Evaluating the Effects of Different Pretreatments on Anaerobic Digestion of Waste Activated Sludge Containing Polystyrene Microplastics

A majority of microplastics (MPs) in wastewater treatment plants are aggregated in waste activated sludge (WAS) and potentially threaten the subsequent anaerobic digestion. Meanwhile, many physical and chemical techniques have been used as pretreatment methods to improve the anaerobic digestion of WAS. However, the effects of frequently used pretreatments on the MPs during WAS anaerobic digestion remain poorly characterized. Here, the impacts of physical (ultrasonic and thermal methods) and chemical pretreatments (alkaline and Fenton methods) on the anaerobic digestion performance of WAS containing polystyrene microplastics (PS-MPs) were investigated. Thermal and chemical pretreatments showed obvious improvements in sludge disintegration, the methane yields of which were increased by 17.5–20.4%. The concentrations of sodium dodecyl sulfate leached from PS-MPs in chemically pretreated groups were 1.7–2.6 times higher than those of the physical pretreatments in spite of the high efficiency of digestion. In addition, the generation of reactive oxygen species among four pretreated groups showed no significant difference (p > 0.05) and a slight increase compared with that of the control. As for the long-term effects of pretreatments on microbial communities, the thermally pretreated digester exhibited the highest proportion of methanogens (phyla Halobacterota and Euryarchaeota) among the four pretreatments, accounting for 13.2%. From the perspectives of the performance of anaerobic digestion and the toxic alteration of MPs, thermal pretreatment is more recommended to improve the anaerobic digestion of WAS containing PS-MPs.