Understanding And Regulation Of The Effects Of Bisphenol A And Bisphenol S On Sludge Anaerobic Treatment

Bisphenols（especially bisphenol A）are typical leachable additives in MPs and are among the world’s most prolific bulk ch emicals.With the widespread use of plastics and other bisphenol-based products such as paper towels,adhesives,flame retardants and paints,the leached bisphenols have been widely found in water,soil,sediment,and even human body,with high content（μg/kg～mg/kg）,which has aroused people’s concern about their potential toxicity.As the last barrier before sewage enters the natural environment,wastewater treatment plants（WWTPs）can remove most bisphenols through aggregation,precipitation,bioadsorption or other biomass-mediated processes.It has been found that WWTPs are the important sources of bisphenols in the environment,with much higher levels of bisphenols than those in water,soil,sediment,and other environmental media.It has been reported t hat the concentration of BPA in waste activate sludge（WAS）can be as high as 36.7-142.2 mg/kg dry weight.However,with increasingly strict restrictions on the use of BPA,the production and use of other bisphenols（e.g.,BPS）are increasing,and their en vironmental concentrations are also increasing.The trapped bisphenols in WAS may pose a risk to its subsequent disposal.Anaerobic treatment is a promising WAS treatment method that can effectively reduce sludge volume,kill pathogens,and produce energy and resources such as short-chain fatty acids,hydrogen,and methane.At the same time,anaerobic treatment is a complex biological process involving many microorganisms.The appearance of bisphenols may affect the activity and growth of these microbes,thereby affecting WAS anaerobic treatment.However,to date,a series of key issues such as the migration,transformation,and impact behavior of bisphenols in sludge anaerobic treatment systems and the underlying mechanisms have not been systematically anal yzed.This study focused on WAS obtained from WWTPs and selected two typical bisphenols（i.e.,BPA and BPS）.Through short-term batch experiments,the study investigated the effects of different concentrations of bisphenols（0,50,100,and200 mg/kg TSS）on the production of key target products（methane）,intermediate metabolic products（short-chain fatty acids,hydrogen）,and toxic by-products（hydrogen sulfide）during sludge anaerobic treatment.The study also used various kinetic models to analyze the i mpact of bisphenols on the kinetic parameters of the anaerobic treatment process.The results showed that BPA and BPS had similar influences on WAS anaerobic treatment process.Low levels of BPA and BPS（50 mg/kg TSS）promoted methane production in the ana erobic treatment process,but high concentrations（100-200 mg/kg TSS）inhibited its production.However,BPA and BPS in concentrations between 50-200 mg/kg TSS promoted the production of short-chain fatty acids and hydrogen sulfide during the anaerobic treatment process.The appearance of bisphenols（especially at high concentrations）reduced methane production and increased hydrogen sulfide production,which had an adverse effect on WAS anaerobic treatment.This study further revealed the migration,transf ormation,and interaction behavior of bisphenol substances in sludge anaerobic treatment systems.The experimental results showed that bisphenols were adsorbed by sludge and difficult to biodegrade during anaerobic treatment.Chemical adsorption and multilayer adsorption on irregular surfaces were the main mechanisms for their adsorption.The pseudo-second-order kinetic model and Freundlich model could well fit the adsorption process of BPA and BPS in sludge.In addition,tyrosine and tryptophan substances in extracellular polymeric substances（EPS）were involved in the interaction between EPS and bisphenols.The hydroxyl group,polysaccharide and amide in sludge EPS may be the main adsorption sites of BPA and BPS,and the hydroxyl group and polysaccharide m ay play an important role in the adsorption of BPA and BPS respectively.Moreover,molecular docking showed that BPA and BPS could spontaneously bind with some functional proteins in sludge extracellular substances（such as those responsible for substrate transport and electron transfer）through hydrogen bonding,Pi-Sigma,or Pi-Pi Stacked interactions.In addition,this study analyzed the effects of bisphenol substances on WAS physicochemical characteristics,typical biochemical processes,and key enzyme activities during anaerobic treatment.The experimental results showed that the addition of bisphenols changed WAS properties and promoted the release of soluble organic matter（such as proteins and amino acids）from the solid phase to the liquid phase.It was found that 200 mg/kg TSS bisphenols increased the reaction rate of the hydrolysis of high molecular weight proteins,the degradation of sulfur-containing amino acids,the acidification of small organic molecules,and inorganic sulfate