The impact of microplastics on the efficacy of urban wastewater treatment processes

Microplastics (MPs), now considered as contaminants of emerging concern (CECs), have gained attention because of their widespread presence in the environment. Even though wastewater treatment plants (WWTPs) can reduce MPs concentrations by up to 99 %, a substantial amount is still released into the environment through sludge (1.69x105 x10 5 MPs/g) and treated wastewater (447 MPs/L). What has not been widely discussed so far, is that the occurrence of MPs can affect process efficacy in WWTPs. This review critically examines the potential impact of MPs on wastewater treatment processes. The discussion first briefly addresses the removal of MPs from treatment trains implemented at WWTPs, to provide background information. Then, the review explores the effects of MPs on preliminary/primary, secondary, and tertiary/quaternary treatment stages, and processes. According to relevant studies, the presence of MPs adversely affects the performance of various treatment stages. Secondary treatment (biological treatment) performance in the presence of MPs is affected by changes occurring in the richness of microbial community, which ultimately affect the enzyme activity of microorganisms and degradation of nutrients. During some advanced oxidation processes, MPs may promote the removal of organic contaminants up to 29.2 %, while in contrast they may reduce bacteria and antibiotic resistant genes removal by up to 2.4 log unit and 18.6%, respectively. Consequently, in many studies MPs were found to pose an additional burden on treatment. Therefore, the scientific community is urged to further explore the knowledge gap regarding the implications of MPs during wastewater treatment and provide effective and sustainable relevant solutions.