Insights into sulfur cycling contribution to nitrogen removal in sulfur autotrophic denitrification combined microbial electrochemical denitrification

In the face of increasing discharge standards, sulfur autotrophic denitrification (SAD) is gradually applied for deep nitrogen removal of electron-donor-deficient waters. However, the product sulfate induces secondary pollution in the effluent, and the proton consumption from alkalinity is detrimental to the stable operation of the system. In this study, a sulfur autotrophic denitrification coupled with microbial electrochemical denitrification system (S-MED) is proposed to reduce the addition of inorganic sulfur, alleviating nitrite accumulation, while enabling the system to self-sustain at a neutral pH. The S-MED system that was operated under the applied potential of −0.5 V achieved a nitrate removal of 97.5 ± 1.57 % and a total nitrogen removal of 90.3 ± 4.27 %, which were 14.6 % and 68.0 % higher than the SAD systems in control under the S/N molar ratio of 0.8. The nitrite accumulation rate was reduced by 69.1 % compared to SAD. The effluent sulfate concentration was also reduced to 5.8 mg/mg-N TN removal. The electrochemically active denitrifying bacterium Thiobacillus was enriched in the coupled system, and the copy numbers of functional genes of narGHI, napAB, and nirK involved in denitrification were increased. According to the microbial analysis and electrochemical characteristics, it suggested that S2- might be generated from sulfate with the presence of electrodes, enabling the cycling of sulfur, providing mobile electron carriers for denitrification, thus allowing bacteria to conduct denitrification by indirectly obtaining electrons from the electrode.