Occurrence, effects, and biodegradation of plastic additives in sludge anaerobic digestion: A review

The retention of microplastics, a complex blend of polymers and plastic additives (PAs), in municipal sludge has been reported. The inevitable release of PAs from microplastics might affect the subsequent biological disposal of sludge, and their final fate are of great public concern. Therefore, this review describes the current knowledge in the occurrence of PAs in sludge and significant advances in their effects on sludge anaerobic digestion (AD) and their biodegradation performance. Specifically, the compositions and contents of plasticizers, stabilizers, and flame retardants in sludge worldwide are systematically summarized. The discrepant impacts of PAs on hydrolysis, acidification, and methanogenesis processes are analyzed and compared, with corresponding trends deduced. Furthermore, the biodegradation performances of PAs during sludge AD are also discussed. For most of the PAs detected in sludge, available data for their fate and effects on AD is yet limited. Moreover, the potential role of AD microbes in the release of PAs from microplastics was still unknown. Especially, the potential effects of PAs released from biodegradable microplastics on sludge AD and their fate should be of concern. The obtained knowledge would update our understanding of the risk assessment and control of PAs in sludge AD. Recommendations for future investigation are made.