Performance Response And Adaptation Mechanism Of Simultaneous Sulfide And Nitrate Removal Process Under Salinity Stress

The"14th Five-Year Plan"and the"double carbon"strategy(carbon peaking and carbon neutral)have sparked the rebirth of the biological denitrification process for wastewater.With extremely strong substrate removal and environmental tolerance,the Simultaneous sulfide and nitrate removal process(SSNAR)uses sulfide as an electron donor to achieve autotrophic denitrification in anoxic conditions.It is an excellent solution for deep nitrogen removal under"double carbon"target.Furthermore,hypersaline nitrogen-containing wastewater is being produced more frequently as a result of the rising industrial and seawater usage sectors.Due to salinity stress,conventional biological treatment technologies struggle to adequately handle this sort of wastewater.In recent years,sulfide driven autotropic denitrification behavior have been discovered in marine and saline lakes,indicating that SSNAR have a great deal of potential for treating hypersaline nitrogen-containing wastewater.However,there are still few studies on its applying.Therefore,this study applied shock and continuous salinity stress on the SSNAR,analyzed the combined effects of salinity and substrate concentration on the substrate removal performance,observed the microscopic morphology,extracellular bio-sulfur,and intracellular K~+/Na~+response patterns to salinity,as well as tracked the structure evolution of microbial community,potential salt-tolerant bacteria and community interactions,which were revealed from multiple dimensions including substrate removal performance,sludge physiology,and microbial community.It would provide a theoretical and technical basis for the stable operation of SSNAR under hypersaline conditions.The main conclusions are as follows:1)The performance response pattern of SSNAR under shocking salinity stress was clarified.For sulfide oxidation and denitrification performance,the tolerable salinity shock levels were 6.09 wt%and 6.64 wt%,respectively.It was determined through factorial analysis that the acute combined toxicity of salinity level and substrate concentration on sulfide oxidation performance depended on the substrate concentration,with antagonistic inhibition at substrate concentration below 480 mg S/L and synergistic inhibition at substrate concentration above 480 mg S/L;whereas the acute combined toxicity of both on denitrification performance depended on salinity,with antagonistic inhibition at salinity below 4 wt%.When the salinity was below 4 wt%,inhibition was antagonistic,and when it was over 4 wt%,it was synergistic.2)The performance response pattern of SSNAR under continuous salinity stress was investigated.The sulfide oxidation and denitrification performance of SSNAR could tolerate continuous salinity levels of 6.64 wt%and 4.67 wt%,respectively.Both performances could recover when the salinity level was below 4.67 wt%.Reducing substrate concentration promoted the recovery of SSNAR under salinity stress.When the salinity level was below 5wt%,the combined toxicity of salinity level and substrate concentration on sulfide oxidation and denitrification were synergistic inhibition.When the salinity level was above 5 wt%,the combined toxicity were antagonistic inhibition and additive inhibition,respectively.3)The physiological adaptation mechanism of the sludge of SSNAR under salinity stress was revealed.The high salinity level(6.64 wt%)stress led to a distinctly depressed or ruptured cell surface,while the low to medium salinity level stress(3.21 to 4.67 wt%)led to a partially ruffled cell surface.Under salinity stress,the sludge of SSNAR responded to the osmotic pressure changes mainly by dewatering,drawing Na~+from the environment and synthesizing intracellular elemental sulfur,which could effectively balance the osmotic pressure below 4.67wt%but not 6.64 wt%.In addition,ionic stress due to salinity is the main limiting factor for performance recovery.4)The microbial community adaptation mechanism of SSNAR under salinity stress was elucidated.Based on LEf Se analysis,Sulfurimonas,Thauera,and Paracoccus were the key functional bacterial genera for the performance recovery,while Candidatus＿Caldatribacterium,Pseudoxanthomonas,Aminivibrio,and Halomonas were the key salt-tolerant bacterial genera.The co-occurrence network results show that salinity stress caused a decrease in network complexity but an increase in stability,resulting in a more rapid response to environmental fluctuations.The molecular ecological network successfully adapted to low to medium salinity level(3.21-4.67 wt%)in 20 days,where highly connected modules of nodes were essential to resist salt and microorganisms within the network resisted or responded to salinity stress through a cooperative strategy.In summary,this paper investigated the performance response of the SSNAR under salinity stress and its salinity adaptation mechanism by examining the changes in substrate removal performance,product type,physiological morphology of sludge and microbial community of the process under shocking and continuous salinity stress,which provide theoretical basis and technical basis for the stable operation of the process under hypersaline conditions.