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Monday, 04 March 2019 06:07

Tyre abrasion – the dark side of microplastics pollution

In an Expert Focus article for Waterbriefing, Jo Bradley, Market Development Manager, SDS Ltd draws attention to an overlooked source of microplastics pollution.

Jo Bradley: We will look back on 2018 as the year the world faced up to the true extent of plastics pollution. With Sir David Attenborough’s heartrending narration of Blue Planet 2 still ringing in our ears, as the year went on we were confronted with remarkable and shocking footage of entangled turtles and birds with stomachs bulging with plastic fragments.

Thank goodness for Sir David, and all those colourful images illustrating how plastic is turning up in even the most remote and unpopulated places on earth. Who could fail to be astonished by its persistent and pervasive global penetration?

But now we understand the plastic pollution we can see, we must work harder to understand, manage, and prevent the plastic pollution that can’t be seen. There is likely to be just as much of it, and it could potentially be more harmful not only to wildlife and ecosystems, but to human health, especially as it’s harder to find and remove.

Simple maths - plastic endures

Plastic endures. The simple maths of how much plastic has been produced since it was invented, compared with how much we can observe, measure, or account for, means there simply has to be a lot more of it out there. Plastics pollution is usually classified into three groups:

  • larger pieces or macroplastics
  • microplastics (particles less than 5mm in diameter)
  • and nanoplastics (with particle sizes down to 1nm)


The ‘Blue Planet Effect’ has steered a public response towards improving waste management and reducing the packaging consumption that leads to macroplastic pollution. Meanwhile pollution from microplastics, including major sources like tyre abrasion, synthetic fibres released through clothes washing, paints, manufactured pellets and beads, have historically attracted less attention. The little blobs and specs stuck in the stomachs of tiny aquatic creatures may be just as harmful, but sadly just don’t make for great social media posts or TV series.

When research from scientists at the University of Vienna measured microplastics in human poo for the first time, it had the potential to make things more personal. Their findings1, published in August, suggested that at least half of us may be digesting and excreting the tiny fragments. Could this be the wake-up call we need?

As water professionals, many of us are familiar with the source, pathway and receptor model. It’s clear that current knowledge, research and public awareness is patchy and insufficient at every stage in the microplastics journey. We need to know a lot more about where microplastics are coming from, how they travel through the environment, transfer through ecosystems, and where they end up. We also need more research, to know when the presence of microplastics is a physical hazard to animals and humans, when they are poisonous, as well as understanding the impact of toxins that attach easily to plastic particles.

There are two ways of estimating the extent of microplastics pollution from different sources: You can sample and measure it in the environment; this is difficult given the scale of the challenge and the variation in the places where microplastics might end up. Some plastics sink to the ocean bed, some float on the surface, some will be deposited in rivers and streams along the way. Alternatively, you can estimate the likely total emissions – the amount produced, discarded or lost to the environment, either intentionally or unintentionally.

One of the most helpful, and readable, reports to date is from Eunomia Research and Consulting. Entitled Reducing Household Contributions to Marine Plastic Pollution,2 it was commissioned by Friends of the Earth (FoE) and published in November 2018. It makes clear, the huge disconnect between the amount of plastics we observe floating in the oceans, compared to the millions of tonnes we could expect to find there.

Tyre abrasion – report concludes vehicle tyre wear and tear is largest source of microplastics pollution

Every driver is familiar with measuring the tread wear on tyres, yet most never stop to think where the lost material goes to. We think of tyres as being made from rubber, but most are made from a mix of synthetic plastic materials.

Microplastics pollution graphic. Eunomia

Source: Eunomia Research and Consulting (Friends of the Earth). Estimates for Key Sources of Plastic Pollution to Surface Waters in the UK from land-based sources.

Drawing on other studies, the Eunomia report reaches the conclusion that vehicle tyre wear and tear is the largest source of microplastics pollution.

The report quotes European Commission figures suggesting half a million tonnes are released in Europe every year, of which they estimate 20% enter surface waters.

