A comprehensive review of synthetic polymers stress on sludge anaerobic digestion resource and value: Current status, mechanisms, and future potential

The presence of synthetic polymers, including water-soluble polymers (WSPs) and microplastics (MPs), presents a challenge to the operational stability and efficiency of sludge anaerobic digestion (AD). This review summarized a decade of scientific literature to elucidate the impact of synthetic polymers on the resource utilization and high-value utilization of sludge AD. It was demonstrated that while synthetic polymers at low concentrations could enhance AD, for instance by promoting sludge granulation, higher concentrations or their toxic leachates consistently impaired the AD process. This impairment manifested as the inhibition of key enzymatic activities, disruption of microbial communities, and reduced yields of valuable products such as methane and hydrogen. A detailed analysis of the underlying mechanisms reviewed that inhibition proceeds via multiple pathways, chiefly the leaching of toxic plasticisers, the induction of reactive oxygen species, and the adverse physicochemical alteration of extracellular polymeric substances, which were vital for microbial aggregation and electron transfer. The review further provided an assessment of diverse mitigation strategies, including thermal hydrolysis, conductive material amendment, and bioelectrochemical systems. The work highlighted that the further effects should clarify the interaction mechanism between synthetic polymers and microbial communities at the molecular level through theoretical methods such as density functional theory and molecular dynamics simulation, and develop new additives or pretreatment technologies to enhance the efficiency and value gained through anaerobic digestion.