Wastewater treatment is responsible for around 1.6% of global anthropogenic greenhouse gas emissions, some 0.77 gigatonnes of CO2 equivalent per year, according to estimates from the Intergovernmental Panel on Climate Change (IPCC). Within this total, the sector accounts for between 7% and 10% of global methane (CH4) and nitrous oxide (N2O) emissions, two gases with a far higher warming potential than carbon dioxide (CO2). Much of this comes from holding lagoons, open systems where treated water is stored for long periods before reuse or discharge and where anaerobic conditions favour gas production.
A team of researchers from RMIT University, Deakin University and the Commonwealth Scientific and Industrial Research Organisation (CSIRO), in Australia, has spent two years assessing the ability of constructed floating wetlands to reduce these emissions under real operating conditions. The study, published in the Journal of Environmental Management, was carried out at the effluent holding lagoon of the Westernport Water treatment plant, on Phillip Island, in the state of Victoria.
To assess the real effect of floating wetlands, the researchers divided the lagoon, which covers 4,500 square metres and can hold up to 14 million litres, into two channels using baffle curtains. One channel included a constructed floating wetland covering 331 square metres, around 7.4% of the lagoon surface, made up of 60 modules planted with native aquatic species such as Baumea articulata and Phragmites australis. After the partial failure of the first species, much of it was replaced in the second year with Bolboschoenus caldwellii. The other channel was left without a wetland and served as the control. A splitter box ensured both channels received similar volumes of water.
Between April 2023 and April 2025, the team continuously monitored CO2, CH4 and N2O fluxes using floating sensors installed at the inlet and outlet of each channel, and collected monthly water samples to analyse total nitrogen, nitrate, phosphorus, dissolved oxygen, pH and temperature.
Significant emission reductions
The results show that greenhouse gas emissions, expressed as CO2 equivalent, were between 22% and 31% lower in the channel with the floating wetland compared with the control channel. The reduction was driven mainly by methane, with falls of between 32% and 66% depending on the sampling point and period considered, followed by CO2, with reductions of between 24% and 36%, and nitrous oxide, down 18%.
One notable finding of the study is that emission reductions became evident between four and seven months after the wetland was installed, well before any significant change in nutrient levels, which only became detectable after twelve months. Specifically, total nitrogen, measured as Kjeldahl nitrogen, fell by an average of 12% in the treatment channel, while nitrate and phosphorus concentrations showed no appreciable difference compared with the control channel.
The results show that greenhouse gas emissions, expressed as CO2 equivalent, were between 22% and 31% lower in the channel with the floating wetland compared with the control channel
The authors note that this gap between the reduction in emissions and the reduction in nutrients suggests that the main mitigation mechanism is unlikely to be simple nutrient uptake by the plants, but rather a combination of factors: microbial and algal consumption of gases within the root associated biofilms, reduced gas exchange with the atmosphere due to lower surface turbulence caused by the partial coverage of the wetland, and, over the longer term, reduced availability of organic matter for microbial respiration.
The study also identified a side effect linked to wetland maintenance. Each time the vegetation was harvested, in November 2023, September 2024 and March 2025, brief, short lived increases in methane emissions were recorded, reaching between 3.3 and 12.5 times those measured in the control channel. These spikes, attributed to physical disturbance of the root network and a temporary interruption of oxygen transport to the rhizosphere, subsided within a month. When these isolated episodes were excluded from the analysis, the reduction in CO2 equivalent emissions attributable to the wetland rose to 31%, compared with 22% when they were included.
Installation and maintenance costs
The researchers also documented the costs associated with installing, monitoring and maintaining the system over the two years of the study, which came to AU$360,016, around US$234,000. Roughly 75% of this expenditure was capital investment, covering the design, manufacture and installation of the wetland, as well as the equipment used to measure gas fluxes and water quality. The remaining 25% was operational cost, mainly labour for maintenance, vegetation harvesting and data analysis. In relative terms, the investment equated to AU$873 per square metre of wetland, and operating costs to AU$107 per square metre per year.
The authors themselves acknowledge several limitations of the study. It is an assessment carried out at a single lagoon, with one particular wetland configuration and plant species mix, so the scale of the reductions observed could vary at other sites with different hydraulic retention times, climates or coverage levels. In addition, the experimental design did not allow a direct calculation of the cost per tonne of CO2 equivalent mitigated at the scale of the whole lagoon, nor a detailed investigation of the specific microbial processes responsible for the emission reductions. The researchers suggest that future work should combine whole lagoon emission measurements with economic models that would better capture the cost effectiveness of these systems.
Even so, the team concludes that constructed floating wetlands represent a nature based solution with the potential to contribute to the water sector’s emission reduction goals, alongside additional benefits such as helping to mitigate contaminants including per and polyfluoroalkyl substances (PFAS), heavy metals and pharmaceuticals, and creating habitat for native wildlife.
