Polluted lakes are being cleaned using floating wetlands made of garbage

When Pradhanang found a mix of trash that could be made into a platform that didn’t disintegrate with age, she started talking to colleagues in Nepal. By 2021, they had secured $78,000 in funding from the Asia-Pacific Network for Global Change Research, a research and policy funder, to conduct the study in Nepal, India, and Bangladesh.

Although the Nagdaha and the Indian and Bangladeshi sites do not provide drinking water for the residents, they have cultural and religious significance. During her many visits to Nepal, Pradhanang saw children swimming in the turbid water of the Nagdaha, dogs lapping at the shore, and women sitting on the banks to wash dishes and clothes. Pradhanang and The Diminutive Earth Nepal wanted to beautify the lake by cleaning it. They chose flowering plants that served both purposes: the Indian shot (Canna indica) and scarlet sage (Shiny Sage), whose radiant red flowers stand out against the grey water.

After the floating platforms were installed in the fall of 2022, scientists tested the water monthly and found results consistent with their predictions. Laboratory testswhich showed a 99 percent reduction in nitrate levels, an 80 percent reduction in phosphates, a 56 percent reduction in iron, and a 55 percent reduction in ammonia. Dissolved oxygen levels increased by half. Their results were sent to Civil Engineering Journal for inspection.

Growing flowering plants can add aesthetic appeal to a body of water, but cooperation means avoiding growing food on the platforms, because the plants accumulate such high levels of pollutants. But humans aren’t the only ones intrigued by floating wetlands; geese and ducks gather at the mats, perch on the platforms, and excrete phosphorus-rich waste into the water.

Pradhanang came up with a clever solution to the mischievous mallards: She tied silver ribbons to the plant stems, the radiant featherlight of which scared the birds away. Although scientists don’t want geese at FTWS, ​​pollinators like bees and diminutive birds are more than welcome—as are the diverse microbial life that lives on plant roots and digests pollutants in the water for energy.

“There’s a real kind of, ‘If you build it, they will come,’ phenomenon that’s associated with floating wetlands,” says Max Rome, who earned his doctorate at FTWS in Boston’s Charles River and now works for the Charles River Watershed Association. “These systems are really effective at creating wetland biodiversity in a place where there’s simply no room for wetlands.”

While trash-based systems solve some water pollution problems, they leave others behind—namely microplastics. Incorporating local trash into the mats shouldn’t make things worse—most trash is collected from the water to begin with—but the high plastic content in the water means it could still pose a health risk to residents, even after it’s cleaned up.

While Pradhanang and outside researchers have raised concerns about incorporating Styrofoam and other plastics into waste-based FTWS, ​​they say the benefits of the mats generally outweigh the costs. “If you can take waste that would normally be in the water,” White says, “and you can use it to make a product that actually helps clean the water, that’s amazing.”