Sewage sludge as a source of microplastics in the environment: A review of occurrence and fate during sludge treatment

Modern wastewater treatment plants (WWTPs) effectively remove microplastics (MPs) from wastewater and unsurprisingly concentrate them in sludge. Hence through its beneficial use and disposal, sludge causes secondary release pathways of an estimated average amount of 106 to 1014 wastewater-based MPs to various environmental compartments yearly. Despite these numbers, studies investigating sludge are scarce. Currently, majority of the studies in the field focus on identifying the magnitude of the problem, whereas research investigating the fate and effects of MPs during sludge treatment are very rare. This review aims to bring together and critically evaluate the limited studies conducted about MPs in the sludge treatment line and bring out the key gaps and research needs in the area. Studies conducted so far indicate that depending on the type, size, and amount of MPs, their effects during anaerobic digestion differ, with some studies demonstrating serious negative impact on biogas production. Possible effect mechanisms are also suggested such as formation of reactive oxygen species (ROS) and leaching of toxic chemicals. Moreover, a potential for sludge treatment processes (thickening, dewatering, drying, stabilization, etc.) to change the characteristics and the number of MPs, which may increase surface area available for adsorption and desorption of pollutants, was observed. Review uncovers that, in the broad universe of MPs, some highly abundant ones in sludge such as polypropylene, polyurethane, polycarbonate, and acrylic are not yet investigated in sludge treatment. Future research should focus not only to investigate the fate/effects but to fully understand the mechanisms behind these, which is missing in many studies reviewed. Besides, new studies show that effect of MPs start from the floc formation stage during biological treatment, which in fact determine the final sludge behavior in thickening and dewatering. Therefore, holistic approaches starting from wastewater till sludge exits WWTP seem necessary. Substantiating from polymer chemistry and response of plastics to stress conditions, review suggests possibilities of deterioration during sludge treatment processes. It becomes evident that some totally uninvestigated aspects such as disintegration conducted before stabilization, can change the fate of MPs during sludge treatment and may bring new perspectives to the solution of the problem.