A floating garden of native plants installed on a wastewater lagoon at Phillip Island has reduced its greenhouse gas emissions by between 22% and 31%, according to a world-first full-scale trial.
The two-year Victorian study recorded particularly strong reductions in methane, one of the most powerful greenhouse gases driving near-term warming. Methane emissions fell by as much as 66% in the part of the lagoon containing the floating wetland.
For anyone who knows Phillip Island primarily for penguins, beaches and weekend traffic, a wastewater treatment plant in Cowes may seem an unlikely source of environmental innovation. Yet the results could have implications for water utilities, farms and polluted waterways far beyond the island.
The results at a glance
| Emission measured | Reduction with floating wetland |
|---|---|
| Total greenhouse gases, measured as CO₂ equivalent | 22% to 31% |
| Methane | 32% to 66% |
| Carbon dioxide | 24% to 36% |
| Nitrous oxide | Approximately 18% |
The findings come from research conducted by RMIT University, Westernport Water and CSIRO. The peer-reviewed study was published in the Journal of Environmental Management in August 2026, before receiving wider national attention in September.
A wetland the size of one and a half tennis courts
The experiment took place at the Cowes Wastewater Treatment Plant between 2023 and 2025.
Researchers installed a constructed floating wetland covering approximately 331 square metres, roughly the area of one and a half tennis courts. The platform was planted with native reeds and sedges, whose roots extended through the structure and into the wastewater below.
That may sound substantial, but the wetland covered only about 7% of the lagoon’s 4,500-square-metre surface. The lagoon itself could hold approximately 14 million litres of treated wastewater.
To create a meaningful comparison, the inlet area was divided into two parallel channels using baffle curtains. One channel contained the floating wetland, while the other served as a control. Both received wastewater from the same source under largely comparable conditions.
Researchers monitored carbon dioxide, methane and nitrous oxide using solar-powered sensors throughout the trial. They also collected regular measurements of nutrients and water quality.
This side-by-side design helped distinguish the wetland’s effect from seasonal changes in temperature, rainfall and wastewater flows.
Why wastewater produces greenhouse gases
Wastewater does not stop affecting the climate when it disappears down a drain.
Treatment plants and storage lagoons contain large amounts of organic material and nutrients. As microorganisms break that material down, they can generate carbon dioxide, methane and nitrous oxide.
Methane is produced particularly readily in oxygen-poor conditions. Nitrous oxide can be released through the biological processes that transform nitrogen compounds in wastewater.
According to the study’s authors, wastewater treatment accounts for approximately 1.6% of human-caused greenhouse gas emissions worldwide. That represents roughly 700 million to 770 million tonnes of carbon dioxide equivalent each year.
These emissions can be difficult to eliminate because the microorganisms producing them are also involved in breaking down waste. Many smaller treatment plants also rely on open lagoons because they are comparatively simple and economical to operate.
How can floating plants reduce emissions?
A constructed floating wetland is not an island of soil. It is a buoyant platform that holds living plants while allowing their roots to hang directly into the water.
The dense root systems create habitat for bacteria and other microorganisms. These microbial communities can absorb nutrients, transform pollutants and alter the biological conditions under which greenhouse gases are produced or consumed.
RMIT researcher Dr Lukas Schuster said the study provided full-scale evidence that supporting microbial communities around wetland roots could reduce wastewater emissions without depending on highly complex equipment.
Importantly, the emissions reductions appeared before researchers detected substantial changes in the lagoon’s nutrient concentrations. Methane reductions became evident after about four months, while changes in carbon dioxide and nitrous oxide emerged within approximately seven months.
This suggests that the result was not simply caused by the plants removing large quantities of nitrogen and phosphorus from the water. Changes in microbial activity and conditions around the submerged roots may have played a more immediate role.
The precise biological mechanisms still require further study.
Why methane reductions matter
The reduction in methane is arguably the trial’s most significant result.
