Researchers had a hunch about how to solve the climate problem caused by wastewater treatment

Researchers have discovered that growing wetland plants on the surface of wastewater treatment lagoons can significantly reduce greenhouse gas emissions from these water bodies. This discovery could provide an elegant solution to address the growing emissions from this sector without disrupting existing infrastructure. Wastewater treatment contributes 1.6% of global greenhouse gas emissions, and as more people gain access to better sanitation, this impact is expected to increase.

Floating wetlands, consisting of native reeds and other marsh plants planted on buoyant structures, are already used to remove excess nutrients like nitrogen and phosphorus from wastewater lagoons. However, researchers suspected these floating wetlands might also lower greenhouse gas emissions. To test this hypothesis, Lukas Schuster and his team constructed a floating wetland the size of one-and-a-half tennis courts on a portion of a wastewater lagoon in southeastern Australia and monitored water quality and greenhouse gas emissions for two years.

The results showed that the floating wetlands reduced carbon dioxide emissions off the lagoon by up to 36%, methane emissions by up to 66%, and nitrous oxide emissions by up to 18% compared to an adjacent portion of the lagoon without a floating wetland. Overall, greenhouse gas emissions were reduced by up to 31%. The researchers were surprised to find that the emissions declines occurred relatively quickly, four to seven months after installation, while nutrient reductions took about a year to become noticeable.

While the mechanism behind the reduced emissions is still not fully understood, the researchers suspect that interactions between plant roots, microbial communities, and greenhouse gas production and consumption may play a crucial role. Additionally, trimming the plants, necessary for regrowth and nutrient uptake, tends to trigger spikes in methane release, which the researchers aim to minimize.

The harvested material, amounting to nearly one metric ton in the study, needs to be converted into a form of long-term carbon storage to ensure the system's climate benefits, such as biochar.

Moving forward, Schuster and his team are exploring the potential of adding floating wetlands to farm ponds, which receive substantial nutrient and organic matter inputs from livestock and can have high greenhouse gas emissions relative to their size. This study will help assess whether floating wetlands can be applied to various aquatic systems as a climate solution.