Microplastics and Biosolids

I have many years under my belt, and during the pandemic I started my eighth decade. Through each of those decades I have retained only vague memories of the mega-forces changing life on Earth, such as toxic chemical releases.  Do not ask me about when I first learned of persistent organic chemicals, as that is a vague memory. While Rachel Carson’s 1962 Silent Spring was in the background of my early response to the environmental movement, not until a flood of DDT appeared in Philadelphia biosolids in 1995 was DDT an issue of salience to me. I had no idea about flame retardants, even though they were ubiquitous, until a fateful moment at the 2004 meeting of the Chesapeake WEA in Ocean City, Maryland, when the first call from a news reporter came in about the flame retardant PBDE in biosolids. When requests occasionally came to me from national organizations looking for willing municipal participants in voluntary analytical surveys of new compounds of concern, as in radioactivity, dioxin and unregulated organics (targeted national sewage sludge surveys), I would raise Philadelphia’s hand each time. I figured that if I were vague about the risks of these things, I was not alone.

Plastics is one of those “things” today about which we in the biosolids profession are vague. Do not ask me about life on Earth before plastic, even though the year of my birth, 1950, is roughly tagged as the turning point for the introduction of plastics to our global consumer culture. I do not recall the first time I wrapped my lunches in plastic wrap, rather than in wax paper bags (about 1968). Neither do I recall as momentous the spotting of my first plastic soda bottle (about 1973).  When did plastic fibers so thoroughly replace natural fibers in the global clothing scene, and did I ever own a polyester leisure suit (the 1970s)? If I am so vague, I believe by and large we are all vague, particularly about the newest plastic issue - microplastics.

Perhaps we are vague because the term “microplastics” is so new.  According to Merriam-Webster the “first known” use of the term “microplastic” was in 1990.  If you check Google NGram Viewer (by the way, this is a super-cool feature), the number of its catalogued publications in which the word “microplastics” is used has shown a straight-line doubling since about 2012.  Microplastics is a term that is highly likely to be soon on everyone’s lips, just as PFAS, the “forever chemical,” is there now. Today’s worry about PFAS in our waters and foods may be tomorrow’s worry about microplastics in our waters and foods.

My vague understanding of pathways of microplastic movement in water and food was underscored by a recent journal article on microplastics in soil protists. I was even vague about what is a “protist.” For those of you who have not been following “systematics,” the arcane discipline of crafting the evolutionary “tree-of-life,” protists are the kingdom of life of non-plant and non-animal organisms that are eukaryotes (with cell nuclei) and mostly unicellular. Soil protists are those amoeba-like organisms living in the soil as opposed to fresh and ocean waters or inside animals. I had been only vaguely aware of this newly classified kingdom of life, but now I understand, through “Soil protists: a fertile frontier in soil biology research,” that protists constitute the first rung of the ladder in the food web and cycle of nutrients and carbon and “protists are the key trophic link facilitating nitrogen-uptake of arbuscular mycorrhizal fungi.”  In Phagotrophic Protists: Central Roles in Microbial Food Webs we learn “phagotrophic protists regenerate a large part of the organic nitrogen and phosphorus in bacterial and algal biomass.”

It cannot be good news that scientists have found microplastics in soil protists, as you do not want to gum up the first loop in the food chain. This could mean that everything above soil protists in the food chain is also accumulating microplastics. In Microplastics and phagotrophic soil protists: evidence of ingestion the authors explain “that most soil protists (>75% individuals) can readily ingest and keep MP within their food vacuoles, even at relatively small MP concentrations (0.1% w/w).…  This is the first report indicating that soil protists can play an important role in the transport and uptake of MPs in the soil food web.”

What does this have to do with biosolids? Very few articles today about microplastics and soil miss the opportunity to cast an eye toward biosolids as a potent source of microplastic, which hints at biosolids harming soil protists. As examples, Microplastics as pollutants in agricultural soils asserts “major causes of MPs pollution include application of biosolids and compost, wastewater irrigation…,” and Evidence of microplastic accumulation in agricultural soils from sewage sludge disposal claims “Sludge is proposed as a primal driver of soil microplastic pollution.”