Methane remains in the atmosphere for less time than carbon dioxide, but it traps much more heat while it is there. Reducing methane emissions can therefore slow warming more quickly than carbon dioxide reductions alone, although both are essential.
The Phillip Island trial recorded methane emissions between 32% and 66% lower in the treatment channel, depending on the location and period measured.
The total climate effect was smaller because methane was only one part of the lagoon’s emissions. Nevertheless, the combined reduction of 22% to 31% is substantial for an intervention that does not require replacing the treatment plant or constructing an entirely new system.
A retrofit rather than a rebuild
One of the technology’s most attractive qualities is its apparent simplicity.
Floating wetlands could potentially be added to existing lagoons while the surrounding treatment infrastructure remains in place. They require no land outside the lagoon and relatively little operational energy once established.
That could make them useful for regional utilities and communities that cannot afford more elaborate treatment upgrades.
Westernport Water describes the project as an investigation into a low-energy intervention capable of producing measurable emissions reductions under genuine operating conditions. Detailed reports from the trial are available through the utility’s floating wetlands project page.
The wetlands may offer other benefits too. Previous research has investigated their ability to capture nutrients and contaminants, including PFAS compounds, while plant-covered platforms can also provide habitat above and below the water.
CSIRO researcher Dr John Awad said the systems could combine emissions reduction with improved water quality before treated water is discharged into downstream ecosystems.
Promising, but not yet a universal solution
The findings are impressive, but they come from one floating wetland in one wastewater lagoon.
Water chemistry, climate, lagoon design, wastewater flow and plant species could all influence the results elsewhere. The wetland also required several months to establish before measurable emissions reductions appeared.
Questions still to be answered include:
- How consistently the reductions can be reproduced in other climates and treatment systems
- How much lagoon coverage delivers the best result
- What the platforms cost to install and maintain over their full lifespan
- How often the vegetation must be harvested or replaced
- What happens to nutrients and contaminants accumulated in plant tissue
- Whether the platforms themselves can be manufactured from more sustainable materials
The researchers have acknowledged that current floating structures can contain plastic components. Future versions may use recycled or plant-based materials to reduce the environmental footprint of the system itself.
It is therefore too early to multiply Phillip Island’s percentage reduction across every wastewater lagoon in the world. The study demonstrates credible potential, not a finished global emissions solution.
Farm dams are the next testing ground
The research is already moving beyond wastewater treatment.
RMIT researchers are working with Melbourne Water and the Bass Coast Landcare Network to install floating wetlands in farm dams across the surrounding region. Those trials are examining their effects on water quality, greenhouse gas emissions and biodiversity.
Australia has approximately 1.8 million farm dams. Individually they are small, but together they occupy a considerable area and can become hotspots for methane when they contain high concentrations of nutrients and organic matter.
Floating vegetation may also provide shelter for birds and other wildlife. One Bass Coast farmer participating in the research told ABC News that ducks were using the platforms as refuge from foxes.
Researchers will now need to establish whether the emissions reductions observed at Cowes can be reproduced across these smaller, more varied bodies of water.
A small island with a potentially global idea
Climate technology is often presented as an expensive piece of machinery, an enormous industrial facility or a breakthrough material developed in a laboratory.
The Phillip Island experiment offers a different model. A floating structure, native plants and the microbial life gathering around their roots altered the emissions produced by an existing piece of infrastructure.
It did not eliminate those emissions. It did not transform wastewater treatment overnight. What it achieved was a measured reduction of nearly one-third under real operating conditions, using a system that could potentially be installed in lagoons already operating around the world.
That is enough to make the Cowes trial more than an attractive local experiment. It is evidence that some climate solutions may involve redesigning industrial environments so that living systems can work inside them.
On Phillip Island, that possibility is already taking root.
Image Source: https://www.westernportwater.com.au/our-community/projects/completed-projects/floating-wetlands-pilot-project/
