Protecting The Great Lakes: Microplastics
Photo source: NOAA
What are microplastics? Plastic litter fragments that are smaller than 5 millimeters are called microplastics. Microplastics can be of primary or secondary origin. Because of their small dimensions, microplastics become available for ingestion to a wide range of marine/freshwater organisms.
Can microplastic bioaccumulate? Yes, microplastics can accumulate within aquatic organisms upon ingestion, potentially resulting in direct effects caused by physical injury in the intestinal tract or even translocation to other tissues or organs. Microplastics have been identified in invertebrates such as mussels, bivalves, fiddler crabs, and shrimps in recent studies. Factors affecting the bioavailability of microplastics include size, shape, density, abundance, and color.
Can microplastic serve as vectors of environmentally persistent chemicals such as PFAS compounds? Yes, microplastic can act as vectors of additives incorporated during manufacture and organic pollutants absorbed from the surrounding water. Microplastics have been recognized as transport vectors for heavy metals and organic pollutants to aquatic environment. According to Wang et.al. (2015), the absorption behavior of some PFAS compounds to the types of microplastics (polyethylene, polystyrene, and polyvinylchloride) are evident and the molecule composition and structure of microplastics play important roles in the absorption processes of organic pollutants.
Photo source: NOAA
Are Wastewater treatment plants (WWTPs) a major route of microplastics to the Great Lakes? Yes, the microplastics present in influent wastewater streams are reintroduced to the environmental via wastewater effluent and biosolids. Most WWTPs are not designed to remove small microplastic particles from the effluent. A very large portion of microplastics is being reintroduced to the soil and water via the legal land application method for biosolids disposal. A large portion of the land-applied microplastics is believed to re-enter the aquatic environment via agricultural runoff.
Can microplastics be captured within the WWTPs? Yes, microplastics can be captured and retained in the sewage sludge with upgrades to the wastewater treatment processes or changes to management practices. The effluent wastewater can be further treated using adsorption and filtration techniques to remove a large portion of the remaining microplastics. The microplastics that are retained in the sewage sludge can be treated using thermal methods that would also provide sustainable and economic benefits to the WWTP operators via renewable energy generation. A current lack of legal obligations to remove microplastics from wastewater effluent or sludge, inhibits WWTPs from investing on process upgrades. However, the emerging contaminants such as Per- and Polyfluoroalkyl Substances (PFAS) are gaining interest and some WWTPs could be under tremendous pressure in the near future to remove these compounds from the treated wastewater or sludge. Fortunately, the technologies that can remove microplastics from the treated effluent also remove the emerging persistent contaminants such as PFAS. Such technologies to remove PFAS from wastewater or sludge include reverse osmosis, adsorption on to activated carbon, gasification, pyrolysis, incineration, etc.
Photo source: Google
Are microplastics in biosolids a concern? Yes, a very large portion of microplastics are being reintroduced to the soil and water via the legal land application method for biosolids disposal. A large portion of the land-applied microplastics are believed to re-enter the aquatic environment via agricultural runoff. According to the Norwegian Institute for Water Research between 110,000 and 730,000 tons of microplastics are transferred every year to agricultural soils in Europe and North America.
Are PFAS compounds present in biosolids? Yes, based on the research conducted by the Environment Ministry of the State of Baden-Württemberg in Germany at 47 WWTPs, PFAS in the range of 102 – 5,136 μg/kg of dry sewage sludge was found. The regulatory limit for PFOA and PFOS for sewage sludge for land application in Germany is 0.1 mg/kg of dry sludge. This suggests that monitoring PFOA and PFOS in biosolids is very important to eliminate potential reintroduction to the environment and food chain.