Scientifically proven effects of microplastics on soil health are vague, and the connection to biosolids even vaguer.  One comprehensive review paper sifted through the science journal articles to date: Source, migration and toxicology of microplastics in soil.  As with other reports, “land application of sewage sludge” and “compost and organic fertilizers” are suspected sources of microplastics in soils.  From a high-altitude viewpoint, the authors assert “The presence of microplastics severely reduces soil quality… the migration and trophic transfer of microplastics in heavily contaminated soils, particularly those in wastewater-irrigated and plastic-film covered areas, pose substantial risks to the ecosystem… Microplastic-induced changes affect soil function and soil microbial communities.”  But the authors admit that few scientific studies have been done. They explain the expected effects in such couched phrases as “the bioaccumulation and biomagnification of microplastics may adversely affect terrestrial food webs,” “uptake of microplastics by organisms can damage important ecophysiological functions that control health and biodiversity,” and “much more researches need to be conducted to address knowledge gaps of influences of microplastics on plants and future studies will be of great interest.” The message is: send more money for research if you want scientifically valid results.

The urgency for research into effects of biosolids-borne microplastics on soil health is rising fast. A barometer of the brewing storm of interest in this topic can be found in Google Scholar. The number of science journal articles that are captured by the search terms “microplastics, sewage sludge” in the year 2010 was eleven. In 2015 the number was 114. By 2019, the number was 730; in 2020 the number is 1,510; and by mid-2021 the number is already 1,410. That is explosive growth in research interest. Note, too, that these numbers are about a factor of 5 higher than the number of citations returned from the search term “PFAS, sewage sludge.” Interestingly, the total Google results for “PFAS” are nearly 50 million, but for “microplastics” less than 5 million. This leads me to presume that the public imagination around the health and environmental effects of PFAS leads by a wide margin the public’s imagination around microplastics, while scientists are far ahead of the public in their imagination of adverse effects microplastics. Scientists seem to be lining up behind microplastics as a significant health and environmental risk.

Even a disinterested spectator might presume that future releases of microplastics is heading upward, not downward. The big trend is an unabated upward production rate for plastics, fostered in part by fracking (Annual production of plastics worldwide from 1950 to 2020: “Global plastics production totaled 368 million metric tons in 2019”). New NGOs, such as Beyond Plastics, are calling attention to the global risk of rising plastic use: “One million plastic bottles are bought around the world every minute.”

For wastewater and biosolids, a principal source of increasing plastic is microfibers released during clothes washing.  Microfiber release from different fabrics during washing reports “Our data suggested that microfibers released during washing of synthetic fabrics may be an important source of microfibers in aquatic environment…Thus, more efficient filtering bags or other technologies in household washing machines should be developed to prevent and reduce the release of microfibers from domestic washing.” Also at play is the overwhelming shift in global fashion, as explored in the documentary The True Cost. The article The global environmental injustice of fast fashion, reports “globally, 80 billion pieces of new clothing are purchased each year.” That is more than 10 items annually for all Earth’s people. Microfiber Masses Recovered from Conventional Machine Washing of New or Aged Garment found “washing synthetic jackets or sweaters as per this study would account for most microfibers entering the environment.” Where does the plastic in jackets come from? Ironically, the principal outlet for recovered plastic bottles is as synthetic fleece, the biggest source of microfibers in wastewater and biosolids.  According to a National Geographic website, “Polartec … has diverted about 1.5 billion PET bottles from landfills and continues to lead the outdoor industry in both performance fabrics and sustainable innovation.” This recycling success does not help the wastewater industry.

To its credit, the clothing industry is worried about environmental releases of microplastics and is in pursuit of “sustainable” options.  A recent positive development is offered by Intrinsic Advanced Materials through its  “CiCLO technology.” This is an “additive that is combined with polyester and nylon [which] creates millions of biodegradable spots in the matrix of the plastic” and thereby “allows plastic-based fibers like polyester to degrade similarly to a natural fiber in marine environments, wastewater treatment plants and landfill conditions.” Polartec will be among the first to use it. We can hope it works.

The wastewater profession seems vague whether microplastics are a problem and whether research ought to be a priority. In June 2019, I attended the IWA Leading Edge Conference in Edinburgh, which featured a full session on microplastics, a global first for this topic, including Are microplastics a challenge for wastewater treatment? The session seemed a plea for research money for the very basics, to figure out how to measure and characterize microplastics in effluent and biosolids. With the overwhelming flood of plastics accumulating on Earth every day, this scope seemed an inadequate response to the global risks from plastics. But, in truth wastewater engineers and operators do not consider in any meaningful way the capture of plastics and microfibers as an objective in their treatment processes, and in truth biosolids managers do not consider in any meaningful way the potential effects of biosolids-borne plastics on soils and crops. Is this beginning to change?

With many years under my belt (a belt most likely made of plastic), I will make a prediction that the future older me will look back some day and only vaguely recall a time when we all were NOT considering Microplastics in Biosolids.