Impact of micro-nanoplastics on biochemical phases of anaerobic digestion in sewage sludge treatment: mechanistic insights and future prospects

Micro-nanoplastics (M-NPs) are generated through the environmental fragmentation of larger plastic waste particles. They subsequently enter the wastewater treatment plants (WWTPs) through domestic, municipal, and industrial wastewater. Although conventional wastewater treatment processes (including primary sedimentation, secondary biological treatment, and tertiary purification) can remove a substantial portion of these particles, complete elimination is not achieved. As a result, M-NPs accumulate in sewage sludge and may be released into the aquatic environment, posing potential ecological and environmental risks. Anaerobic digestion (AD) is a widely effective method for sludge treatment in WWTPs, where they serve as substrates for the generation of biogas. Consequently, the objective of this review is to elucidate the mechanisms underlying the influence of M-NPs on AD, delineated both antagonistic and synergistic effects. It explores M-NPs influence on biochemical pathways, key enzymes, functional genes, and microbial populations based on current research findings. This review also complies strategies to reduce the detrimental effects of M-NPs on AD, along with potential technologies to remove M-NPs from WWTPs. Despite variations in their characteristics, M-NPs have been shown to promote the proliferation on antibiotic resistance genes, inhibitory effects on bacteria, and interference with functioning enzymes in AD. The inhibitory mechanisms of M-NPs in AD include direct contact with microorganisms, cooperative interactions with other pollutants, leaching of toxic additives, and the production of reactive oxygen species. Extensive study is required to develop effective methods for reducing the retention of M-NPs in sludge, as current treatment methods are inadequate at removing M-NPs from WWTPs.