Sludge Thermal Hydrolysis for Mitigating Oxidative Stress of Polystyrene Nanoplastics in Anaerobic Digestion: Significance of the Solids Content

Microplastics and nanoplastics (MPs/NPs) have been extensively detected in sewage sludge. They inevitably transfer to anaerobic digestion (AD) used for sludge processing and induce oxidative stress on the anaerobic microbiome. The thermal hydrolysis process (THP) is a widely implemented sludge pretreatment process to boost methane generation and solids reduction in AD. Although few recent reports indicated that the THP could alleviate oxidative stress of MPs/NPs in AD, little is known about how different solids contents of sludge would influence its effectiveness. This study scrutinized how the THP (160 °C, 60 min) affects AD of primary sludge with 4, 8, and 12% total solids (TS) when exposed to polystyrene nanoplastics (PsNPs). The presence of PsNPs (150 μg/L) substantially enhanced reactive oxygen species (ROS) levels at lower TS (4 and 8%) compared to a higher TS of 12% (16.20–16.71% vs 8.79%). Consequently, methane production decreased by 7.25–15.07% for 4–8% TS. Nonetheless, applying the THP could effectively mitigate ROS-induced stress and the propagation of most antibiotic resistance genes (ARGs). Moreover, a positive correlation was observed between the changes in extracellular polymeric substances due to the THP and the impact of PsNPs. These results provide new insights into understanding the significance of the sludge solids content in the THP for coremediation of PsNPs-induced oxidative stress and ARGs propagation in AD.