Beyond simple inhibition: Unveiling the non-monotonic impact and multi-level mechanisms of aged microplastics on sludge anaerobic digestion

Polypropylene microplastics (PP-MPs) in waste activated sludge (WAS) pose challenges to anaerobic digestion (AD), yet the impact of their aging degree is poorly understood. This study employed plasma treatment to precisely control PP-MPs aging, investigating their effects on WAS AD performance and underlying mechanisms. Unaged PP-MPs significantly inhibited methane production (38 % of control). However, aging remarkably alleviated this, exhibiting a novel non-monotonic relationship: optimal mitigation occurred at a carbonyl index (CI) of 0.219, recovering methane yield to 83 %. Mechanistically, this optimal aging enhanced hydrolysis and acidogenesis (improved kinetics, organic degradation, short-chain fatty acid (SCFA) production) and favorably shifted microbial communities (e.g., promoting hydrolytic/acidifying bacteria (Bacteroides), favoring hydrogenotrophic methanogenesis (Methanobacterium)). Crucially, these benefits were linked to enhanced cellular viability and mitigated oxidative stress (reduced reactive oxygen species (ROS), increased glutathione (GSH), restored cell integrity in sludge and Escherichia coli models). These multi-level non-monotonic responses precisely correlated with changes in PP-MPs' surface properties (hydrophilicity, zeta potential, crystallinity, nitrogenous functional groups), all peaking at CI = 0.219, which modulated microbial interactions. This study provides unprecedented mechanistic insights into the complex non-monotonic impact of PP-MP aging on WAS AD. Our findings underscore the critical importance of considering MPs' specific aging degree for environmental risk assessment and optimizing AD processes.