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Microplastics: What You Need to Know

Cayla Cook, PE - Hazen and Sawyer

For decades, concerns over visible plastic pollution increased as the proliferation of plastic waste internationally continued to grow. From the Great Pacific Garbage Patch to the roadside litter of rural communities, we are now beginning to understand one very important message: plastic pollution doesn’t just disappear—it continues to get smaller. Moreso, this emerging contaminant class is the visible equivalent of a “forever chemical” as estimates project hundreds of years for decomposition. This critical combination of increasingly smaller particle sizes and recalcitrant properties has led an exponential burst of research to seek answers to some of the most urgent questions concerning a material that many cannot imagine living without.

It is critical to first understand the various mechanisms through which microplastics may pose harm to ecology, humans, or aquatic species. These include the potential harm of the physical particles, adsorbed contaminants, and the microbial communities they harbor. First, research does indicate that the physical particles may be toxic due to the size range and shape in which they occur; asbestos is similar in that it is toxic due to the size range and shape in which it occurs. Second, the adsorbed—or even manufactured—contaminants associated with microplastics range from various perfluoroalkyl substances to pharmaceuticals. Lastly, the plastisphere, or microecosystem which plastics create, provides a high surface area when weathered and leach potentially desirable compounds for microbial communities to proliferate.

Considering the potential size range-dependent toxicity, the size range of microplastics which we can quantify today is perhaps the most important characteristic. Currently, the lower detection limit of approved standard operating procedures is approximately 10 µm. While studies indicate a vast percentage of microplastics in the 10-100 µm size range may represent the largest portion of detectible particles, particles that cannot be detected may account for up to 98% of all microplastics in wastewater. These are possibly the more toxic size ranges, so researchers are working on filling this informational gap with different characterization technologies and techniques.

While stormwater concentrations of microplastics are orders of magnitude higher due to tire wear particles, wastewater biosolids and effluent are a top concern for regulators. Sources for microplastics into wastewater range from microfibers that synthetic clothing releases when washed to wet wipes and plastic or textile industrial effluent. This myriad of sources lends to a variety of shapes, sizes, and densities of microplastics which occur in wastewater influent, effluent, and biosolids and creates a more complex challenge for mitigation strategies. Currently, wastewater facilities are estimated to remove a majority of microplastics from the liquids stream; however, this shifts the particles into the biosolids. Early research indicates that this leads to high concentrations of microplastics in agricultural runoff and may impact agricultural crops themselves.

Microplastics removal rates

Perhaps the most heated debate regarding microplastics is related to whether concentrations in drinking water pose any public health concerns. Current treatment technologies effectively remove microplastics greater than 100 µm and potentially even smaller depending upon the unit process. However, concentrations of microplastics in source waters will undoubtedly rise as legacy plastics in the environment continue to build up and begin to breakdown. Moreover, early toxicity studies confirm that microplastics pose reproductive harm and may pose impacts through neurotoxicity and reproductive harm.

Additionally, microplastics may create unique challenges for the water industry considering all of the different ways in which our treatment and conveyance systems interact with plastic. Due to low removal rates in reverse osmosis, some researchers surmise that membranes may be microplastic sources. This is further supported by research into membrane technologies as they age and continuously encounter caustic chemicals which may breakdown the plastics they’re comprised of over time. Similarly, oxidation may be breaking down microplastics into nanoplastics due to increased fragility of the plastic particles themselves. Last but not least, concentrations of microplastics in wastewater recycle streams are similar to concentrations in wastewater influent indicating that processes may be recycling some particles until they’re small enough to escape in the effluent.

Microplastics technologies timeline

It is clear that microplastics may create unique challenges for the water industry considering all of the different ways in which our treatment and conveyance systems interact with plastic. These studies indicate a strong need to rethink some of our processes, and—minimally—to rethink our facility’s interactions with plastic.

The length at which the smallest, most persistent particles may travel in our environment, water supplies, and bodies continues to surprise scientists, consultants, utilities, and even industries. For a material with such a profound and distinct legacy as plastic, even today, it is valid to deeply consider what the road ahead will look like. While it is tempting to point to bioplastics as a quick-fix, many of these materials may be harmful as well creating a bigger mess with the all-too-common regrettable substitution.

As research continues to improve our understanding of contamination, multidisciplinary solutions across industries provide much-needed hope for what the future will bring.