The largest source of microplastic pollution in the UK - tyre wear
By Jenny Hughes and Jonathan Windeatt
Society is facing a collective awakening to the impacts of plastic pollution on human health, on our oceans, and on the broader environment. The initial focus of the public eye during the last couple of years was on the impacts of relatively large plastic particles, waste on our oceans and wildlife. The narrative and focus are shifting and expanding as there is a growing awareness of the issue of microplastics, which are tiny synthetic polymers that resist breakdown and have a range of associated impacts.
As the understanding of the different sources of microplastic pollution grows, there is increasing recognition of the contribution of pollution from vehicles, and last year, DEFRA and DfT released a call for evidence on “Brake, Tyre and Road Surface Wear” to further explore this issue. The consultation aimed to get a better understanding of the quantitative background to microplastics, greater awareness of source pathways, technologies on car parts and in wastewater treatment plants and more standardised testing for tread abrasion rates and other tyre features.
What is the issue and why is this important?
Plastic pollution can be in the form of large visible pieces, macro (>5mm), micro (<5mm) or nanoplastics (≤100nm). Plastics are broken down in the environment via UV radiation or physical abrasion and resist biodegradation.
Contrary to public perception, most tyres are made from a mix of synthetic plastic materials rather than conventional rubber. Microplastic pollution from vehicles is predominantly associated with tyre wear and braking systems.
There is an increasing concern around the impacts of microplastics as they are particularly hard to detect, trace from source and treat, making the task of quantifying their impact a challenge. A recent report by Eunomia draws on a number of studies and concludes that, surprisingly, automotive tyres are the largest source of microplastic pollution in the UK ahead of cosmetics, paints and clothing. The report claims that tyre wear accounts for 68,000 tonnes of microplastics in the UK, with 7,000-19,000 tonnes entering surface waters annually.
It is understood that microplastic pollution is associated with numerous health impacts and contributes to the total annual deaths linked to air pollution in the UK, estimated between 28,000-36,000 deaths. Health impacts include respiratory, cardiovascular and mental illnesses.
Pathways and methods of treatment
The two main pathways which microplastics can be transported through the natural environment are water and air.
Water - microplastics become suspended in water. Rainwater flows directly into surface waters or sewer waste treatment plants. This carries microplastics, which are difficult to filter. Subsequently, they can be deposited into river beds or if light enough transported to oceans.
Air - as plastic particles degrade, they can become airborne. Microplastics in the air result from the fragmentation of larger plastic particles and fibres through friction, heat or light.
Microplastics in water can be treated and removed. Removal of wastewater treatment works is dependent on the density and size of particles. Dense microplastics that settle on surfaces can be periodically removed from tanks, but buoyant particles remain in suspension and escape further treatment filters. Very little specific treatment and filtration strategies for micro/nano-plastics exist, and this is a relatively emerging area of research. Conventional waste water treatment plants do have high-efficiency rates of up to 98% removal of micro-plastics in the UK, but 2% still relates to a relatively large quantity of microplastics. Treatment plants struggle to filter out particles which are particularly buoyant and are on the micro/nano size boundary.
Importance to stakeholders
As previously highlighted, brake, tyre and road surface wear are major sources of plastic pollution and this is a significant concern to a range of stakeholders, including water companies, governmental departments such as DEFRA, DfT, Highways Agency, Public Health England and sewage/waste management businesses.
The recent government consultation and “call for evidence on brake, tyre and road surface wear” was published in July 2018. Outcomes of the DEFRA consultation are summarised below:
“We received over 50 responses to this call for evidence, from academia, industry, local government, regulators and individuals. We will use these responses to inform the continuous improvement programme of the National Atmospheric Emissions Inventory and the review of non-exhaust emissions by the Air Quality Expert Group, which make recommendations to government. The responses will also inform our work with international partners to research the development of new standards for tyres and brakes with an aim to address these emission sources.”
An example of research commissioned by DEFRA is a project currently carried out by the University of Plymouth: “Investigation of Sources and Pathways of Microplastic Pollution into the Marine Environment”.
Addressing the issue
The main opportunity for tackling this issue is at the source of pollution and the automotive industry bears a large part of this responsibility. Electric cars with regenerative braking, more efficient braking systems and smarter tyre materials would all contribute to reduced pollution. Policy measures, incentives and greater awareness among the general population can also have the potential to significantly reduce microplastic pollution and its associated issues.
There are also, for example, a range of solutions that can be deployed on highways and major trunk road drainage systems to remove suspended solids. A Waterbriefing article highlights the potential for focusing on more effective highways maintenance and drainage:
“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.”
The outcome from the DEFRA consultation will assist the research and development of new standards for tyres and brakes with an aim to address these emission sources.
Focused research and action are needed going forward, as industrial and governmental bodies now have a strong awareness of the impacts, but detection, treatment and reduction at source remain as areas where knowledge and action are superficial.