Polymer-specific effects of biodegradable plastic–derived microplastics on anaerobic digestion performance and microbial pathways

Biodegradable plastics (BPs) are increasingly entering anaerobic digestion (AD) systems with organic waste, where incomplete degradation can lead to microplastic formation and alter process performance. This study systematically investigated microplastics generation from eight commercially available BPs and evaluated their short- and long-term effects on methane production, intermediate metabolism, and microbial community in mesophilic AD. Distinct polymer-specific responses were observed. Microplastics derived from poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) were transient and promoted methanogenesis, resulting in higher methane yields and production rates compared with the glucose-fed control. In contrast, polylactic acid and cellulose diacetate microplastics caused pronounced inhibition of AD, as evidenced by significantly extended lag phases, markedly reduced methane production, substantial hydrogen accumulation, and persistent propionate buildup. Polybutylene succinate and polybutylene adipate-co-terephthalate exerted moderate inhibitory effects on AD, as reflected by slightly reduced methane production rates and increased butyrate formation. In comparison, polycaprolactone exhibited limited fragmentation and yielded intermediates and final products with compositions and yields like those of the glucose control, indicating no impacts on AD performance. Integrated analyses of methane kinetics, volatile fatty acids, carbon flow distribution, and microbial community composition revealed that BP-derived microplastics selectively reshape acidogenic, acetogenic, and methanogenic pathways, thereby governing AD efficiency and process stability. Shifts in bacterial and methanogenic archaeal communities were closely associated with changes in metabolite profiles and methane production behavior. Overall, these results demonstrate that polymer chemistry critically determines their impact on process performance and microbial functionality. This study provides mechanistic insights for evaluating the impacts of BPs on bioenergy recovery systems.