Microplastics (MPs)/nanoplastics (NPs) have been widely detected in wastewater treatment plants (WWTPs). They are captured mainly by sludge and unavoidably move into the anaerobic digestion (AD) process. Recent studies suggested that MPs/NPs could induce oxidative stress to the microbiome in digesters. The thermal hydrolysis process (THP) has been implemented for sludge pretreatment in many full-scale WWTPs. To date, there is limited knowledge about how THP can influence MPs/NPs-induced stress during AD. This study systematically investigated the impact of THP (80 and 160 °C) on AD of sewage sludge exposed to different levels (50–150 μg/L) of polystyrene nanoplastics (PsNPs), one of the most found MPs/NPs in WWTPs. Compared to the control, higher PsNPs levels of 100 and 150 μg/L decreased methane yields by 17.98 and 29.34%, respectively. Moreover, reactive oxygen species (ROS) levels increased by 17.18 and 34.84%. Our results demonstrated that THP counteracted the suppression of methane production imposed by such PsNPs concentrations, with decreased ROS levels. Also, THP reduced antibiotic resistance gene (ARG) propagation that can be encouraged by PsNPs, thus minimizing the ARG transmission risks of digestate biosolids. These findings suggest that THP holds a high promise to further develop as a remediation method for MPs/NPs in WWTPs.

The positive impact of the thermal hydrolysis process (THP) of sewage sludge on antibiotic resistance genes (ARGs) removal during anaerobic digestion (AD) has been reported in the literature. However, little information is available on how changes in different extracellular polymeric substances (EPS) due to THP can influence ARG propagation during AD. This study focused on systematically correlating EPS components and ARG abundance in AD of sewage sludge pretreated with THP (80 °C, 110 °C, 140 °C, 170 °C). THP under different conditions improved sludge solubilization followed by improved methane yields in the biochemical methane potential (BMP) test. The highest methane yield of 275 ± 11.5 ml CH4/g COD was observed for THP-140 °C, which was 40.5 ± 2.5% higher than the control. Increasing THP operating temperatures showed a non-linear response of ARG propagation in AD due to the rebound effect. The highest ARGs removal in AD was achieved with THP at 140 °C. The multivariate analysis showed that EPS polysaccharides positively correlated with most ARGs and integrons, except for macrolides resistance genes. In contrast, EPS protein was only strongly correlated with β-lactam resistance genes. These results suggest that manipulating THP operating conditions targeting specific EPS components will be critical to effectively mitigating the dissemination of particular ARG types in AD.

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