Bioelectrochemical reactor improved by assembling anode with rice husk for treating nitrate-contaminated groundwater

Abstract

To maximize the hydrogenotrophic denitrification and minimize the negative impact of anode by-products, a rice husk-enhanced anode bioelectrochemical reactor (RABER) was constructed by assembling anode with rice husk, and the denitrification performances and mechanisms involved were investigated. It was demonstrated that rice husk can be used as a slow-release carbon source and microbial carrier in RABER and weakened the anodic by-products. RABER obtained the highest contribution of hydrogenotrophic denitrification as 33.7% and achieved the fastest nitrate removal rate of 0.27 mg-N/(L·d) at 200 mA/m2. The relative lower accumulation of dissolved oxygen (1.99 ± 0.11 mg/L) and hydroxyl radicals were obtained at 200 mA/m2. Moreover, stable extracellular polymeric substances and higher activities of denitrification-related enzymes, i.e., around 2.78-fold for nitrate reductase and 1.96-fold for nitrite reductase those of 0 mA/m2, promoted denitrification in RABER at 200 mA/m2. At an optimal current density of 200 mA/m2, the coexistence of cellulose-degrading bacteria (Cellvibrio), aerobic denitrifier (Brevundimonas), anaerobic denitrifier (Flavobacterium), and hydrogenotrophic denitrifier (Alicycliphilus) was achieved, and the inorganic carbon generated by heterotrophic denitrifiers and electrochemical mineralization was sufficient to meet the demands of hydrogenotrophic denitrifiers, conductive to the heterotrophic‑hydrogenotrophic synergistic denitrification. This study provided a theoretical basis for the enhancement of hydrogenotrophic denitrification in BER as well as minimizing the side effects of the anode and providing a new strategy for the disposal of agricultural waste.