Applying UK traffic statistics to tyre wear data, the study then calculates that the UK accounted for 68,000 tonnes of microplastics with 7,000 – 19,000 tonnes entering surface waters annually.

Actual evidence of emissions from tyre abrasion and how they travel through river systems into the environment is extremely limited. Monitoring and control of all suspended solid discharges from highway runoff is frequently inadequate in any case (see Waterbriefing - Urban and transport run-off significant contributors to water quality failures).

Nevertheless, it’s clear the sheer volume of emissions from tyres has to make it a pollution source of extremely serious concern. Yet public attention is focused on large scale plastic waste management – which we already do relatively well – and on ocean pollution. Emissions from tyre particles are highly likely to be polluting the environment much closer to home, both in the air and in the water environment.

Should we be worried?

Waterbriefing tyre abrasion image Tyres are manufactured using a mix of natural rubbers and synthetic rubber compounds that are kept a closely guarded secret by manufacturers. As tyres wear down on the road, they create small particles which build up on the road surface and at the edges of the road, and then get washed into the drainage network when it rains, mixed in with all the suspended solids particles from abrasion of brake pads, road markings and the road surface. We do know that tyres also contain other chemicals, binders and fillers including carbon black, which itself has been indicated as a possible carcinogen.

The silts and sediments suspended in highway runoff when it rains are carriers for metals such as copper and zinc, and Polyaromatic Hydrocarbons (PAHs). The carbon-based structures of microplastics attract and adsorb PAHs, in particular, very successfully. Benzo[a]pyrene is carcinogenic, mutagenic, toxic for reproduction, bioaccumulative and persists in the environment. The European Union classes it as a ‘Substance of Very High Concern’ under REACH, and as a Priority Hazardous Substance under the Environmental Quality Standards Directive.

Pathways to Pollution

Recent field studies have begun to indicate how microplastics might be travelling through the water environment. In March, research by a team from the University of Manchester discovered the highest microplastic pollution levels yet recorded in river bed sediments anywhere in the world, on an otherwise unremarkable river in a highly-populated urban location. Their study, published in the journal “Nature Geoscience”3, also clearly demonstrated how high concentrations of microplastic pollution were flushed out into the sea after the 2015 Boxing Day floods.

Then, the first UK study of microplastics in river insects led by The University of Cardiff School of Biosciences4 found microplastics in half the organisms they studied. The findings were the same both upstream and downstream of wastewater treatment works, indicating they were entering the river system via a variety of pathways. One of these is highly likely to be highway runoff.

According to the study’s authors, the concentration and impact of microplastics on river ecosystems will be affected by the upstream land use, urban runoff, and effluent discharges from wastewater treatment plants. Once ingested, microplastics affect aquatic organisms in different ways, including inhibiting their digestive and reproductive ability. They can also provide a route for secondary toxicity of pollutants including polychlorinated biphyenyls (PCBs) through the environment and along the food chain.

It’s clear from both these studies that more evidence is needed of the sources, pathways and impacts of microplastics pollution as they are transported from land to sea along rivers. The way in which microplastics are transferred across food chains, and potentially into human consumption, also needs a great deal of further investigation.

In the meantime, it should be impossible for all responsible stakeholders to ignore a significant problem – albeit an invisible one. The Eunomia/FoE study goes on to outline the actions needed to tackle and reduce tyre pollution. The recommendations include a testing and labelling scheme for tyres according to their resistance to wear and tear and encouraging manufacturers to review and improve the materials they use. Improved driver behaviour and reduced vehicle usage to reduce pollution could be important factors.

A levy on each tyre sold could pay for further research and implement mitigation solutions for microplastics pollution. These are laudable and important aspirations but will take time and political will to implement.

Immediate, practical mitigation is possible

The Eunomia/FoE study also calls for the most efficient use – and regular emptying - of roadside gully pots to catch debris, sediment and microplastics. It’s clear that using effective highway drainage as close as possible to the source of the pollution is a key consideration where action can be taken using existing technologies and regulatory instruments.

