Microplastics in Estonian wastewater treatment plants: First evaluation of baseline concentrations and stage-wise removal efficiency

Abstract

Wastewater treatment plants (WWTPs) are important pathways of microplastics (MPs) into the environment. To date, the extent of MPs contamination from Estonian WWTPs, located at the Baltic Sea, is not known. To establish MPs baseline levels in the Estonian wastewater treatment system and evaluate MPs removal efficiency, six WWTPs were selected for evaluation. From each plant, 24 h composite samples were collected from raw influent, after primary treatment, and from secondary effluent using an automated sampler with a three-layered sieve system. Upon Fenton-H₂O₂ digestion of organic matter, ≥ 300 µm MPs were quantified by microscope and categorized by size, shape and color. At least 50 % of microscopically identified MPs were analyzed by µFTIR, identifying at least 78 % of these as artificial polymers.

The results showed that MPs concentrations in the WWTPs’ influents were 205 – 520 MPs/L of which 36 – 94 % was removed during mechanical treatment. As a result of the overall MPs removal efficiency of 99.6 – 99.8 % compared to the influent, 0.5 – 1.4 MPs/L was quantified in the final effluent of the WWTPs. Fibers, fragments and films were recorded in the influent whereas the effluents were dominated by fibers. Fragments and films were mainly composed of polypropylene (PP) and -ethylene, while fibers had more diverse polymeric compositions incl. PP, polyethylene terephthalate and polyacrylonitrile. Despite high MPs removal rates, in total, the six studied WWTP discharge about 9,7E+07 MPs/day in the environment. The obtained results are significant for future regulatory and research endeavors.