Nitrate removal from groundwater by cooperating heterotrophic with autotrophic denitrification in a biofilm–electrode reactor

► Intensified biofilm–electrode reactor using cooperative denitrification is developed. ► IBER combines heterotrophic and autotrophic denitrification. ► CO 2 formed by heterotrophic denitrification is used by autotrophic bacteria. ► Optimum running conditions are C/N = 0.75, HRT = 8 h, and I = 40 mA. ► A novel degradation mechanism for cooperating denitrification process is proposed. An intensified biofilm–electrode reactor (IBER) combining heterotrophic and autotrophic denitrification was developed for treatment of nitrate contaminated groundwater. The reactor was evaluated with synthetic groundwater (NO 3 −–N50 mg L −1) under different hydraulic retention times (HRTs), carbon to nitrogen ratios (C/N) and electric currents ( I). The experimental results demonstrate that high nitrate and nitrite removal efficiency (100%) were achieved at C/N = 1, HRT = 8 h, and I = 10 mA. C/N ratios were reduced from 1 to 0.5 and the applied electric current was changed from 10 to 100 mA, showing that the optimum running condition was C/N = 0.75 and I = 40 mA, under which over 97% of NO 3 −–N was removed and organic carbon (methanol) was completely consumed in treated water. Simultaneously, the denitrification mechanism in this system was analyzed through pH variation in effluent. The CO 2 produced from the anode acted as a good pH buffer, automatically controlling pH in the reaction zone. The intensified biofilm–electrode reactor developed in the study was effective for the treatment of groundwater polluted by nitrate.