Study On The Efficiency Of Sulfur-sulfide Co-driven Autotrophic Denitrification

Under the increasing pressure of stricter sewage discharge standards announced by local governments in China,upgrading or renovating sewage treatment plants has become a necessary measure.Microbial denitrification is an eco-friendly and cost-effective technology widely used in biofilters behind secondary biological treatment to effectively remove total nitrogen（TN）.However,heterotrophic denitrification requires organic substrates（electron donors）and carbon sources,leading to high operating costs and carbon dioxide emissions.Therefore,the sulfur autotrophic denitrification（SAD）process that utilizes low-cost sulfur as the electron donor is becoming a promising alternative.Various reduction states of sulfur,such as S^2-,S⁰,S₂O₃^2-,and SO₃^2-,can be effectively utilized by SAD microorganisms.Therefore,solid particulate elemental sulfur（S⁰）is mainly used as a biofilm carrier or electron donor to construct S⁰ packed bed（S⁰PB）.However,the fixed capacity of sulfur（electron donor）in the packing limits the maximum denitrification efficiency,making it difficult to cope with the increasing nitrate load and decreasing temperature in practical applications.Sulfide,as the lowest valence compound of sulfur,has the potential to enhance the denitrification performance of S⁰PB and can be used as an external supplementary electron donor.However,the response of the biofilm to the electron donors from elemental sulfur（inside to out）and sulfides（outside to in）is still unknown,and studies on the efficacy of monomeric sulfur-sulfide synergy to drive autotrophic denitrification are still lacking,especially considering the potential biotoxicity of sulfide.In this study,we investigated the effects of different doses of sulfide on S⁰-based biofilm（S⁰PB）to explore the mutual influence of sulfide and elemental sulfur in driving autotrophic denitrification.When the dose of sulfide was increased to 3.6 kg/m³/d,the nitrate removal efficiency continued to increase,and the effluent nitrate was reduced to2.7 mg N/L.However,when the sulfide dose exceeded 0.9 kg/m³/d,nitrite accumulation became severe,exceeding 6.5 mg N/L,leading to a decrease in total nitrogen removal efficiency.Kinetic studies confirmed that increasing the external sulfide dosage to the S⁰PB system could accelerate the nitrate reduction rate（k:0.04-0.27）,but excessive sulfide usage would harm the nitrite reduction rate（k:0.09-0.04）.Further calculation results showed that increasing sulfide usage would conflict with in situ sulfur,manifested as the increasing contribution of sulfide to the total nitrogen removal rate,up to 85.5%,while the contribution of in situ sulfur decreased continuously.In addition,although the important sulfur-oxidizing bacteria Thiobacillus was enriched twice in the study,CLSM observations showed significant biomass reduction and biofilm destruction,which was attributed to the biological toxicity of sulfide and accumulated nitrite.Therefore,this study emphasizes the importance of optimizing external sulfide dosage to improve the denitrification performance of the SAD system and highlights the importance of considering the potential biological toxicity of external electron donors when developing efficient wastewater treatment strategies.