After observing the benefits of natural floating islands, SPEL developed a product to mimic this natural phenomenon. Now their floating treatment wetlands (FTWs) are enhancing the environment in numerous ways.
By now the benefits of floating islands are well-established. They deliver a range of benefits including:
- Removing nutrients from catchment water flows.
- Reducing the land take compared with constructed wetlands and conventional treatment such as bioretention.
- Reducing pollutant loads in water bodies that have high background concentrations of pollutants.
- Creating new habitat to act as safe refuge for local wildlife, including nesting birds and amphibians as well as providing a habitat below the water that can shelter fish and frogs from larger predators.
“Floating wetlands can do everything from treating water to creating an ecosystem that helps to keep habitats sustainable and thriving,” says Sam Kowald, Senior Environmental Scientist at SPEL Stormwater.
“Below the water surface, the root structure of the floating wetlands becomes a habitat for micro and macro-organisms, and above the surface there is an ecosystem that supports various creatures.”
Floating islands form in nature in one of two ways. Plants growing from the bank over the water’s surface may become dislodged during a storm, or thick layers of floating plants may clump together and start to accumulate soil and biomass.
Comparison to constructed wetlands
Constructed wetlands use three main removal pathways to treat nutrients. These are: attached nutrients settling out in the sediment, nutrient uptake by plants, and microbiological processes. Microbial action includes nitrification in the biofilm, which releases nutrients from the water column as a gas.
Floating wetlands enhances each of the three removal pathways. The exposed roots in the water column provide surface area for biofilm growth and increase sedimentation. The floating nature of the wetland structure also encourages strong plant growth by providing optimal growing conditions.
Consequently, floating treatment wetlands reduce the footprint of land required to meet water quality objectives, by up to 60 per cent when compared to traditional constructed wetlands. This improves land useability for the developer, whilst providing more open water and potential increases to land value.
Lessons from floating islands
Through closely observing these natural systems, SPEL Stormwater has created floating treatment wetlands that enhance the benefits of traditional wetlands.
To create floating treatment wetlands, SPEL Stormwater installs a modular floating structure that replicates a natural floating wetland. This allows SPEL Stormwater’s engineers to place the wetland in an ideal location, before planting selected wetland species in a stable position in the water column. As these plants grow, their roots are exposed to the water which increases biofilm formation compared to constructed islands.
Biofilm supercharges the treatment of the water flows. When sediment in the water passes through the root mass, it is captured by the biofilm. As it gets heavier, the sediment falls off and settles to the bottom of the water body, providing better removal of TSS (Total Suspended Solids) than a constructed wetland relying on gravity and attenuation times alone.
In many cases, the biofilm transforms the nitrogen into a gaseous form that can then escape the water column. This nitrogen can then either assist with floatation or be released into the atmosphere.
“The micro-organisms in the biofilm help to drive nitrification. That creates nitrogen which is either absorbed by the plants to assist with floatation or is released into the atmosphere,” says Kowald.
“Removing nutrients is important as their presence can lead to algae blooms when nutrient levels are too high. Floating wetlands enhance the removal pathways for nutrients.”
Beyond creating pathways for nutrients and TSS to leave the wetland, research currently being conducted is exploring the potential of floating treatment wetlands to deliver sustainability benefits by reducing the amount of methane released into the atmosphere. This results from rotting leaves in lakes, and the CSIRO is researching the potential for PFAS removal and looking at the performance of particular plant species.
Building a floating treatment wetland
Each floating wetland design is unique, and SPEL Stormwater tailors site-specific stormwater solutions based on its client’s requirements.
“We look at the balance of pollutants and nutrients, and assess whether they are well balanced for effective removal, or if we need to take additional steps prior to constructing a floating wetland to tip that balance. Often, we need to move the balance from ammonia towards nitrate. We would also aerate the water in applications that have a higher ratio of ammonia within the total nitrogen (TN).” says Kowald.
To construct a floating treatment wetland, SPEL runs an assessment to determine the most suitable locally available plants. Local guidelines, clients and authorities will typically provide a list of approved plants.
“From this list of plants, we will select the ones that are most suited for the project, considering factors such as salt tolerance, and whether they can be permanently in water,” says Kowald.
“It’s a collaborative process to develop a really effective floating treatment wetland.”
SPEL Stormwater’s water quality data is backed by independent local and international research, and their device performance is peer-reviewed by industry experts.
Installed using anchor lines connected to a block or Platypus anchor, floating wetlands are a flexible solution that can be detached and moved as required. This allows for simple and cost-effective maintenance.
To learn more about floating treatment wetlands, register for Engineers Australia’s Thought Leaders webinar in partnership with SPEL.