A novel pattern of coupling sulfur-based autotrophic disproportionation and denitrification processes for achieving high-rate and precisely adjustable nitrogen removal

Sulfur-based autotrophic disproportionation (SADP) has been accidentally discovered during employing sulfur-based autotrophic denitrification (SADN) bioreactor to treat low C/N ratio wastewater. The SADP products, including sulfide (SADP-S²⁻) and polysulfide (SADP-S_n²⁻), can supply additional electrons to improve the denitrification efficiency. However, the in-situ SADP process was challenging to control, resulting in the unstable enhancement efficiency and excessive sulfide emissions, thereby impeding its scalability in practical engineering. In this study, an independent SADP process was achieved in a sulfur-packed bed bioreactor, with a controllable sulfide production rate ranging from 0.24 ± 0.02 to 0.83 ± 0.03 kg-S/m³/d by adjusting the empty bed contact time between 1 and 5 h. Subsequently, the SADP bioreactor was connected upstream of a SADN bioreactor to create a sulfur-based autotrophic disproportionation-denitrification (SAD²) system. The SAD² system realized an enhanced denitrification rate by up to 2.38 times, with flexibility to adjust between 0.65 ± 0.06–1.55 ± 0.20 kg-N/m³/d. One reason for this enhancement was that SADP-S²⁻ provided additional electrons to the SADN process and exhibited higher biocompatibility than chemical-S²⁻ (Na₂S) at similar doses. Additionally, it stimulated the sulfur bioavailability by 1.66 times as a result of stoichiometry matrix calculation. SADP-S_n²⁻ was only detected in the biofilm, as indicated by a linear increase in the S_n proportion from 8.1 % to 30.1 %, but with minimal releases (352 μg-S/L) into the water. Another reason for the enhancement was bioaugmentation, with increases in protein (30.0 %), ATP (6.9 %), live cells proportion (10.1 %) and total relative abundance of denitrificans (87.7 %). These findings imply that the novel SAD² system facilitates adjustable and high-rate nitrogen removal without the risk of sulfide emission, surpassing the capabilities of the independent SADN system. This offers a new pattern for practical applications.