Effect of Microplastics on Activated Sludge Systems

Microplastics (MPs) are emerging pollutants that enter wastewater treatment plants (WWTPs) and may interfere with biological treatment processes, subsequently disrupting the sustainability of ecosystems. The number of studies on the effect of microplastics on biological wastewater treatment has been increasing. The aim of this work is to provide an overview of the most recent developments in research on the effects of microplastics on activated sludge systems, in particular on the biochemical activity of microorganisms, floc morphology and composition, and changes in microbial communities. This review shows that, among macropollutants, nitrogen removal is most often affected by the presence of MPs in wastewater, in particular, the inhibition of denitrification processes. This results from the decrease in the abundance of denitrifying bacteria. The microbial diversity of activated sludge exposed to MPs usually shifts, but the changes in species richness and evenness are often statistically insignificant. The presence of MPs influences floc morphology and composition and may lead to a decline in floc formation and stability. This work reveals that studies on floc morphology and the biochemical activity of activated sludge are scarce and should be developed in the future. The vast majority of reports on the effect of MPs on activated sludge have been conducted in sequencing batch reactors (SBRs) at a laboratory scale. Thus, there is a need to apply other bioreactors in future experiments, particularly the continuous-flow bioreactors with recirculation that are commonly used in WWTPs.