Where do tire particles go?
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Once again, there is no clarity on where these particles go, once released. Some research by the tire industry suggests that very little reaches the oceans because the dense particles tend to sink as soon as they enter a watercourse. Independent reports from the IUCN, for example, put tires as the primary source of microplastics in the oceans.More recent research by The Norwegian water institute says the ability of a particle to sink or float depends on both size and density. Large, dense particles tend to sink to the bottom of a collection pond quite quickly, but smaller or less dense particles will remain in the water column, even after days of settlement time in completely still water.
One of the difficulties in the TIP analysis of the situation is that TIP appears to be deliberately ignoring these smaller particles, and focussing all research on the larger particles that will sink to the bottom over time. These smaller particles remain in the water column almost indefinitely.
One multi-year research programme ‘Tyre-Loss’ at the University of Plymouth in the UK aims to put a figure on this, but the research is not yet complete.
Outside the water research programme, there are a number of transport mechanisms for the particles.
Once generated in the tire-road contact patch, the larger particles fall to the ground and then, through a combination of active brushing with water spray and passive rainfall, they find their way into drainage channels.
Some of that drainage material goes into collection ponds. Other parts go into the national sewage treatment system.
Some countries simply allow the water run-off (with any particulate pollutants) to drain into surrounding fields and onto the land.
Other countries have installed collection ponds close to the road network to avoid the expense of transporting water run-off to central treatment facilities. The water in these ponds is recycled for agricultural irrigation and sometimes for road cleaning.
The fate of these particles is unclear, but some inevitably finds its way into water courses. Others find their way onto fields where crops are grown.
Where the road run-off goes into the sewage system, the larger particles settle down into the sewage sludge. Finer particles do not settle down and remain in the water column. Many of these are returned to rivers and other water courses.
The sewage sludge is often used by farmers as fertiliser, and thus, the particles are once again transferred to fields.
Daniel Venghaus, Research Assistant Urban Water Management Tire Wear and Microplastics at the TU Berlin presented the next paper at the CSR Europe event. We have previously featured the work of TU Berlin in the context of a joint research project with Audi, in which some road run-off drainage systems were fitted with particulate filters to gather the TRWP after and during rainfall.
He said it is very difficult to separate TRWP from other road run-ff debris. You can only separate the total suspended solid material from the liquid materials.
He said there are two main regimes in place. In one, the run-off and suspended solids are fed into the city sewers and processed effectively in existing waste water treatment plants.
In the other, the water is directed away from the road but is not treated in any way as the water drains away into the environment.
He said that of all the TRWP arisings, about half find their way into surface water. About 10% go into sewage systems for treatment and about 2% are eventually separated from the water and sewage.
He said it is expensive and difficult to treat every drainage point and gulley, so the focus should be on hot spots where large amounts of TRWP are generated.
He said that each hot spot will need a different management regime, due to the pre-existing arrangements of drains and other urban infrastructure.
