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 of 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 to alleviate nitrite accumulation, while making the system self-sustaining at neutral pH. The electrochemically active denitrifying bacterium Thiobacillus was enriched in the coupled system, and the copy numbers of functional genes of narG/H/I, napA/B, and nirK involved in denitrification were increased. According to the microbial analysis and electrochemical characteristics, it suggested that sulfate 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.