Tyre wear accounts for the largest share of microplastic input into the environment

This figure is surprising: every year in Germany, about 1.2 kilogrammes of tyre wear, i.e. particles produced by the rubbing of car tyres on the road surface, are released into the environment per capita. This makes tyre wear the number one source of microplastic emissions.

This means that urgent measures must be taken – however, in order to develop suitable strategies, we need reliable data on where and how much tyre abrasion is produced - as well as its distribution in the environment. In the project "Tyre Abrasion in the Environment" (RAU), researchers from TU Berlin, along with cooperation partners from science and industry, have investigated where particularly large amounts of tyre abrasion are produced in road traffic, the routes by which it enters the environment and how it could be effectively removed. The results were presented at a webinar in summer 2021.

When rubber and road surface rub against each other, abrasion occurs. Depending on the weight of the vehicle, speed, type of surface and other influencing factors, approximately half tyre rubber and half road surface. The abrasion is transported by rainwater and collects in the sewage system. The resulting wastewater poses a major problem, as stormwater overflows in areas with segregated systems usually discharge water directly into bodies of water - along with the abrasion. In cities with mixed systems, where rainwater and wastewater are discharged together, this may also occur if the sewage treatment plants are overloaded during heavy rainfall.

Tyre abrasion does not occur evenly. Researchers in the RAU project investigated different traffic situations on inner-city roads in Berlin: Curves, straights, inclines, roundabouts and traffic lights. For this purpose, novel and elaborate sampling techniques had to first be developed. Dr. Jens Reiber from the Wessling company explained the adapted sample preparation and the novel analysis methods.

The researchers found a particularly high amount of tyre wear around bends and at traffic lights, i.e. at places where the tyres are particularly stressed. Curves and intersections, where drivers typically make a lot of turns, are significantly more stressed than traffic lights, where not every car comes to a halt and has to start up again. On straights and uphill slopes, on the other hand, only little abrasion is generated. This is, naturally, in proportion to the amount of traffic: The more cars are on the road, the more tyre wear occurs.

RAU cooperation partner and tyre manufacturer Continental has investigated which factors have the greatest influence on how much tyre wear occurs. They found that individual driving behaviour is the most important factor.. This becomes evident immediately when you think of braking and acceleration marks on the asphalt. Other important factors are, listed in order of decreasing importance:: the course of the bend, the road surface, the vehicle, the tyre design, the temperature and the wetness or dryness. According to Conti expert Dr. Frank Schmerwitz, the friction between tyre and road serves another important purpose: it creates grip and more safety in road traffic.

During rainfall, between 12 and 20 per cent of tyre wear ends up in the aquatic environment. Ideally, the road sweeper comes along first.. The project also investigated this scenario in detail.. In tests conducted both on the road and indoors, the RAU project partner Sieker analysed how much waste the machines pick up and which parts of the tyre wear are picked up in the process and which are not. The indoor test already showed: the sweeper swallows up to about 80 percent of the particles, and about 55 percent of the microcomponents. The figures are worse on the road, partly because of cracks in the road surface and the fact that the sweeper often is unable to sweep directly along the edge of the road because of parked cars. The project partner has reached the following conclusion: In principle, street sweeping has a positive effect, but there is room for improvement. Dr. Harald Sommer, Sieker expert, reported that an enormous amount of tyre abrasion accumulates along the roadsides, which is often not picked up.

Where does the tyre wear end up? About 60 per cent ends up in our soils, 20 per cent in surface water, i.e. it enters into bodies of water via rainwater. A portion of this - 2 to 5 per cent - ultimately reaches the sea by way of the river mouth.. That may not sound like much at first, but it is: at 1.2 kilogrammes per German citizen per year, it is 24 to 60 grammes and thus a total of 1.92 to 4.8 million kilogrammes of tyre abrasion with which Germany alone pollutes the oceans every year.

At the end of the webinar, he question arose as to where the greatest potential lies for reducing tyre wear in the environment.. Daniel Venghaus recommended focusing on the road surface: He said it was best to keep tyre wear there as far as possible and to remove it by means of road cleaning. The fine particles in particular must be collected more efficiently, added Dr Harald Sommer. Dr. Frank Schmerwitz focused on individual responsibility, i.e. driving behaviour, and an intelligent influence on traffic flows. RAU project coordinator Prof. Matthias Barjenbruch had a practical tip: Clean the streets before it rains! The tours of the street cleaning should be scheduled more precisely. And of course, , as a society, we should reduce the use of cars and instead rely increasingly on public transport and bicycles.

Written by Wiebke Peters