The Impact and Mechanism of High-Temperature Aged Polyethylene Terephthalate (PET) Microplastics on Anaerobic Sludge Digestion
Prior to entering a wastewater treatment plant, plastic waste undergoes a complex aging process in the natural environment. This process alters the physical and chemical properties of microplastics (MPs), thereby influencing their environmental behavior and ecological risks. Consequently, the aged of MPs is a critical factor that must be considered when evaluating their environmental behavior and biological effects. This study investigates the impact of high-temperature aged polyethylene terephthalate (PET) microplastics on methane production during sludge anaerobic digestion and elucidates the underlying mechanisms. The results indicate that at a concentration of 60 mg/L, the methane production from PET aged for 15 days and 30 days decreases by 2.31% and 3.43%, respectively, compared to primal PET. Material characterization reveals that the surface of aged PET develops micro-pores and cracks, with a slight increase in O/C ratio. Biochemical and microbial community analyses suggest that the primary mechanism involves aged PET inducing oxidative stress and releasing trace leachates, which reduce enzyme activity. Furthermore, high concentrations of aged microplastics can shift methanogenic communities from acetoclastic to hydrogenotrophic pathways, affecting methane production. However, long-term aged microplastics at high concentrations may partially mitigate these inhibitory effects through modulation of electron transfer processes. This study elucidates the mechanisms by which aged microplastics influence anaerobic digestion via multiple pathways, including chemical leaching, oxidative stress, electron transfer, and microbial community shifts, offering new insights into optimizing sludge treatment processes.