For those of you reading this who are not of a certain age and artistic bent, there was a scene in The Graduate, a coming of age movie with Dustin Hoffman, that should be seen and remembered by all, long after “Meet the Fockers” is forgotten. Hoffman plays a lost soul, newly graduated from a highly prestigious East Coast university. At a party thrown by his parents, he receives career advice from one of his dad’s colleagues: ‘Plastics’. Dustin Hoffman the actor did just fine, whether he listened to that advice or not. If, however, you didn’t have a successful acting career and still listened, you most likely could have bought the studio by now. In the last 50+ years, plastic has become an indispensable part of our lives, in everything from our cars, computers, kitchens to our clothes. And you know what that means – plastic is also now a concern in our wastewater treatment plants. This month’s library is focused on plastic, starting with a general article and ending with some work on the limited information on plastics in soils.
The first article makes the extent to which we have come to depend on plastic very clear. Plastics are primarily derived from petrochemicals, a/k/a fossil deposits, a/k/a the same stuff as oil and gas. Zalasiewicz et al. (2015) focus on using plastic as a signature of our time. It is worth looking because of some of the general information presented. There are 15 - 20 major types of plastic currently in large scale production. The world produced less than 2 million tons of plastic in 1950 and makes 300 million tons annually today. As plastic doesn’t go away, that means that worldwide there is about 5 billion tons of plastic still around, or enough to wrap the earth in a layer of Saran wrap. Current global production amounts to about 40 kg per person with a total of 7 billion people. You need to realize that some of that plastic will end up in the toilet or the storm sewer.
The article goes on to divide plastics into two general categories. Macroplastics are > 5 mm, and we can see them. Microplastics (<5mm) are too small for the naked eye, or for the grit screen. Some of these microplastics are directly from products containing microplastics, such as cosmetics and toothpastes. Others come from washing those no-wrinkle shirts and pants: up to a thousand fibers can be released from a single pair of pants in one wash cycle. You know where those fibers end up.
So, the next question with regards to these microplastics is whether they sink or swim. Do they get released with the effluent or end up with the solids. That goes to article number 2. Here the authors tested microplastic beads like those used in toothpastes and cosmetics. I should point out here that this type of plastic was recently banned (http://oceanservice.noaa.gov/facts/microplastics.html). But even if microbeads are not in cosmetics, we will still be dealing with plastic fibers from clothes and carpets. One thing that is important in this paper is how the researchers tested for the plastics. They used different sized sieves and took pictures. These are pieces, not compounds, so you can’t test for them using standard lab procedures. You have to look for them with microscopy. This article focuses more on effluent than solids, and samples are taken primarily from secondary water plants. But the authors conclude that most of the particles are removed from tertiary effluent either because they sink or because they are removed during skimming. And, so the presumption is that the plastic microbeads and microfibers end up in the solids.
That takes us to the third article. If microplastic ends up in the solids and if the solids are land applied, can we see these microplastics in the soil? Here the authors looked for fibers from clothing in a range of biosolids and in soils, for up to 15 years after land application. They use polarized light microscopy as a way to see the tiny threads. They found them. They found them in proportions predicted based on what they had seen in the lab. They also found them in proportion to metals that were elevated because of biosolids application. The threads look pretty much the same after the time in the soil as they had in the fresh cake. So, microplastic is in the biosolids, and it ends up in the soil. Does microplastic hurt anything once in the soil? That question is left to the last two papers in this month’s library.
The first of these (4th paper) is a short editorial. The author notes that the focus of concern about these compounds has primarily been in aquatic environments but that terrestrial environments should also be considered. He says that the concern related to these particles is two-fold. 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. The author notes the lack of data on this topic and suggests that additional research should be a priority.
The final paper in the library tests the impact of microplastics on worms. The authors used polyethylene added to leaf litter at very high rates: 7% - 60% dry weight, which is equivalent to 0.2% to 1.2% after mixing with soil. They measured changes in growth rate and mortality in the worms and the characteristics of worm castes. At the higher rates, the worms were smaller, showing an impact. The threads lodge in the gastric system, and worms have reduced appetites. Other mechanisms are likely factors as well. The authors also found that the worm castes contained high quantities of the microplastics, and that the worm’s digestion process yields smaller microplastics. Of all plastic particles in the soil, 50% were smaller than 50 mm. But in the castes, this size fraction had increased to >90%.
Very little research directly focuses on microplastics in soils and biosolids. Much of the focus and concern seem to be rightly directed towards aquatic environments. If microplastics do become an area of concern in soil, remember that microfibers from clothing and carpet have been around since that famous scene in the Graduate or at least Saturday Night Fever. I would guess that at least some of the polyester in Travolta’s white pantsuit ended up in the biosolids. All long-term application sites provide indirect proof that the presence of these particles have not resulted in measurable negative impact on soil or crop health.
Sally Brown, University of Washington