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When a river flows into the ocean, so does plastic. Lots of it. Globally, rivers transport as much as four million tonnes of plastic out to sea every year.

Engineers have developed barriers that trap plastic and reduce this flow, but they often impede boats and animals, limiting where the barriers can be used. But now, a Dutch team has created a barrier that it claims can block plastic without affecting other river traffic, and they’ve just tested their prototype in its first large-scale trial.

Francis Zoet, a former energy and sustainability consultant, designed the Great Bubble Barrier with two friends in response to the plastic they saw flowing through Amsterdam into the North Sea. “We thought, we need a barrier. We need something to stop this,” she says.

But you can’t just put a wall across the watercourses of a maritime nation like the Netherlands. So that rules out solutions such as Baltimore’s Mr. Trash Wheel which, while very effective, blocks part of the river with a containment boom, similar to the ones used to trap spilled oil. Instead, Zoet and her colleagues turned to another oil industry mainstay: bubbles.

The oil industry uses barriers of bubbles to trap oil after spills and contain sediment during dredging. Zoet wondered if bubbles could also catch plastic.

In concept, the Great Bubble Barrier is simple. Compressors pump air into two perforated pipes that lie diagonally across the riverbed, angled slightly upstream. The pipes create two curtains of bubbles that trap plastics and lift them to the surface.

The team built a model to refine their idea, and in June 2016 won the Plastic Free Rivers Makathon. Their win earned them a chance to conduct a pilot trial on the river IJssel, funded by the Netherlands’ Ministry of Infrastructure and Water Management and the water company PWN. The prize also included funding to further develop their idea and test it under different conditions at Deltares, a Dutch water research institute.

In November, Zoet and her partners installed a bubble barrier across a 200-meter-wide stretch of the IJssel, near the city of Kampen. Over the following three weeks, they monitored the plastic trapped by the barrier. They also floated oranges, small flags, and pieces of fabric down the river to test how the barrier handled known quantities of waste.

Their test runs were successful. The bubble barrier caught a wide range of plastics, from bags and bottles to much smaller particles, averaging two to three centimeters long. According to Zoet, the bubbles acted like a containment boom, trapping plastics on the surface.

Grim Eidnes, a scientist at SINTEF, a Norwegian research institute, helped pioneer the use of bubble curtains for containing oil spills. He says bubbles can trap plastics floating on the river’s surface, but those below the surface are more difficult to contain. Rather than staying trapped, he says, these plastics will continue to sink and rise in the stream of bubbles, and may eventually bounce their way through the bubble curtain.

There are other potential problems to look out for, too. In the industrial world, bubble barriers, combined with light and sound, are used to keep fish away from dams and power station structures.

Peter Sorenson, a fish biologist at the University of British Columbia, worries about the potential ecological effects. Sorenson investigates the use of bubble barriers for controlling invasive fish species and says bubbles create sounds and visual stimuli that fish often notice, with the responses varying between species.

“It would be naive just to stick [bubble curtains] in the water and think that they are not having any effect on organisms,” he says. Any unintended consequences would depend on how many bubbles are being produced, the barriers’ locations, and the fish species, he says.

Carlos Garcia de Leaniz, director of the Centre for Sustainable Aquatic Research at Swansea University in the United Kingdom, says the bubble barrier is a “terrific idea” worth exploring. But he agrees that more research is needed. “Air barriers alone are not usually effective at deflecting migratory fish, but where is the data to say that this specific solution is fish friendly?” he asks. He adds that migratory fish, such as salmon, are more likely to be affected, possibly delaying migration and increasing their risk of predation as they pause at barriers.

However, Garcia de Leaniz thinks a bubble barrier would be much less of a concern than the many other barriers already on our rivers, such as dams. “It would be tragic if these people were not allowed to develop their system … on the grounds that it might create a barrier, when we have barriers littering rivers that … we accept as a necessary evil,” he says.

Zoet says she and her colleagues are aware of possible effects on fish, but have measures to mitigate them. For example, they’ve raised the bubble-blowing pipes 20 to 30 centimeters off the riverbed to allow fish to swim underneath. She says the team’s research suggests that while fish are initially scared by the bubbles, they do get used to them and will eventually swim through.

For now, Zoet and the rest of the team at the Great Bubble Barrier are focusing on analyzing the results from the pilot project and their research at Deltares, and setting up the first permanent barrier. To do this, they need to find a city that is willing to host it and are hoping to raise US $120,000 through crowdfunding to pay for it.

They also need to figure out how to remove the trapped plastic from the water. For now, the plastic is pushed by the current to the riverbank. Zoet says they still need a system to separate small fish and animals that have been trapped by the barrier from the waste, and are researching the best ways to do this. Zoet says a conveyor belt system, similar to that used in Baltimore’s garbage wheel, is likely their best bet.