. Scientific Frontline: Blind spots when monitoring plastic waste

Wednesday, November 2, 2022

Blind spots when monitoring plastic waste

The researchers used river models that were filled with plastic waste for their investigation
Photo Credit: Daniel Valero, KIT

Whether in drinking water, in food or even in the air: plastic is a global problem - and the full extent of the pollution may not be known yet. Researchers at the Karlsruhe Institute of Technology (KIT), together with partners from the Netherlands and Australia, have reviewed conventional assumptions for the transport of plastic in rivers. The actual amount of plastic waste in rivers could therefore be up to 90 percent larger than previously thought. The new findings are intended to help improve monitoring and remove plastic from water. They report on their results in the journal Water Research.

Rivers play a key role in the transportation of plastic in the environment. "As soon as plastic gets into a river, it is transported at high speed and spread in the environment," says Dr. Daniel Valero from the KIT Institute for Water and Water Development and lead author of the current study on plastic transport. “But depending on the size and nature, plastic can behave very differently. It can be dipped, swimming or stopped by obstacles. "Current methods for estimating plastic pollution in rivers are based as standard but mainly on surface observations. “This is the only way to effectively monitor large rivers from bridges. However, the underlying assumptions have not yet been adequately reviewed,” said Valero.

Plastic particles are transported very differently

Together with its research partners, Valero now examined the behavior of 3,000 particles in the size range of 30 millimeters up to larger objects such as plastic cups in flowing waters. In flow models in the laboratory, each individual particle was tracked with millimeter precision in 3D using a multi-camera system, whereby the entire water column - from the water surface to the floor - was recorded. With the experiment, the researchers were able to prove statistically that plastic particles behave very differently depending on where they are in a river. Plastic that is transported under the water surface therefore behaves as common models for turbulent flows predict. “The particles are scattered like dust in the wind. Only a few particles get back to the water surface,” says Valero. As soon as plastic appears, however, the situation changes fundamentally: “When in contact with the free water surface, the particles are captured by the surface tension like flies in a spider web. They then continue to float together. "This adhesive effect is just as relevant for superficial transport in rivers as the specific buoyancy of a plastic particle.

Better models for visual surveillance

The results of the experiment show, on the one hand, that it is not enough to estimate the amount of plastic in rivers to consider only floating plastic on the surface. “The distortion is significant. If the turbulent nature of transporting plastic particles under the water surface is not taken into account, the amount of plastic waste in rivers can be underestimated by up to 90 percent,” says Valero. The results, on the other hand, confirm that existing knowledge of the behavior of particles in turbulent flows is relevant for the transport of plastic in rivers and that it can help to estimate the total amount more realistically. For this purpose, the researchers quantified the balance of the concentrations of plastic particles on the water surface and at greater depths with different transport conditions. On this basis, monitoring can continue to be carried out by visual observation of the water surface and the quantity actually transported can be calculated relatively precisely. In addition, the results can help in practice, namely in the development of new approaches to removing the plastic: "If you can estimate where most of the plastic is, then you also know where cleaning is most effective," said Valero.

Source/Credit: Karlsruhe Institute of Technology

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