Constructed floating wetlands cut greenhouse gas emissions from wastewater lagoons.

Wastewater treatment is a significant, yet often overlooked, contributor to global greenhouse gas emissions, accounting for ∼1.6% of anthropogenic emissions (∼0.77 Gt carbon dioxide [CO2]-equivalent per year), including 7-10% of methane (CH4) and nitrous oxide emissions (N2O). However, scalable mitigation options remain scarce. Constructed floating wetlands (CFWs) are widely used to reduce nutrient loads in wastewater systems, but their potential to reduce greenhouse gas emissions at full operational scales remains unclear. We conducted a two-year, full-scale trial of a CFW in a wastewater holding lagoon in southeastern Australia. The lagoon was divided into paired treatment (with the CFW) and control channels using baffle curtains. At the start and end of each channel, we continuously monitored greenhouse gas fluxes using hourly chamber concentration measurements and collected monthly water samples to assess water quality. On average, CO2-equivalent emissions in the treatment channel were 22-31% lower than in the control, primarily driven by reduced CH4 emissions (32-66%), with additional reductions in CO2 (24-36%) and N2O emissions (18%). Total Kjeldahl nitrogen levels (primarily organic nitrogen) declined by 12%, whereas nitrate and phosphorus remained unaffected. Notably, emission reductions emerged within four to seven months of CFW installation, preceding detectable nutrient reductions, which only became evident after 12 months. Altogether, our findings provide one of the first full-scale demonstrations that CFWs can substantially reduce greenhouse gas emissions from wastewater lagoons and support their potential as a nature-based pathway for decarbonising the water sector.