Effects of microplastics on substance transformation, sludge characteristics, toxicological effect, and microbial communities in different biochemical sludge systems: A review

In recent years, microplastics (MPs) have attracted worldwide attention as emerging pollutants, and wastewater treatment plants are among the environment's most important sources of MPs. This study aimed to summarize MPs effects on various aspects of sludge systems. The results showed that MPs inhibited substance transformation in sludge systems, with greater inhibition observed at higher concentrations and smaller MP particle sizes. Moreover, low concentrations of MPs promote extracellular polymeric substance (EPS) secretion, whereas high concentrations suppress EPS secretion, destroying the sludge structure. Granular sludge systems exhibit higher resistance to MPs than activated sludge systems owing to their layered structures. Micrometer-sized MPs primarily inhibit the dewatering performance of activated sludge through physical crushing, while nano-sized MPs primarily affect sludge dewatering through biological effects. However, for granular sludge system, micrometer-sized MPs were unable to penetrate the granular sludge interior, their affinity and aggregation capabilities enabled them to accumulate on the sludge surface. In contrast, nano-sized MPs can enter the interior of granular sludge and impair mass-transfer pathways, ultimately resulting in toxic effects. Furthermore, MPs induce various toxic effects in sludge systems (single and combined toxic effects). These toxic effects affect the expression of key enzymes and functional genes, leading to changes in microbial communities. Because of the layered structure of granular sludge systems, MPs may initially affect the microbial community structure outside the granules, with less impact on the internal microorganisms. However, the specific mechanism still needs to be explored. Finally, this study presents research questions and directions that require further investigation.