reduction,but in hibited the reaction rate of methane production.Moreover,200 mg/kg TSS bisphenols increased the activity of key enzymes in the hydrolysis（protease）,acidification（phosphotransacetylase and acetate kinase）,and sulfate reduction process（adenosine-5’-phosphosulfate reductase）,but inhibited the activity of coenzyme F ₄₂₀ in the methanogenesis process.Mechanistic studies have shown that bisphenols increase the charge in organics,weakens the mutual repulsion between extracellular hydrolytic enzymes and organicsr.In addition,bisphenols disrupt the hydrogen bond network of proteins,making protein structures loose,providing more usable reaction sites,improving the adsorption and interaction between extracellular hydrolytic enzymes and organics.This may be an important reason for bisphenols to increase hydrolytic enzyme activity.Moreover,bisphenols induce reactive oxygen species（ROS）production.An increase in ROS to a certain extent can stimulate cell activity,but excessive ROS can cause cell membran e damage and increase cell membrane permeability.Differences in EPS characteristics and cell tolerance to ROS among different functional microorganisms（such as acid-producing bacteria,sulfate-reducing bacteria,and methane-producing bacteria）may be an important reason for differences in enzyme activity of different functions.Based on the above research,this study analyzed the impact of bisphenols on the macro and micro characteristics of microbial communities in sludge anaerobic treatment systems by running long-term semi-continuous reactors.Experimental results showed that bisphenols did not significantly change the overall number of OTUs and alpha diversity of the microbial community in the system,but there were significant differences in the compo sition of OTUs,phyla,and genera among different samples.200 mg/kg TSS bisphenols increased the relative abundance of microorganisms involved in hydrolysis,acidification,and sulfate reduction,but reduced the relative abundance of methanogens.In addit ion,200mg/kg TSS bisphenols significantly increased the relative abundance of genes involved in membrane transport,carbohydrate metabolism,and fatty acid biosynthesis,as well as the relative abundance of genes involved in organic sulfurs degradation and sulfate reduction,but reduced the relative abundance of genes involved in acetoclastic and hydrogenotrophic methane production.Finally,this study used calcium peroxide to regulate the impact of bisphenols on sludge anaerobic treatment,enhancing the degradation of bisphenols and promoting WAS anaerobic treatment performance,while revealing the related mechanisms,providing an initial exploration for the regulation of bisphenols in sludge treatment processes.Experimental results showed that calcium p eroxide not only effectively promoted the degradation of bisphenols,reducing their estrogenic activity,but also destroyed WAS structure,promoted WAS degradation,and enhanced the transformation of refractory substances（e.g.,humic substances,lignin）,providing more available substrates for sludge anaerobic fermentation,overcoming the bottleneck of poor bioavailability of sludge,and significantly increasing the production of short-chain fatty acids during sludge anaerobic fermentation.In addition,calcium peroxide increased the p H of sludge system and increased the release of soluble metal ions from sludge,resulting in a change in the form of hydrogen sulfide from H₂S to HS^-and S^2-during anaerobic treatment,which accumulates in sludge system and even generates metal sulfide precipitation,greatly reducing the emission of hydrogen sulfide gas.In summary,the regulation of calcium peroxide can greatly eliminate the adverse effects of bisphenols on anaerobic systems and reduce their potential environmental risks.This study systematically elucidated the impact of bisphenols on sludge anaerobic treatment,investigated the adsorption and degradation behavior of bisphenols in sludge anaerobic systems and their interactions with sludge,and revealed the mechanism of bisphenols’impact on sludge anaerobic treatment process from aspects such as sludge physicochemical properties,typical biochemical processes,key enzyme activities,microbia l community succession,and functional gene abundance changes.On this basis,calcium peroxide was used to regulate the impact of bisphenols on sludge anaerobic treatment,enhancing the degradation of bisphenols and promoting sludge anaerobic treatment per formance.These results fill the theoretical gap in the impact of bisphenols on sludge anaerobic treatment and provide necessary theoretical basis for enhancing the adsorption and degradation of bisphenols in sludge and effectively controlling the ecotoxicity of bisphenols in the environment.Especially,BPS,as the main substitute for BPA,cannot effectively reduce the ecotoxicity of bisphenols in anaerobic environments.Therefore,this study has certain theoretical and practical value.