By Sally Brown, University of Washington
Abstracts of these resources are available in the searchable Information Portal offered to Northwest Biosolids members.
Are agricultural soils dumps for microplastics of urban origin?https://nwbiosolids.org/resource/are-agricultural-soils-dumps-microplastics-urban-origin
Organic fertilizer as a vehicle for the entry of microplastic into the environment https://nwbiosolids.org/resource/organic-fertilizer-vehicle-entry-microplastic-environment
Polyester textiles as a source of microplastics from households: A mechanistic study to understand microfiber release during washing
Microplastics in sewage sludge: effects of treatment
Impacts of microplastics on the soil biophysical environment
I am getting old for sure. Old enough that I am not getting upset about microplastics (MP) in biosolids. Old enough to know that microplastics are nothing new. After all, polyester is a form of plastic. If you think that John Travolta’s white suit in Staying Alive was made from natural fibers, I have a bridge I’d like to sell you. This is not to say that I don’t think that plastic is a major environmental issue, it most certainly is. Large scale plastic contamination of aquatic systems is an even bigger problem than the great Pacific garbage patch (https://www.theoceancleanup.com/great-pacific-garbage-patch/). Waste plastic on land is also a huge issue. Take a look at the first article in the July 2017 library if you want proof. In that the authors argue that plastic contamination is the signature contaminant of the Anthropocene. As of 2015 we have generated 5 billion tons of the stuff- enough to wrap the planet in Saran. In other words, plastic pollution is an enormous problem. Microplastics are just very small pieces of plastic (< 5 mm). Some are visible to the naked eye and many are not. Basically, as long as plastic has been around, little pieces of plastic have also been around. Are microplastics in biosolids a real concern?
The first article says absolutely. It is a two pager from the front section of Environmental Science and Technology. This is a very well- respected journal that often breaks critical stories. It is also a journal that features articles that focus on the sky is falling approach to science. Their big article about zucchini plants on carbamazepine is just one example of this. In many cases when a new contaminant is identified scientist test the worst case- high (unrealistic) concentrations of contaminant not in a matrix (added straight not with biosolids). While demonstrating that something is possible in theory, this approach very often has no bearing in real life. For example- in theory I could run a marathon in under 3 hour. In reality, I was able to jog a significant portion of the Biofest 2-mile fun run with only periodic walk breaks.
In this piece the authors argue that land application of biosolids is a major pathway for introduction of microplastics into the environment. Sources that are identified include small pieces from tires, household dust and water from washing machines, and erosion of paints. It would seem that the authors are focused on combined sewer systems. The authors say that between 1270 and 2130 tons of micro plastics are generated by every million people annually. That comes to up to 8 mg of microplastics per hectare per year. They come up with this meaning that somewhere between 44 000- 300 000 tons of microplastics are applied to farmlands in N America each year through the land application of biosolids. Those numbers sound a bit fishy to me. So let’s do a quick calculation. If you figure that half of the microplastics that are generated end up in the wastewater treatment system and 90% of those end up in the biosolids and each person makes about 30 kg of dry biosolids a year, that means that about 5% of the biosolids is made up of microplastics. Sounds off by an order of magnitude or three to me. Five percent is what you typically see in terms of nitrogen content- not plastic content.
For those reading this who work with compost- the second article in the library suggests that you aren’t off the hook on this one. This article looked at digestates from 4 plants and two composts and found microplastics in all but the energy crop digester. Between 10 and 1bout 150 particles (between 1 and 5 mm) were found for each dry kg of material tested. Dry weights of the plastic particles were not given. In other words, anything that comes from people will contain plastic- including biosolids, digestates and composts. These were all screened materials. It is just that the particles are small enough to pass through screens.
The third article takes a hard look at the premier pathway for microplastics from the home to the treatment plant: the washing machine- about 1900 fibers per wash (from the 4th article). Did you ever think that you would be looking at the spin cycle for contaminants in biosolids? The authors looked at MP release from polyester textiles in a laboratory version of a washing machine. They considered different types of fabrics, different detergents, temperatures, and length of cycle. Using detergent resulted in more MP being released. Most of the fabric pieces released were between 100-800 µm long. You are welcome to read this article to get a detailed description of what types of fabrics shed the most fibers and factors in the textiles that result in loss of threads. The authors point out that installing a filter in the washer would miss most of the microplastics and suggest that the way to reduce loss is to better engineer fabrics. Forget flow diagrams, go to engineering school to design better polyester.
The particles leave the clothes in the washing machine and enter into the sewer system. The fourth article traces the fate of these particles during wastewater treatment. It turns out that different biosolids stabilization systems produce different amounts of MP. The systems that involve the harshest environments; lime stabilization and thermal drying, tend to produce the most MP (up to 13 675 per kg dry matter). In contrast, anaerobic digestion, a relatively gentle process produces fewer MP (2000-4000 per kg dry matter). The article includes lots of pictures of different particles if you want to see the variety.
The final article looks at impact in a soil system. Again- from Environmental Science and Technology- and again looking at scenarios that are completely removed from the real world. The authors added microplastics to a sandy loam soil at concentrations of up to 2%. With a typical soil bulk density, that is the equivalent of 20 tons of microplastics per acre. They found that polyester increased soil water holding capacity and that polyester, polyacrylic and polyethylene all decreased bulk density. That makes sense- you add plastic to soil at such high rates- it weighs a whole lot less than soil minerals (aka rocks). They also found that two of the three decreased microbial activity, although if you look at the data, that conclusion is not so clear. And if you look at so many of the long- term studies of both compost and biosolids application to soils you see benefits. More recent studies have confirmed health of the soil microbial community in long- term sites. It turns out that the organic matter and nutrients provided by the composts and biosolids do much more to help the soils in ‘Staying Alive’ than any white pant suit fibers do to harm them.