A garden that floats on water while cleaning it at the same time, is the potential offered by constructed floating wetlands (CFWs), an emerging technology now backed by new international research as both effective and economically viable.
Led by scientists from Australia’s national science agency CSIRO and the University of South Australia, the study analysed 11 floating wetland projects in Australia, Canada, the United States and Pakistan.
These engineered systems consist of buoyant rafts planted with wetland vegetation, whose roots dangle into lakes, rivers or wastewater ponds. The roots form an underwater habitat for microbes that naturally filter out excess nutrients, sediments and pollutants.
The research provides the first large-scale comparison of costs. Capital expenses ranged widely – from as little as A$15 m² in Pakistan to over A$3,800 in parts of Australia and North America – reflecting differences in design, labour and materials. Annual operating costs varied from under A$1 to more than A$270 m².
Despite this variation, floating wetlands were shown to be competitive with traditional treatment systems, particularly for nitrogen removal. Across projects, the cost of removing nitrogen ranged from about A$10 to A$183 per kilogram, while phosphorus removal was more expensive, costing A$15 to nearly A$5,000 per kilogram.
Lead author, CSIRO Research Engineer and UniSA Adjunct Research Fellow Dr John Awad said: ‘Floating wetlands mimic the functions of natural ecosystems, filtering nutrients and pollutants through plant roots and microbial communities.
‘But until now, there’s been limited data on what they actually cost to build, maintain and operate at scale. Our study shows that while costs vary, floating wetlands can be competitive with other engineering treatment options, especially for nitrogen removal.’
Larger systems were found to be more efficient, lowering costs per unit of pollutant removed.
Dr Awad explained: ‘Scale matters. Larger wetlands [are] more economical over time. Climate also plays a role. Wetlands in warmer regions had longer growing seasons and higher pollutant removal rates.’
‘Understanding these variations helps governments and planners adapt the technology to local conditions.
‘It also shows that floating wetlands could be a particularly valuable option in low- and middle-income countries, where affordable, low-energy treatment systems are urgently needed.’
Study co-author, UniSA Professor Simon Beecham, points out that there are benefits beyond water quality: ‘Floating wetlands can be retrofitted into existing lakes and stormwater ponds without the need for costly land acquisition.
‘They also provide habitat for birds and aquatic life, improve urban amenity, and may even contribute to carbon sequestration.’
While not a silver bullet, the researchers argue that floating wetlands should be part of a broader toolkit for sustainable water management.
The full research can be read here.
Photo: CSIRO