Microplastics (MPs) in wastewater treatment plants sludges: Substrates, digestive properties, microbial communities, mechanisms, and treatments

Microplastics (MPs) are introduced to the environment upon the breaking down of the big plastic wastes into small particles. MPs then enter the wastewater treatment plants (WWTPs) from domestic, municipal, and industrial wastewater. Primary, secondary, and tertiary units are the main stages for the treatment of wastewater. Large solids in the wastewater are settled at the bottom of the primary tank via sedimentation forming primary sludge (PS). Residual wastewater (i.e., upper layer) is then transferred to a secondary unit where it is biologically treated and filtered using a trickle filter forming secondary sludge (SS). Comparatively clear wastewater from secondary tanks is finally tossed to tertiary units for advanced treatment and discharge into the environment. While PS and SS transferred to anaerobic digestion (AD) to be used as substrates for biogas production. Thus, this review aims to cover the identification, distribution, and abundance of different MPs types in all the units of WWTPs. MPs impact on microbial communities and functional enzymes of AD were deliberated. Mechanisms of MPs impact on AD are highlighted. Potential technologies to remove MPs from WWTPs are also summarized along with approaches to minimize negative impacts of MPs on AD. MPs showed inhibitory effects on microorganisms, functional enzymes, and proliferative effects on antibiotic resistance genes in AD, despite differences in their properties. Direct contact with microorganisms and synergistic interactions with other contaminants are main MPs mechanisms in AD. Existing treatments and their removal capacity of MPs in WWTPs are insufficient and extensive research is needed to explore approaches to reduce the retention of MPs in sludge.

Automated summary

This review covers the identification, distribution, and abundance of different types of microplastics (MPs) in all units of wastewater treatment plants (WWTPs). It discusses the impact of MPs on microbial communities and functional enzymes of anaerobic digestion (AD). The mechanisms of MPs' impact on AD are highlighted.