Microplastics and antibiotic resistance genes nexus in sewage sludge: impact of thermal hydrolysis process- anaerobic digestion

Sewage sludge is increasingly recognized as a major reservoir of emerging contaminants, notably microplastics (MPs), antibiotic resistance genes (ARGs), and biofilm-embedded microbial communities. Their persistence during wastewater treatment poses environmental and public health risks, particularly when treated biosolids are applied to land. This review synthesizes current understanding on the interactions between MPs, ARGs, and biofilms in sludge treatment, with emphasis on thermal hydrolysis process (THP) integrated with anaerobic digestion (AD). MPs accumulate in sludge and undergo physical and morphological changes during THP and AD, yet they rarely degrade completely, thereby continuing to act as carriers for ARGs and microbial colonization. THP, through high-temperature and pressure processing, effectively lyses microbial cells, degrades DNA, and solubilizes extracellular polymeric substances (EPS). THP can reduce total absolute abundance of ARGs and MGEs up to 11.09 and 2.33 log copies/g sludge, respectively, from raw sludge. However, ARG rebound during subsequent AD remains a persistent challenge (2.27-7.39 log copies/g for ARGs; 0.70-2.21 log copies/g for MGEs rebound in total absolute abundance), but THP coupled AD systems still demonstrate the lowest final absolute abundances of ARGs/MGEs in digested sludge, thereby minimizing HGT potential and achieving superior overall ARG/MGE mitigation despite inevitable rebound. This ARG persistence is often linked to resistant microbial groups such as Proteobacteria and Firmicutes, and driven by horizontal gene transfer (HGT) within biofilms and MP-associated microbial consortia. MPs further influence digestion performance by restructuring microbial communities, suppressing methanogenesis, and intensifying ARG dissemination, with wastewater-derived MPs exerting stronger inhibitory effects than those introduced during AD. Collectively, these insights highlight the dual role of THP-AD systems in mitigating yet simultaneously reshaping risks linked to MPs and ARGs. Future directions should focus on optimizing pretreatment conditions, regulating microbial dynamics, and implementing targeted monitoring of MPs and ARGs to ensure safe sludge valorization and minimize downstream ecological and health impacts.