Pradhanang built floating wetlands from trash as proprietary mats cost a year's pay

In a July 2024 WIRED report, hydrologist Soni Pradhanang of the University of Rhode Island describes her floating treatment wetland systems, 4-by-6-foot buoyant platforms topped with plants. WIRED says cities such as Baltimore, Boston and Chicago have used proprietary versions, but Pradhanang calculated that bases costing $600 each in 2018 would equal roughly 70,000 Nepalese rupees apiece, "somebody's one-year salary."

Her answer is a method she calls "trash for cash," where the cash is improved water quality. She first tested trash-based mats in a Rhode Island lab in 2017, then worked with The Small Earth Nepal, a Kathmandu Valley research and community group. At Nagdaha lake, they used styrofoam and bamboo mats woven with zip ties and coconut fiber, with 55 flowering plants per platform. They chose flowering plants so the lake would also look better.

By 2021 the team had $78,000 from the Asia-Pacific Network for Global Change Research for studies in Nepal, India and Bangladesh. After installing the platforms in fall 2022, Nagdaha has five clusters of eight platforms each. Silver ribbons tied on the plant stems drive off geese and ducks whose droppings add phosphorus.

Proprietary bases were unaffordable for the lakes that most needed them.

Local waste, much of it gathered from the water, makes the platforms close to free.

Flowering plants gave residents a visible benefit as well as cleaner water.

Clusters of mattress-sized platforms can scale to treat larger areas.

When the proven solution is priced out of reach, rebuild its expensive component from local waste and measure that it still works.

The researchers say monthly testing found a 99 percent reduction in nitrates, 80 percent in phosphates, 56 percent in iron and 55 percent in ammonia, with dissolved oxygen up by half; the results were submitted to the Journal of Civil Engineering for review. WIRED notes the mats leave microplastics unaddressed, that styrofoam use drew concern from outside researchers, and that they need maintenance or the pieces turn back into trash.