Floating Wetlands Cut Greenhouse Gas Emissions From Wastewater Lagoons

Floating wetlands installed in wastewater lagoons could provide water utilities with a relatively low-complexity way to reduce greenhouse gas emissions while supporting water treatment, according to results from a two-year trial in southeastern Australia.

Researchers from RMIT University, Westernport Water and Australia’s Commonwealth Scientific and Industrial Research Organisation, or CSIRO, tested a constructed floating wetland at a wastewater holding lagoon on Phillip Island in Victoria.

The peer-reviewed study, published in the Journal of Environmental Management in August 2026, found that total greenhouse gas emissions measured in carbon dioxide equivalent fell by approximately 22% to 31% over the two-year monitoring period compared with a control section of the lagoon.

Individual gases showed larger reductions. Methane emissions declined by between 32% and 66%, carbon dioxide emissions fell by approximately 24% to 36%, and nitrous oxide emissions were reduced by about 18%, according to the researchers. Emission reductions emerged within roughly four to seven months after installation.

The findings are significant because wastewater treatment is an often overlooked source of climate emissions. The study estimates that wastewater treatment accounts for around 1.6% of global human-caused greenhouse gas emissions, equivalent to approximately 0.77 gigatonnes of CO₂ equivalent annually. Wastewater systems are also estimated to contribute around 7% to 10% of global methane and nitrous oxide emissions.

Native Plants Create Habitat for Beneficial Microbes

The trial used a floating platform covering approximately 330 square meters, roughly the area of one-and-a-half tennis courts. It was planted with native reeds and sedges, with their roots extending directly into the wastewater below.

Rather than functioning simply as vegetation, the root systems create habitat for microbial communities. These microorganisms can consume nutrients and pollutants in the water and may also influence the biological processes responsible for producing or consuming greenhouse gases.

Floating treatment wetlands have previously been used to remove nutrients from wastewater and stormwater. What had been less clear was whether they could significantly reduce greenhouse gas emissions when deployed at operational scale.

According to RMIT researcher Lukas Schuster, the results provide evidence that enhancing microbial communities around wetland roots can reduce emissions without requiring highly complex treatment technology.

The wastewater lagoon was divided into treatment and control sections, allowing researchers to compare emissions under similar operating and environmental conditions.

Emissions were continuously monitored over two years using Pondi, a solar-powered sensor system developed by RMIT with partners including Leading Edge Engineering Solutions, Deakin University and the University of Queensland.

Methane Reductions Could Be Particularly Important

The methane reduction is potentially one of the most relevant findings for wastewater operators developing net-zero strategies.

Wastewater environments can create oxygen-poor conditions that encourage microorganisms to break down organic matter and generate methane. Although methane remains in the atmosphere for less time than carbon dioxide, it has a substantially stronger warming effect over shorter climate accounting periods.

Reducing methane emissions can therefore provide relatively rapid climate benefits, particularly for utilities where biological wastewater processes represent a significant part of their operational emissions.

Nitrous oxide is another important concern. It can be produced during biological nitrogen treatment processes and has a much higher warming impact per unit of gas than carbon dioxide.

The study's finding that a floating wetland reduced multiple greenhouse gases simultaneously suggests the technology could complement other emissions-reduction measures rather than targeting only one source.

Retrofitting Existing Infrastructure

One potentially important advantage is that floating wetlands can be installed in existing wastewater lagoons.

Westernport Water Managing Director Dona Tantirimudalige said the ability to retrofit the systems could allow utilities to reduce emissions without making major changes to existing treatment infrastructure. The organization said further trials across different lagoon designs and operating environments will be necessary to determine how widely the results can be replicated.

That distinction is important for the economics of wastewater decarbonization. Replacing or extensively modifying treatment plants can require significant capital investment, while a floating treatment system may be capable of being added to infrastructure already in operation.

Costs would still depend on factors including wetland size, plant selection, lagoon conditions, maintenance requirements and local climate.

The Phillip Island trial required maintenance including weed management and measures to deter birds while the plants became established. Long-term commercial assessments will therefore need to consider operating and maintenance costs alongside emissions reductions.

Water Quality Provides an Additional Benefit

Climate mitigation is not the only potential advantage.

CSIRO researcher John Awad said floating wetlands can also remove contaminants and improve water quality before treated wastewater enters downstream ecosystems.

This combination could make the approach particularly relevant to utilities seeking investments that address several environmental objectives simultaneously, including greenhouse gas reduction, nutrient management, water quality and ecosystem protection.

Other research into floating constructed wetlands has similarly identified their potential as low-energy biological treatment systems. A separate 2026 study of a floating wetland configuration for municipal wastewater treatment reported substantial removal of suspended solids, organic pollution, nitrogen and phosphorus without relying on electromechanical treatment equipment.

However, the Phillip Island results should not yet be interpreted as evidence that identical reductions will occur at every wastewater facility. Lagoon design, wastewater composition, temperature, plant species, microbial communities and hydraulic conditions can all influence performance.

Researchers Expand Trials to Farm Dams

RMIT researchers are now extending the approach beyond wastewater treatment plants.

A separate project with Melbourne Water and the Bass Coast Landcare Network is testing floating wetlands in farm dams across Victoria's Bass Coast. Researchers are examining their effects on greenhouse gas emissions, water quality and biodiversity.

Australia has approximately 1.8 million farm dams, suggesting a potentially much larger application if the technology proves effective across agricultural water systems.

For the water sector, the immediate significance of the wastewater trial is more focused. Utilities face increasing pressure to identify credible ways of reducing direct emissions from treatment operations, including methane and nitrous oxide sources that cannot be addressed simply by purchasing renewable electricity.

Floating wetlands could offer another tool in that process. The technology uses existing lagoon space, requires relatively little mechanical infrastructure and may generate additional water-quality benefits.

Further studies will be needed to establish performance across different treatment systems and climates, determine costs per tonne of CO₂ equivalent avoided and assess durability over longer periods.

The Australian trial nevertheless provides full-scale evidence that nature-based infrastructure can play a measurable role in reducing emissions from an industrial process that remains difficult to decarbonize.

Source: www.sustainabilitymatters.net.au