Microplastics in the Environment

Just One Word: Biosolids

“Just one word: plastic…. There is a great future in plastics” is the famous line from the 1967 film The Graduate. Dustin Hoffman stars in the role of Ben, an aimless college graduate, who is taken aside by the man whom he will soon cuckold and receives this famous, fatherly advice.  We may have all forgetten just how true that observation was in 1967; plastics were at that time an innovation. Today we have the contradiction: the future is NOT great in plastics.

Problems with plastics were “top of mind” this month, now some 50+ years after The Graduate. The news article  Whale Is Found Dead in Italy With 48 Pounds of Plastic in Its Stomach: described how “[m]ore than 48 pounds of plastic, including disposable dishes, a corrugated tube, shopping bags and a detergent package with its bar code still visible, were found inside a dead sperm whale in Italy, the World Wildlife Fund said on Monday.” In going through this news story, I learned that the WWF has issued the report Out of the Plastic Trap: Saving the Mediterranean from Plastic Pollution which, while lamenting macro-plastic pollution as a danger to mega-fauna, asserted the bigger concern “is the microplastics, smaller and more insidious fragments that reach record levels in the Mediterranean Sea: the concentration of microplastics is almost four times higher than in the “plastic island” found in the North Pacific Ocean.”  The WWF claims that microplastics, “by entering the food chain, these fragments threaten an increasing number of animal species as well as human health.”

We in the biosolids profession are now “woke” to microplastics.  One review article (Transport and fate of microplastics in wastewater treatment plants: implications to environmental health) provides such eye-popping phrases as: “WWTPs constitute an essential route for MPs [microplastics] entering surface water systems…. MPs were found in high concentrations ranging from 260 to 320 particles L-1 in raw wastewater… During the treatment process, around 95% of Microplastics is retained in biosolids.” An opinion piece rhetorically asked “Are Agricultural Soils Dumps for Microplastics of Urban Origin?” and then answered with “Based on new MP emission estimates in industrialized countries, we suggest that widespread application of sewage sludge from municipal wastewater treatment plants (WWTPs) to farmlands is likely to represent a major input of MPs to agricultural soils, with unknown consequences for sustainability and food security.”  

So, what gives with this flurry of attention and concern? Is biosolids are major "input" of microplastics to soil? Should we in the biosolids profession be alarmed?

Science journalists have justifiably caused this flurry or attention  Popular media has increasingly reported on accumulating piles of plastic waste on land and water, and the media point to the sad fact humanity has no effective response.  For me, the scope of the challenge was compellingly laid out last year in Production, use, and fate of all plastics ever made. From this report we learn that plastic production was negligible at the time of The Graduate, and since then over 8 billion metric tons of plastic have been produced, and the rate of plastic production has continued to increase. Between 1950 and 2015, 8.3 billion metric tons were produced, of which 6.3 billion still exists as waste: on the land surface, in landfills, along roadsides and coastlines, and in oceans. In 2019, over 0.4 more billion metric tons will be produced, and at the same time 0.3 billion tons will be discarded, and an estimated 12 million tons will end up in the oceans, some of it as “macro” plastics, but most if it as microplastics.

This plastics accumulation has been going on for over six decades, but only in the last decade or so have scientists asked how plastics, particularly microplastics, is affecting the environment and human health. I checked out Google Scholar and determined that in 65 years of research between 1950 and 2015, 3,960 research papers were published on microplastics, but in just the last two years the number of published science articles on microplastics is 6,310. The amount of discarded plastic in our environment increased by 10 percent since 2015, but the number of research papers has increased 160%. Small surprise, then, that for environmentalists and biosolids managers the plastic waste issue has moved forward in the priority of issues, even as the overwhelming issue of climate change claws its way to center stage.

The immediate question for us biosolids managers is how much of a threat may plastics generally, and microplastics specifically, pose for the future of biosolids land application?

Thankfully, Dr. Sally Brown, the consummate biosolids researcher at the University of Washington and author of MABA’s monthly Research Updates, has been vigilant on the microplastic topic for several years. Sally has issued three “blurbs” on the topic. Her assessment overall is that no alarm bells need to be ringing in terms of evidence of plant and soil harm, but we need to be paying attention to this as an area of concern, because the science is young.

In her May 2015 blurb, Dr. Brown explains that: “It turns out that microplastics, typically in the form of small beads, are now found in many types of cosmetics [Review of microplastics in cosmetics], [and] studies have found them in aquatic environments.  One of the prime sources of these materials is wastewater [again, the article Transport and fate of microplastics in wastewater treatment plants: implications to environmental health]….  They potentially have a negative impact, although it is still not clear.” She also points out that microplastics added to soil is not new, and in fact one of the first articles to connect microplastics, biosolids and soil is the 2005 article Synthetic fibers as an indicator of land application of sludge.

Dr. Brown turns to microfibers in her July 2017 blurb. Although federal legislation is causing microbeads to be withdrawn from personal care products, Dr. Brown says “we will still be dealing with plastic fibers from clothes and carpets [Polyester textiles as a source of microplastics from households: A mechanistic study to understand microfiber release during washing].”  As with beads, the potential harm of microfibers is not known.  She reviews journal articles that explores two pathways; [First,]“microplastic can be eaten and can harm the critters that eat them by physically disrupting the digestive process. Second, as organics, plastics can potentially sorb other organics in soils and become hiding areas for toxic organics.  Worms and other creatures that eat soil are likely the most sensitive individuals.” Yet, for neither pathway was the research definitive about negative effects of microplastics.

