Polluted Lakes Are Being Cleansed Using Floating Wetlands Made of Trash

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ON THE BANKS of Nagdaha, a polluted and lotus-infested lake in Nepal, Soni Pradhanang is putting trash back into the water—on purpose.

She carefully assembles a platform of styrofoam and bamboo mats, then weaves it together with zip ties and coconut fiber, refuse from nearby tech stores. Then, she pokes 55 plants lush with red flowers through 2-inch holes in the platform, each plant set 6 inches apart. Though Pradhanang’s creation isn’t high-tech, it is effective, and one of the most affordable water-filtration systems available. “I’m cheap,” she says, laughing.

Pradhanang, a hydrologist at the University of Rhode Island who studies water-quality monitoring and modeling, has spent the past seven years working on her trash-based contraptions. Called floating treatment wetland systems (FTWS), these are 4-foot by 6-foot buoyant platforms topped with plants. When their roots are submerged in contaminated water, the plants suck pollutants into their stems and leaves as they grow. In turn, they leave behind dissolved oxygen captured during photosynthesis, which supports life beneath the surface.

The hope with these devices is two-fold: that they will cleanse waters and recycle rubbish in parts of the world where budgets for either are incredibly tight.

Pradhanang is also the chief scientific technical adviser to The Small Earth Nepal, a Kathmandu Valley–based research and community engagement group. It has spent around five years building, testing, and implementing the first known trash-based wetland water-cleaning systems here in Nagdaha’s polluted waters, which contain high concentrations of nitrates and phosphates due to agricultural and urban runoff. Pradhananag and The Small Earth Nepal also operate sites with collaborators in Ajmer, India, and Dhaka, Bangladesh.

These floating treatment systems aren’t huge—each mat is around the dimensions of a full-size mattress—but they can connect to form clusters that treat larger areas of water. In Nagdaha, where FTWS have been bobbing lazily for nearly two years, there are five clusters, each with eight platforms.






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