Effects of electrical stimulation on purification of secondary effluent containing chlorophenols by denitrification...

The coexistence of chlorophenols (CPs) and total nitrogen (TN) is common in advanced purification of industrial secondary effluent, which brings challenges to conventional denitrification biofilters (DNBFs). Electrical stimulation is an effective method for the degradation of CPs, However, the application of electrical stimulation in DNBFs to enhance the treatment of secondary effluent containing CPs remains largely unknown. Herein, this study conducted a systematic investigation towards the effects of electrical stimulation on DNBF through eight lab-scale reactors at room and low temperatures and different hydraulic retention times (HRTs). Results showed that the electrical stimulation effect was not greatly affected by temperature and the optimal applied voltage was 3 V. Overall, the removal rates of TN and CPs were increased by 114%-334% and 2.68%-34.79% respectively after electrical stimulation. When the influent concentration of NO₃^--N, COD and each CP of 25 mg/L, 50 mg/L and 5 mg/L, about 15 mg/L of effluent TN could be achieved and the removals of p-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol were increased by 10.58%, 5.78% and 34.79% respectively, under the voltage of 3 V and HRT of 4 h. However, the reduction rate of biotoxicity was decreased and could not achieve low toxicity grade in general. Electrical stimulation promoted the elevation of Hydrogenophaga and thus enhanced the removal of TN, and the increase of Microbacterium and Ahniella was significantly associated with the improvement of CPs removal rate. In addition, the obvious accumulation of nitrite was found to be significantly negatively correlated with the abundance of Nitrospira. This study highlighted a further need for the optimization of electrical stimulation in DNBFs treating industrial secondary effluent containing CPs to achieve the goal of pollutant removal and toxicity reduction simultaneously.