Abatement of microplastics from municipal effluents by two different wastewater treatment technologies

Abstract

This paper discusses the role of two different wastewater treatment technologies in the removal efficiency of microplastics from the final effluent of an urban wastewater treatment plant; i.e., a membrane bioreactor as an advanced wastewater treatment technology and rapid sand filtration as a tertiary treatment after an activated sludge process. Fourteen different polymer types were identified in all wastewater samples, mainly represented by low-density polyethylene (71.89%), high-density polyethylene (5.44%), acrylate (5.24%), polypropylene (5.22%), polystyrene (4.24%), and nylon (2.56%). The main forms isolated were fibres (61.2%), followed by films (31.5%), fragments (6.7%), and beads (0.6%). The main size interval corresponded to 1 and 2 mm, accounting for 28.2%, 37.3%, and 36.8% for the influent, membrane bioreactor, and rapid sand filtration, respectively. A total of 480 microplastic particles were isolated in all wastewater samples, with an average concentration of 2.16 ± 0.42 items l-1, and removal rate percentages of 79.01% for the membrane bioreactor and 75.49% for rapid sand filtration. Both technologies proved to be more efficient removing particulate microplastics (98.83% and 95.53%, respectively) than fibres (57.61% and 53.83%, respectively), showing a clear escape into the aquatic environment for fibres. The average microplastic size displayed a statistically significant increase from the influent of the sewage treatment plant (1.05 ± 0.05 mm), to rapid sand filtration effluent (1.15 ± 0.08 mm) and membrane bioreactor (1.39 ± 0.15 mm) (F-test = 4.014, p = 0.019) indicating the fibre selection made by advanced treatment technologies previously discussed.