Wetland plants grown on the surface of wastewater treatment lagoons can reduce greenhouse gas emissions from these water bodies by more than one-quarter, according to a new study. These so-called “floating wetlands” may be an elegant solution that works with existing infrastructure to address a growing emissions hotspot.
Wastewater treatment is responsible for 1.6% of global greenhouse gas emissions. As economic development brings better sanitation to more of the world’s population, this climate impact is also increasing. “Wastewater treatment solves one environmental problem by cleaning dirty water, but creates another by emitting greenhouse gases,” says study team member Lukas Schuster, who studies nature-based climate solutions at RMIT University in Melbourne, Australia.
Floating wetlands, in which native reeds and other marsh plants are planted on buoyant artificial structures with their roots dangling in the water, are becoming a widespread tool for soaking up excess nutrients such as nitrogen and phosphorus in wastewater lagoons. These open-air ponds temporarily store treated water before it is released into adjacent waterways, reused for irrigation, or undergoes further treatment to be recycled into drinking water.
Researchers have suspected that floating wetlands might also reduce greenhouse gas emissions from the surface of wastewater lagoons. In the first large-scale, real-world test of this hunch, Schuster and his collaborators constructed a floating wetland the size of one-and-a-half tennis courts on portion of a wastewater lagoon in southeastern Australia. They monitored water quality and greenhouse gas emissions from the surface of the water for two years.
The floating wetlands reduced carbon dioxide emissions off the surface of the lagoon by up to 36%, methane up to 66%, and nitrous oxide by up to 18% compared to an adjacent portion of the lagoon without a floating wetland, the researchers found. Overall greenhouse gas emissions are reduced by up to 31%.
“One of the biggest surprises was how quickly greenhouse gas emissions declined after the floating wetland was installed,” Schuster says. The researchers expected that emissions would only be dampened after nutrients were removed from the water. In fact, greenhouse gas emissions declines were seen four to seven months into the experiment, but nutrient reductions only emerged after about a year.
It’s not yet clear exactly how the plants work their magic. “We suspect that interactions between plant roots, microbial communities, and greenhouse gas production and consumption may be important, but targeted measurements are needed to test these mechanisms,” says Schuster.
And there are still some kinks to be worked out. Trimming the plants—necessary to promote regrowth and maximize nutrient uptake—tends to trigger spikes of methane release, so the researchers want to figure out how to minimize that result. The harvested material, which amounted to nearly one metric ton in the current study, also needs to be converted into some form of longer-term carbon storage to make sure the system truly benefits the climate (the researchers suggest biochar).
Schuster and his team are now looking at adding floating wetlands to farm ponds. “These systems receive substantial inputs of nutrients and organic matter from livestock and can have high greenhouse gas emissions relative to their size,” Schuster says. The new study will help assess whether floating wetlands are a climate solution that could be applied to various types of aquatic systems, he says.
Sources: Schuster L. et al. “Constructed floating wetlands cut greenhouse gas emissions from wastewater lagoons.” Journal of Environmental Management 2026.
Image: Courtesy of Lukas Schuster.