Dr. Brown points out in her October 2018 blurb that scientists, with the hope of attracting research funds, have begun ‘amping up’ the urgency of microplastic pollution ahead of convincing research results.  The two-page opinion essay, Are agricultural soils dumps for microplastics of urban origin?, mentioned above, is Dr, Brown's example of how “In many cases, when a new contaminant is identified, scientist test the worst case, meaning high (unrealistic) concentrations of contaminant are tested, not in a matrix (added straight not with biosolids).” Dr. Brown examined the authors’ estimates of microplastic loadings to agricultural soils and concluded that their estimate “sounds off by an order of magnitude or three to me.” That is no small amount of exaggeration.

Science is still in the very early, hypothesis-building stages of exploring the environmental risks of biosolids-borne microplastics. Examples of such “thought pieces"are: Microplastic in Terrestrial Ecosystems and the Soil? and Microplastics in the Terrestrial Ecosystem: Implications for Lumbricus terrestris (Oligochaeta, Lumbricidae).

For the biosolids profession, we need to place the risks of biosolids-borne plastics into the larger context of sources and sinks.  The incidence of plastics, both macro and micro, are emblematic of today’s world, as they are in our air, water and lands.  One paper asserts The geological cycle of plastics and their use as a stratigraphic indicator of the Anthropocene. Aquatic life is most vulnerable to plastics, and sources are diverse, including rainfall, ocean vessel releases, and fishing (Primary microplastics in the oceans: a global evaluation of sources).  Indeed, plastics are in the air we breathe, coming from fabrics and carpets, and still the effects on human health are not known, as explained in Microplastics in air: Are we breathing it in? Smaller, nano-size particles may be worse (Micro(nano)plastics: A threat to human health? ). Plastic-containing dust in urban areas, abraded from tires and brake pads, create urban air exposures (Various forms and deposition fluxes of microplastics identified in the coastal urban atmosphere). This dust can be then washed through storm sewers into surface waters, where it can mix with floodplain soils in environmentally sensitive riparian corridors (Microplastics in Swiss Floodplain Soils).  Last year we saw reports of microplastics in bottled drinking water, apparently coming from the cap (WHO launches health review after microplastics found in 90% of bottled water). Microplastics are indeed ubiquitous.

Biosolids is not even the major source of plastics to farm soils, as we learn in An overview of microplastic and nanoplastic pollution in agroecosystems.  Agriculture itself directly introduces plastic to the terrestrial environment.   One big source is plastic film mulch, with over 7 million tons used annually across the globe, and China is the largest user of this kind of mulch (Policy considerations for limiting unintended residual plastic in agricultural soils). Such mulch is typically plowed into the soil after a single use, as costs for its removal and disposal are high, and, as with other fates of plastic in the environment,the effects of this practice on soil health are not known. Scientists are asking Plastic mulching in agriculture. Trading short-term agronomic benefits for long-term soil degradation?  A stab at an answer is included in some preliminary reports, such as Microplastic effects on plants and Impacts of microplastics on the soil biophysical environment.  Interestingly, microplastics also impact organic farmers, as their sources of organic matter and nutrients may be contaminated (Organic fertilizer as a vehicle for the entry of microplastic into the environment).

When biodegradable plastic film mulch became available a decade ago, it was held out as a solution to soil contamination, and its quick adoption was foreseen. But then scientists raised concern for the qualities of such film, the veracity of the claims, and its actual fate in real soil conditions. Some of this research is going on at Washington State University, in fact by friends of Dr Brown’s (Is Biodegradable Plastic Mulch the Solution to Agriculture’s Plastic Problem?). So, sadly, we can't even count on plastics built on bio-based polymers. 

Yet, for all these sources and sinks, our job as biosolids professionals is to study the sources and fate of plastics in biosolids. This is a big job, even for this narrow part of the issue, as analytical procedures are very challenging. Scientists are beginning to report results of studying wastewater influent (Inflow possibility of microplastics to sewage treatment facilities through sewerage) and effluent (Wastewater treatment plant effluent as a source of microplastics: review of the fate, chemical interactions and potential risks to aquatic organisms).  Advanced treatment systems are being studied for their capacity to achieve superior plastic removal from effluent (Solutions to microplastic pollution – Removal of microplastics from wastewater effluent with advanced wastewater treatment technologies). The partitioning of microplastics to biosolids during conventional treatment  is a principal means of their removal from effluent (Mapping microplastics in sludge) and may account for up to 95% removal from effluent. Solids handling systems seem to have a surprising (at least to me) effect on microplastics concentrations, even beyond the concentrating effect of volatile solids destruction during digestion, or dilution during lime stabilization (Microplastics in Sewage Sludge: Effects of Treatment).  And once applied to soil, biosolids-borne microplastics are subject to several pathways: they may move through or across soil; they may be ingested by soil micro and macrofauna; and they may (though this is, in my mind, a remote possibility) be taken up by plant roots (Microplastics as an emerging threat to terrestrial ecosystems).  This is all subject to future scientific research, which the biosolids profession ought to support financially.

As with some of our other recent toxicity challenges (e.g., flame retardants, non-stick coatings and prescription drugs), public sewerage and treatment plants are not sources of plastics in themselves, but rather are a conduit for microplastics. At the end of the day, much of the responsibility for decreasing loading in the biosolids will have to come from changing production and distribution patterns for consumer goods. The 2015 ban on microbeads may yet result in lower biosolids microplastics, and perhaps we can turn consumers back to natural fabrics to reduce plastic microfibers in our influent. And we may have other positive surprises, as in this recent story about biodegradable fabrics (Hello, Little Microbe. Doesn’t This Jacket Look Yummy?).

We biosolids professionals nevertheless have a role to play in understanding how our treatment systems can be managed to reduce loadings in biosolids and in ensuring that our practices of land application do no harm and are shown to have countervailing soil health benefits. I still believe there is a great future in biosolids.