A step closer to European objectives thanks to innovative wetlands
The Flanders Environment Agency (VMM) has taken an important step towards a climate-resilient water system within the Interreg Vlaanderen-Nederland project Aquatuur in completing the construction of a hybrid filter system. More specifically, a treatment wetland (with reed) was combined with an adsorption filter (with iron-coated sand). This nature-based solution is located at the confluence of the Ringbeek and Hertsbergebeek in Oostkamp. The wetland will not only improve water quality but also contribute to a resilient ecosystem.
Why a treatment wetland in this spot?
The Ringbeek does not meet the objectives of the Water Framework Directive and suffers from pollution peaks. High concentrations of nutrients such as nitrogen and phosphorus cause excessive algal blooms and turbid water during warmer periods. Moreover, they disrupt the balance of the ecosystem, causing fish and aquatic plants to disappear while the oxygen levels are controlled by the abundant, fast-growing algae.
Especially after long dry periods followed by intense rainfall (which occur more frequently due to climate change), combined sewer overflows and runoff from diffuse discharges lead to elevated nutrient concentrations, which in turn cause drops in oxygen levels in the watercourse.
The treatment wetland consists of two parts: the reed-bed filter, which removes suspended solids and nitrogen, and the iron-sand filter, which adsorbs phosphorus.
© Aquatuur en Sweco
Many benefits
A treatment wetland offers many advantages: it is cheaper than many grey solutions, almost energy-neutral, boosts biodiversity, provides water storage, enhances the experience of water in the landscape, and is fully integrated into its surroundings. In addition, the reed plants convert CO₂ from the air into biomass through photosynthesis. This specific treatment wetland focuses on stimulating the stream’s natural self-purification capacity.
You might see this wetland as a purely end-of-pipe measure, but nothing could be further from the truth. Even when collective and individual wastewater treatment (sewer systems + small wastewater treatment units) is fully developed in the future, the installation will remain relevant for tackling diffuse pollution. We are therefore working along two tracks: source-oriented measures and the treatment wetland as an effect-based measure.
Eliminated fish migration barrier
Fourteen fish passages were constructed on the Hertsbergebeek and Ringbeek. These ensure that fish can once again migrate upstream, as a key migration barrier has been removed. They also raise the minimum water levels, allowing the reed-bed and iron-sand filters to continue operating by gravity.
In addition to the fish passages, a winter bed was created to ensure that the frequency of flooding on adjacent parcels and the maximum water levels do not increase.
Smart control, higher efficiency
A network of sensors supports the installation. These sensors continuously measure parameters such as nitrate, oxygen and conductivity. A phosphate and ammonium sensor will soon be added. Based on this data, several mathematical models are being developed and tested, which we will validate against the system in operation. This will help us to better understand the various processes in the reed-bed filter and to optimise the system’s control. Such optimisation increases the filter’s purification capacity without expanding its surface area. In short: more efficiency on a smaller footprint!
A concrete example of this smart control: the timing of surface water intake into the reed-bed filter will be determined by the combination of sensors and an AI- and/or process-driven model. The aim is for the system to (i) predict when a ‘polluted pulse’ of surface water is approaching, (ii) take that water into the filter, and (iii) release it again downstream afterwards. This allows us to focus on capturing the highest pollution loads and to optimise the residence time, which is essential for reed-bed filters.
Working smarter together
The project is the result of close collaboration. Within VMM, the Digital Monitoring Department is responsible for installing and maintaining the sensors, while the Sampling and Water Quality Inventory Department carries out additional sampling. Ghent University then processes all the data, contributes to the modelling work and conducts research into the filter’s effectiveness. The first results were somewhat limited due to the dry summer, but they still provided valuable insights. For example, we observed clearly oxygen-depleted conditions after a rain event (first-flush effect). The nitrogen parameter showed a dynamic pattern during that flush, in contrast to phosphorus, which remained stable. The higher the pollution load, the more organic nitrogen (KjN) was detected. Some metals (such as cobalt, manganese, titanium and iron) also exhibited peak patterns.
The Core Management and Investments Team will use these insights to optimise the filter’s efficiency by steering on both water quality and water quantity.
Impact and future outlook
The reed-bed and iron-sand filter is a showcase project that can be replicated in other regions of Flanders. It contributes to the objectives of the Water Framework Directive and strengthens ecosystem services such as water purification, water storage and biodiversity. The lessons learned will be compiled into a practical guide, and VMM will continue monitoring even after the project ends in order to further improve the system’s performance. In the course of 2026, the project will also be presented to the wider public during a site visit.
You can find more information about Aquatuur and the project results here.