We can estimate there are 1 million discharges of road runoff via drainage outfalls across the UK. At point-source discharges, it’s easy to intervene, capture suspended solids (including microplastics) and treat surface water before it enters into the environment.

However, the difficulty with using gully pots is that during intense storm events – now expected to be more frequent as a consequence of climate change – the sediments collected can be flushed out again, continuing their onward journey into the environment. As microplastics are likely to have a lower specific gravity than other silts, sediment and grits, it might be reasonable to assume they would be resuspended more easily. Emptying gully pots very regularly at the roadside is expensive, because it may require lane closures and high levels of manpower to complete.

Fortunately, there are technologies and techniques available that can be designed to capture and retain suspended solids from road runoff with less frequent and easily-managed maintenance.

Vortex separator

Hydrodynamic vortex separators are increasingly being deployed on highways and major trunk roads to remove suspended solids. A hydrodynamic separator is similar in size and shape to standard gully pots, but its internal components are designed to capture and retain sediment. Depending on the size of the device, and the engineering design requirements of the location, several tonnes can be collected before easy removal by a vacuum tanker at predictable maintenance intervals.

There would appear to be no specific tests that have looked at microplastics removal in isolation. However, the SDS Aqua-Swirl™ hydrodynamic vortex separator achieved 80% removal of Total Suspended Solids in internationally-recognised field tests in which a large unit was installed to allow for infrequent but large storm events. When smaller units are installed to treat more frequent, smaller rainfall events, removal efficiencies of 50% can be achieved. This is a significant improvement on the pre-treatment situation.

Using such devices provides additional reassurance of measured, predictable and independently-verified performance, and can also enable above-ground vegetative features to be integrated in a stormwater treatment train as part of Sustainable Drainage Systems. Without such a device, the solids pass forward and settle out in the pond or basin. The maintenance interval is unknown, the cost can be high, hard to budget for and require specialist service providers.

Maintenance plans can be set and budgets planned according to the manufacturer’s recommendations. In future, level sensing and telematic technologies will also enable more remote monitoring of SuDS devices to manage and predict maintenance interventions.


The UK Government has committed to a 25-year plan for the environment, post Brexit, and has indicated, it will be committed to taking regulatory action necessary to protect the environment.

Our environment – and even our own bodies – may be steeped in microplastics. Greater awareness is vital to fuel public conscience and drive further research interests. We must know more about its sources, pathways through the environment and its effects on ecosystems, aquatic and plant life, and ourselves.

In the meantime, there are no excuses for ignoring the obvious scale of an invisible problem. We have the knowledge, technologies and expertise to design and implement effective solutions to remove microplastics from runoff, along with other harmful and polluting substances. Whilst some steps are being taken, let’s hope the plastics debate gets ‘down and dirty’ in 2019 and considers the impact of less visually-arresting pollution.


1: Liebmann, B, Köppel, S, Königshofer, P, Bucsics T, Reiberger T, and Schwabl, P. (2018). Assessment of Microplastic Concentrations in Human Stool – Final Results of a Prospective Study, Monte Verità, Ascona, Switzerland, Conference on Nano and microplastics in technical and freshwater systems, Microplastics 2018.

2: Hann S, Darrah C, Sherrington C, Blacklaws K, Horton I, and Thompson A, Reducing Household Contributions to Marine Plastic Pollution, Friends of the Earth, 20 November 2018.

3: Hurley, R., Woodward, J and Rothwell, J. Microplastic contamination of river beds significantly reduced by catchment-wide flooding. Nature GeoScience, 12 March 2018

4: Windsor FM, Tilley Rosie M, Tyler CR and Ormerod SJ, Microplastic ingestion by riverine macroinvertebrates. Science of the Total Environment, 20 July 2018

Further reading:

Jan Kole, Löhr, Van Belleghem & Ragas: Wear & tear of tyres: A stealthy source of microplastics in the environment. Int. Journal of Environmental Research & Public Health. 2017 Oct; 14 (10): 1265

Boucher, J. and Friot D. (2017). Primary Microplastics in the Oceans: A Global Evaluation of Sources. Gland, Switzerland: IUCN

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