Nitrate Removal from Groundwater by Heterotrophic and Electro-Autotrophic Denitrification

Abstract

A heterotrophic and autotrophic denitrification (HAD) system shows satisfactory performance for groundwater with nitrate contamination. In this study, an HAD system combining solid-phase heterotrophic denitrification and electrochemical hydrogen autotrophic denitrification (SHD-EHD) was developed for the treatment of nitrate-contaminated groundwater, in which polycaprolactone (PCL) was used as the carbon source to enhance the nitrate removal performance and prevent secondary pollution of the electrochemical hydrogen autotrophic denitrification (EHD) system. The denitrification performance, microbial community structure and nitrogen metabolism were investigated. The results showed that a high nitrate removal rate of 99.04% was achieved with an influent nitrate concentration of 40 mg/L, a current of 40 mA and a hydraulic retention time (HRT) of 4 h. By comparing the performance with the EHD system, it was found that the HAD system with PCL promoted the complete denitrification and reduced the accumulation of NO2−-N. Analysis of the microbial community structure identified the key denitrifying bacteria: Dechloromonas, Thauera and Hydrogenophaga. A comparison of microbial communities from SHD-EHD and solid-phase heterotrophic denitrification (SHD) demonstrated that electrical stimulation promoted the abundance of the dominant denitrifying bacteria and the electroactive bacteria. Analysis of the nitrogen metabolic pathway revealed that the conversion of NO to N2O was the rate-limiting step in the overall denitrification pathway. The SHD-EHD developed in this study showed great potential for groundwater nitrate removal.