodic by-products and higher bioavailability of rice husks, as well as higher microbial metabolic activity, such as stable extracellular polymeric substance, the maximum electron transport system activity as 11.63 ± 0.14 μg O₂·g^-1·min^-1·mg protein^-1 and the highest activity of nitrate reductase (3.15-fold that of control check). The application of current realized the coexistence of heterotrophic and hydrogenotrophic denitrifiers, and multiple functional bacteria such as anaerobic denitrifiers Flavobacterium, aerobic denitrifiers Comamonas, hydrogenotrophic denitrifiers Thermomonas and electron transfer-related Enterobacter coexisted at 200 mA/m², thereby improving RCBER's adaptability to the complex microenvironment. This study provides the theoretical basis for realizing a win-win situation of environmental pollution remediation and agricultural waste disposal.