Sulfur autotrophic denitrification as an efficient nitrogen removals method for wastewater treatment towards lower organic requirement: A review

The sulfur autotrophic denitrification (SADN) process is an organic–free denitrification process that utilizes reduced inorganic sulfur compounds (RISCs) as the electron donor for nitrate reduction. It has been proven to be a cost–effective and environment–friendly approach to achieving carbon neutrality in wastewater treatment plants. However, there is no consensus on whether SADN can become a dominant denitrification process to treat domestic wastewater or industrial wastewater if organic carbon is desired to be saved. Through a comprehensive summary of the SADN process and extensive discussion of state–of–the–art SADN–based technologies, this review provides a systematic overview of the potential of the SADN process as a sustainable alternative for the heterotrophic denitrification (HD) process (organic carbons as electron donor). First, we introduce the mechanism of the SADN process that is different from the HD process, including its transformation pathways based on different RISCs as well as functional bacteria and key enzymes. The SADN process has unique theoretical advantages (e.g., economy and carbon–free, less greenhouse gas emissions, and a great potential for coupling with novel autotrophic processes), even if there are still some potential issues (e.g., S intermediates undesired production, and relatively slow growth rate of sulfur–oxidizing bacteria [SOB]) for wastewater treatment. Then we present the current representative SADN–based technologies, and propose the outlooks for future research in regards to SADN process, including implement of coupling of SADN with other nitrogen removal processes (e.g., HD, and sulfate–dependent anaerobic ammonium oxidation), and formation of SOB–enriched biofilm. This review will provide guidance for the future applications of the SADN process to ensure a robust–performance and chemical–saving denitrification for wastewater treatment.