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Can Purification Technologies End Microplastic Pollution?

In one of the more fondly remembered hits of the 1980s, a group called Buggles sang that “Video Killed the Radio Star.” They may have been right. However, it was their less well-remembered follow-up that was truly prescient. We are indeed "Living in the Plastic Age."

According to some geologists, the Earth is now in an epoch known as the Anthropocene—a period where humanity’s impact on the planet will be obvious to future generations from examining geological samples. The term is not yet formally recognized, but geological societies around the world are considering it and using it informally in publications and conferences. While some believe that the epoch started with the Industrial Revolution and that its identifying characteristics will stem from the increase in atmospheric carbon dioxide, one of the most obvious markers will certainly be the presence of plastics from the 1960s onwards.

Every region of the globe, from the deepest oceanic trenches to the tops of the highest mountains, has been found to contain traces of synthetic polymers.

Synthetic polymers are now ubiquitous. Every region of the globe, from the deepest oceanic trenches to the tops of the highest mountains, has been found to contain traces of them. One of the most worrying categories of plastics are microplastics. Microplastics are defined by the U.S. National Oceanic and Atmospheric Administration as any particle of plastic less than 5 mm in length, although many are much smaller. They are generally formed through the mechanical breakup of larger pieces of polymer, with notable mechanisms being laundering of clothing made from synthetic fibers and disposal of plastic waste in the oceans.

Microplastic waste in the oceans can interfere with marine ecosystems. They accumulate in the stomachs and tissues of fish, other marine life and creatures that feed on them, alter marine behavior, reduce growth, and restrict reproduction. Although microplastics have not been proven to be directly toxic to humans, they are certainly non-nutritious and are bio-accumulative. As levels build up in the food chain, the amount of plastics in the human gut will likely increase. Aside from possible toxic effects of microplastic bioaccumulation, free-floating pollutants (such as polychlorinated biphenyls, heavy metal compounds and polycyclic aromatic hydrocarbons) tend to stick to the surface of microplastics, allowing these harmful substances to enter the body when the particles are ingested.

Plastic pollution breaks down and enters the environment through water.   ©Alexander Schimmeck/Unsplash
Plastic pollution breaks down and enters the environment through water. ©Alexander Schimmeck/Unsplash

These circumstances may well lead to negative health effects in the future. For this reason, health authorities are eager to reduce the amount of microplastic within the food chain. One way to remove microplastics from the food chain is to clean up the habitats of animals that occupy the food web.

However, implementing and maintaining such a comprehensive removal process may be difficult and laborious. A more feasible means of removing microplastic waste from the environment may be to concentrate on reducing levels found in water supplies. As many of the sources of microplastics are domestic, removing them at water treatment plants serves a double purpose: it eliminates microplastics from drinking water, and also from treated water that is disgorged into rivers and the sea.

A review of research into removal of microplastics by wastewater treatment processes was published in January 2021 in the journal, Environment International. Carried out by environmental scientists from three Chinese universities: Beijing University of Chemical Technology, the Beijing Technology and Business University, and Henan Normal University, the review centered around a meta-analysis of twenty-three primary papers covering microplastics in global wastewater treatment plants.

Microplastic pollution comes from a variety of sources including nylon, synthetic clothes, polystyrene, and tires.

The study looked at incoming waste streams containing from 0.28 particles per liter to 3.14×10,000 particles per liter and considered both the liquid and sludge effluents from the treatment plants. They found that filter-based treatment technologies performed best at removing microplastics. Fibers and particles of size 0.5 to 5 mm were easily separated by primary settling, while polyethylene and small size microplastics particles (less than 0.5 mm) were trapped by bacteria in the activated sludge of bioreactor systems.

The papers analyzed in the Chinese study identified twenty-nine types of polymers in microplastic waste; of these, six polymer types were dominant: i) polyamides; ii) polyethylene terephthalate and polyester (that mainly originated from textiles and synthetic clothing); iii) polyethylene; iv) polypropylene, v) polystyrene and solid polyesters (that originated from the mechanical crushing of plastic products, and tire and textile manufacturing); and vi) rubber particles in road dust (that mostly originated from tires). Some other types of microplastic waste were region-specific; for example, wastewater specifically from Glasgow, Scotland, contained alkyds, which are widely used in industrial coatings.

Wastewater treatment processes prove to be very effective at removing microplastic.   ©Ivan Bandura/Unsplash
Wastewater treatment processes prove to be very effective at removing microplastic. ©Ivan Bandura/Unsplash

The Chinese study looked at the removal efficiency of a variety of treatment processes. These processes ranged from primary-stage procedures, such as grit and grease removal and settling procedures; secondary-stage processes such as A20 (anaerobic/anoxic/oxic tanks to remove nitrogen and phosphorus), biofilters and other bioreactors; and tertiary stage treatments such as ultraviolet, ozone, chlorination, biologically active filters, disc filters and rapid sand filters.

Of the three stages, the primary and secondary methods were found to be approximately equally efficient at removing microplastics, but the tertiary methods yielded limited removal efficiencies. Filter-based technologies were found to be the most effective, although not without problems, such as when rapid sand filters broke microparticles into smaller pieces.

Specific studies have shown that secondary stage membrane bioreactors can remove 99.9% of microplastic particles from water that had already gone through preliminary processing. A study at Aalto University in Finland found that the commonly used activated sludge process, a secondary stage procedure in which air or oxygen is blown into unsettled sewage to break down solid lumps and develop a biological 'soup' that digests the organic content, removed 99% of particles 20 micrometers to 5 mm in size. This appeared to be effective regardless of the type of polymer or the shape of the particles.

Breaking down the treatment process, this study found that primary treatments removed 99% of ‘microlitter’, and 88% of the residue was removed by activated sludge treatment. Additional processes reduced the residue even further: membrane bioreactors removed an additional 99.9%; sand filtration, 97%; dissolved air flotation, 95%; and disc filtration, 40% to 98.5%. Biologically active filtration did not have any impact, the Finnish researchers added.

Microplastics successfully removed from wastewater end up contaminating agricultural fields and the food supply when collected waste materials are used as fertilizer. Using this solid waste in brick production instead may be a key solution.

However, removing microlitter, as the Aalto team calls it, from sewage waters does not eliminate the problem. The sludge resulting from wastewater treatment is often spread onto agricultural land. Abbas Mohajerani of the School of Engineering at the Royal Melbourne Institute of Technology published a paper in the journal, Waste Management, in April 2020 stating that a total of 62,192 tons of microplastics are spread onto farmlands in the US, European Union, China, Canada, and Australia annually. These microplastics decompose in the soil to form nanoplastics.

Mohajerani explained that nanoplastics are an even greater risk to health as their huge specific surface area allows them to transport significant quantities of toxic pollutants, such as those listed above, into the food chain. Mohajerani’s preferred solution to this problem is to mandate the addition of seven percent biosolids by weight into brick production worldwide. This would lock the microplastics into construction materials, where they cannot enter the food chain.

Interestingly, this technique would have the added advantage of reducing the energy required for brick firing by over 12.5%. Some 1.5 billion bricks are produced every year worldwide. Considering that a recent study found that every thousand bricks contains over 5300 MJ of embodied energy and accounts for almost six tons of emitted carbon dioxide in its manufacture, this energy reduction might be a worthwhile goal in itself.


*Stuart Nathan is a London-based freelance writer, specializing in science, engineering and technology.


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