Tyre microplastics pollution: Ignore it or remove it?

Scientists are gathering mounting evidence that microplastics (particles <5µm) are everywhere – from the remotest mountains and the deepest parts of the ocean to our local rivers and even the soil in our gardens.

They are in the food we eat, the water we drink, the air we breathe and they have even been found in our faeces. They are likely to persist across all stages of the water and wastewater treatment cycles.

Global plastics production stands at over 300 million tonnes a year, is rising exponentially, and about half is produced for single-use purposes. Not all plastics are inherently bad. But those which create emissions into the environment – either intentionally or unintentionally – must be controlled or eliminated.

In the case of microplastics, intentional sources like recently banned cosmetic microbeads are easy to stop altogether, while unintentional emissions, like packaging broken down into ever smaller pieces, can be reduced through public awareness and good waste management. Microscopic fibres from clothing are the latest to come to public attention – adding filters into washing machines and tumble dryers might be one way of at least reducing their emissions.

Tyre wear on roads has been calculated as the second largest contributor to microplastics pollution in terms of volumes of emissions. Perhaps it is just too much for a society in love with the automobile to even contemplate the impact this might be having. It’s hard to see how a public awareness campaign could create the momentum for change. The solutions might be complex, but does that make it a problem we should ignore?

How much is out there?

Reducing Household Contributions to Marine Plastic Pollution, a report commissioned by Friends of the Earth (FoE) and published late last year, calculates that 68,000 tonnes of microplastics are released from tyre abrasion in the UK every year with 7,000-19,000 tonnes entering surface waters annually. Yes, it’s an estimate, made by applying UK traffic statistics to tyre wear data. But we can all appreciate that our tyres wear down and that plastic has to go somewhere.

How do they end up in the water environment?

As tyres abrade on the road surface, they create small particles that build up on the surface and at the edges of the highway, then get washed into the drainage network when it rains. Highway drains, in turn, discharge the collected water directly into rivers and streams.

Although they did not isolate tyre microplastics specifically, several recent field studies have built up a picture of the pathways for microplastics pollution through the water environment. A team from the University of Manchester found the highest microplastic pollution levels yet recorded in riverbed sediments anywhere in the world, on an otherwise unremarkable river in a highly populated urban location in North West England.

The first UK study of microplastics in river insects led by The University of Cardiff School of Biosciences 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.

Tyre manufacturers have claimed “only 2 per cent of tyre microplastics have the potential to get into oceans, because they’re heavier than water so sink to the bottom of rivers” (What Car, 29 May 2019).

Are tyre microplastics harmful?

Once ingested, microplastics affect aquatic organisms in different ways, including inhibiting their digestive tracts. Microplastics have also been shown to be carriers for other poisonous pollutants that are released and transferred into the bodies of organisms once they are ingested.

Tyre microplastics get added to the silt and sediment load on a highway surface and carried in the runoff. Suspended solids settle out in receiving rivers and streams, clog the gravel beds where fish breed and spawn and limit the light penetration. If tyre particles are more likely to settle out through this route – as manufacturers are suggesting – they are contributing to this problem.

Tyres are manufactured using a mix of natural rubbers and synthetic rubber compounds that are kept a closely guarded secret by manufacturers. They also contain other chemicals, binders and fillers including carbon black, which itself has been indicated as a possible carcinogen. Road runoff contains a cocktail of other pollutants created by the dust from brake and clutch pads, engine wear, exhaust emissions, oil and fuel leakages. Pollution includes poisonous heavy metals, particularly copper and zinc, and a spectrum of organic pollutants, chemicals known as polyaromatic hydrocarbons (PAHs), including benzo[a]pyrene.

It’s known that the carbon-based structures of microplastics are ‘sticky’ and good at attracting and adsorbing PAHs. PAHs are known to be harmful to plants, animals and humans. Benzo[a]pyrene is carcinogenic, mutagenic, toxic for aquatic reproduction, bioaccumulative and persists in the environment.

Can drainage control the pollution?

The Eunomia/FoE study suggests there should be more and better use of gully pots to catch debris, sediment and microplastics at the roadside. But gully pots do not prevent the captured sediment from being flushed out again during heavy storms. To avoid this, gully pots need to be emptied frequently – an expensive option requiring lane closures and high levels of manpower to complete.

A hydrodynamic vortex separator, such as the SDS Aqua-Swirl, is a far more effective device for removing pollutants. It is larger than a gully pot and 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 of sediment can be collected before being removed by a vacuum tanker. Maintenance is easy and at predictable maintenance intervals, so it can often be completed without the need for lane-closures and traffic management.

Using a vortex separator ensures measured, predictable and independently verified performance and can enable vegetative SuDS features, such as ponds and wetlands, to be integrated in a stormwater treatment train, so that local habitats can be enhanced for wildlife.

If highway pollution is already regulated, then what’s the problem?

Although there is a regulatory framework in place to control an estimated 1 million discharges of highway runoff in the UK through Environmental Permits, they are not used, so outfalls remain unregulated and are not monitored at all. The extent of the pollution that these outfalls cause is unknown. Yet the Environment Agency (EA) has reported that only 14 per cent of rivers in England reach ‘good’ ecological status and that urban and transport pressures are a significant contributor to those failures.

The EA does not routinely monitor for copper, zinc and benzo[a]pyrene across the water environment in England, so elevated levels are not consistently recorded. Some water bodies are classed by the EA as ‘Do Not Require Assessment’ (DNRA) but given ‘good’ Water Framework Directive chemical status, nevertheless, even though they are not monitored at all.

So, while there are some excellent drainage schemes preventing pollution in places, the overall coverage is patchy, and pollution is inconsistently monitored and poorly controlled (see my WWT article Getting to Grips with Highway Pollution).

Tyre microplastics are an added contributor to an already known problem, which the water industry should be concerned to address with more consistency and commitment. Microplastics travel along roads, through rivers and even into groundwaters, aquifers and rivers where drinking water is extracted, yet hardly anyone is watching.

If you want to learn more about how microplastics are affecting the water industry, attend the WWT Drinking Water Quality Conference pre-conference workshop on microplastics, taking place 26 November, more information here: https://event.wwtonline.co.uk/drinking/