Different responses of mesophilic and thermophilic anaerobic digestion of waste activated sludge to PVC microplastics.

The effect of microplastics (MPs) retained in waste activated sludge (WAS) on anaerobic digestion (AD) performance has attracted more and more attention. However, their effect on thermophilic AD remains unclear. Here, the influence of polyvinyl chloride (PVC) MPs on methanogenesis and active microbial communities in mesophilic (37 °C) and thermophilic (55 °C) AD was investigated. The results showed that 1, 5, and 10 mg/L PVC MPs significantly promoted the cumulative methane yield in mesophilic AD by 5.62%, 7.36%, and 8.87%, respectively, while PVC MPs reduced that in thermophilic AD by 13.30%, 18.82%, and 19.99%, respectively. Moreover, propionate accumulation was only detected at the end of thermophilic AD with PVC MPs. Microbial community analysis indicated that PVC MPs in mesophilic AD enriched hydrolytic and acidifying bacteria (Candidatus Competibacter, Lentimicrobium, Romboutsia, etc.) together with acetoclastic methanogens (Methanosarcina, Methanosaeta). By contrast, most carbohydrate-hydrolyzing bacteria, propionate-oxidizing bacterium (Pelotomaculum), and Methanosarcina were inhibited by PVC MPs in thermophilic AD. Network analysis further suggested that PVC MPs significantly changed the relationship of key microorganisms in the AD process. A stronger correlation among the above genera occurred in mesophilic AD, which may promote the methanogenic performance. These results suggested that PVC MPs affected mesophilic and thermophilic AD of WAS via changing microbial activities and interaction.