Effects And Mechanisms Of Polyamide 6 And Polycarbonate Microplastics On Methane Production In Anaerobic Digestion Of Waste Activated Sludge

Microplastics,as a new type of pollutant ubiquity in aquatic environments,have received increasing attention from all over the world.Meanwhile,a large amount of microplastics have also been detected in wastewater and waste activated sludge（WAS）in the wastewater treatment plants（WWTPs）.However,the effects and underlying mechanisms of the existed microplastics on anaerobic digestion of WAS have not been fully documented.In this study,the effects of two common microplastics in sludge,Polyamide 6（PA6）and polycarbonate（PC）microplastics,on anaerobic methane production from WAS were investigated using biochemical methane production（BMP）and long term semi-continuously testing,and further explored the underlying mechanisms by monitoring the degradation of model substrates,the activities of enzymes,the effects of leaching chemicals,as well as the structure of microbial community.The results indicated that the existence of PA6 microplastics（d=0.5-1.0 mm,5-50particles/g TS）significantly increased methane production during the anaerobic digestion of WAS.The presence of 10 particles PA6 microplastics/g TS achieved the highest methane production of 173±8 L CH₄/kg VS,increased by 39.5%compared with control（124±6 L CH₄/kg VS）.PA6 microplastics had no effect on hydrolysis coefficient,while promoted methane production potential of WAS.The mechanistic studies showed that PA6 microplastics affected anaerobic methane production mainly due to the leaching of caprolactam（CPL）.The CPL binds with enzyme molecules to change the active site and other conformations of the enzyme,thereby enhancing the affinity with the substrate and the catalytic activity of the enzyme,hence improved acidification and methanogenesis processes,and increased methane production.Research on PC microplastics showed that the lower concentrations（10-60 particles/g TS）of PC microplastics（d=165-212μm）also improved methane production,while higher concentrations（100 and 200 particles/g TS）of PC microplastics inhibited the methane production.The highest methane production obtained at 30 particles/g TS of PC microplastics,arriving 286±4 L CH₄/kg VS and increased by 24.7%compared with control group of 230±5 L CH₄/kg VS.A mechanisms study revealed that the leached bisphenol A（BPA）rather than phenol played a role in PC microplastics.The leached BPA from PC microplastics improved the key enzyme activities by combined with enzyme proteins,and increased methane production by promoting the relative abundance of microorganisms（bacteria and archaea）.In addition,the induction of reactive oxygen species by high levels of PC microplastics resulted in the decline in cell viability,thereby inhibiting the production of methane.This study revealed the effects and potential mechanisms of two common microplastics PA6 and PC on methane production of WAS anaerobic digestion.These findings provide an important reference for deeply understanding the effect behaviors and mechanism characteristics of microplastics on wastewater and WAS treatment,and have important theoretical and practical guiding significance for avoiding and eliminating the negative risks of residual microplastics on sludge treatment processes and ensuring the stable operation of WWTPs.