Cleaning up polluted tertiary wastewater from the agri-food industry with floating wetlands

In 2018, IMT Atlantique researchers launched the FloWAT project, based on a hydroponic system of floating wetlands. It aims to reduce polluting emissions from treated wastewater into the discharge site.

Claire Gérente, researcher at IMT Atlantique, has been coordinating the FloWat¹ decontamination project, funded by the French National Agency for Research (ANR), since its creation. The main aim of the initiative is to provide complementary treatment for tertiary wastewater from the agri-food industry, using floating wetlands. Tertiary wastewater is effluent that undergoes a final phase in the water treatment process to eliminate residual pollutants. It is then drained into the discharge site, an aquatic ecosystem where treated wastewater is released.

These wetlands act as filters for particle and dissolved pollutants. They can easily be added to existing waste stabilization pond systems in order to further treat this water. One of this project’s objectives is to improve on conventional floating wetlands to increase phosphorus removal, or even collect it for reuse, thereby reducing the pressure on this non-renewable resource.

In this context, research is being conducted around the use of a particular material, cellular concrete, to allow phosphorus to be recovered. “Phosphorus removal is of great environmental interest, particularly as it reduces the eutrophication of natural water sources that are discharge sites for treated effluent,” states Gérente. Eutrophication is a process characterized by an increase in nitrogen and phosphorus concentration in water, leading to ecosystem disruption.

Floating wetlands: a nature-based solution

The floating wetland system involves covering an area of water, typically a pond, with plants placed on a floating bed, specifically sedges. The submerged roots act as filters, retaining the pollutants found in the water via various physical, chemical and biological processes. This mechanism is called phytopurification.

Floating wetlands are part of an approach known as nature-based solutions, whereby natural systems, less costly than conventional technologies, are implemented to respond to ecological challenges. To function efficiently, the most important thing is to “monitor that the plants are growing well, as they are the site of decontamination,” emphasizes Gérente.

In order to meet the project objectives, a pilot study was set up on an industrial abattoir and meat processing site. After being biologically treated, real agri-food effluent is discharged into four pilot ponds, three of which that are covered with floating wetlands of various sizes, and one that is uncovered, as a control. The experimental site is entirely automated and can be controlled remotely to facilitate supervision.

Performance monitoring is undertaken for the treatment of organic matter, nitrogen, phosphorus and suspended matter. As well as data on the incoming and outgoing water quality, physico-chemical parameters and climate data are constantly monitored. The outcome for pollutants in the different components of the treatment system will be identified by sampling and analysis of plants, sediment and phosphorus removal material.

These floating wetlands will be the first to be easy to dismantle and recycle, improved for phosphorus removal and even collection, as well as able to treat suspended matter, carbon pollution and nutrients.

Photograph of the experimental system

Improving compliance with regulation

In 1991, the French government established a limit on phosphorus levels to reduce water pollution, in order to preserve biodiversity and prevent algal bloom, which is when one or several algae species grow rapidly in an aquatic system.

The floating wetlands developed by IMT Atlantique researchers could allow these thresholds to be better respected, by improving capacities for water treatment. Furthermore, they are part of a circular economy approach, as beyond collecting phosphorus for reuse, the cellular concrete and polymers used as plant supports are recyclable or reusable.

Further reading on I’MTech: Circular economy, environmental assessment and environmental budgeting

To create these wetlands, you simply have to place the plants on the discharge ponds. This makes this technique cheap and easy to implement. However, while such systems integrate rather well into the landscape, they are not suitable for all environments. The climate in northern countries, for example, may slow down or impair how the plants function. Furthermore, results take longer to obtain with natural methods like floating wetlands than with conventional methods. Nearly 7000 French agri-food companies have been identified as potential users for these floating wetlands. Nevertheless, the FloWAT coordinator reminds us that “this project is a feasability study, our role is to evaluate the effectiveness of floating wetlands as a filtering system. We will have to wait until the project finishes in 2023 to find out if this promising treatment system is effective.”

Rémy Fauvel