Amsterdam is trying out a "bubble barrier" to help remove plastic from canals by capturing trash hidden beneath the surface of the water.

While the project launched in November is just a small-scale test right now, the non-profit group behind it hopes it could be deployed elsewhere, if successful.

"A bubble barrier is basically a tube that we place on the bottom of the river or canal, that has holes in it and we press air through it; that creates a bubble curtain," said Francis Zoet, the Great Bubble Barrier project's technical director.

Because the tube lies diagonally across the canal, the bubbles work with the flow of water in the canal to float the waste and then shuttle it into a collector on the side.

The project's proponents say the bubbles don't interfere with passing boats, and most fish readily pass through or "take a short time" to pass through.

"We nevertheless implement a fish passage in each design, either under the catchment system or under the bubble tube itself," the Great Bubble Barrier site says.

While Amsterdam already has four boats that collect around 42,000 kilograms of plastic a year, the boats can only pick up the rubbish on the surface and some smaller pieces are missed altogether.

"What people don't realize is that every piece of plastic which falls into the water in the canals is eventually flowing out to the North Sea. We want to prevent that," said Roy Leysner of Waternet, part of the local water authority which is funding the project with the City of Amsterdam.

Rivers are a major conduit of plastic pollution into the world's oceans, carrying up to four million metric tonnes of plastic to the sea each year, according to estimates by the Hemholtz Centre for Environmental Research in Germany.

A second Dutch non-profit has also launched a system to collect surface river trash using floating barriers.

Zoet of the Amsterdam project said the two concepts are complementary. "We support "basically every initiative that is focused on reducing the plastic soup," she said.