Waste not, recover more: Flanders leads the way in wastewater innovation

Flanders is pioneering wastewater innovation, transforming urban drainage into valuable bio-based fertilisers. However, obstacles such as economic viability and regulatory hurdles persist, warns Aquafin's Lennert Dockx.

Few hurdles in human civilisation have been as enduring as the need to manage urban wastewater. From medieval streets to modern metropolises, city drainage has always carried a toxic mix of chemicals and nutrients. If untreated and released into the environment, it poses a threat to public health—as well as to our rivers, lakes, and shorelines. Historically, the goal has been simple: dispose of wastewater safely and decontaminate as much as technology allows. But today, the conversation has shifted. Instead of just managing wastewater, scientists and engineers are looking at how to turn the waste contained in the drainage into a resource, most notably bio-based fertilisers. Aquafin, a company involved in the WALNUT project and responsible for wastewater treatment in Flanders, Belgium, is one of the institutions exploring this new approach.

The company's demo site is located at the Aartselaar urban wastewater treatment plant, about 50 kilometres from Brussels. This facility includes a dedicated research hall for testing new technologies. Leading these efforts is Lennert Dockx, one of the lead R&D engineers at Aquafin, with nearly four years of experience at the company. Before that, he worked as a researcher at the University of Antwerp, specialising in biochemical engineering and sludge treatment. At Aquafin, Lennert focuses on resource recovery, particularly nitrogen and phosphorus recovery from wastewater and fly ashes resulting from sludge incineration. He also works on the inventory of greenhouse gas emissions and mitigation strategies.

Can you tell us something about Aquafin?

Aquafin is the company in charge of wastewater treatment in Flanders, Belgium. The Flemish government set it up in 1990 to design, finance, build, and operate the region's wastewater treatment infrastructure. Today, it runs over 300 treatment plants and handles the entire inter-municipal wastewater network.

Why was the Aartselaar site chosen to test these new technologies?

We picked Aartselaar mainly because it's a tough environment for testing. The sewer system there gets a lot of dilution from rainwater runoff, especially during heavy rainfall, which means that the wastewater is more diluted. This makes it harder to recover nutrients like nitrogen because they're spread out in larger volumes of water. But that's actually a good thing—it gives us a "worst-case scenario" to really test how well the recovery methods work. If they work here, they'll be more likely to work in other, less complex environments too.

What are the leading technologies used on this demo site?

One of the main technologies being tested uses a two-step process to get nitrogen out of wastewater. First, it uses high-rate activated sludge, where specialised bacteria can break down organic matter and remove suspended solids. Then, the water goes through filters filled with natural adsorbents, like zeolites, that trap nitrogen by swapping them with harmless ions. This method is being tested alongside more traditional techniques like stripping and scrubbing to see which works best. If it succeeds, it could really boost how efficiently nitrogen is recovered.

Could this technology be widely implemented once the testing is successful?

Resource recovery, especially nitrogen recovery in sidestream treatments, is definitely doable. The main challenges are setting up a reliable business case and navigating the administrative complexities, which make it rather challenging to implement on a larger scale. That said, Aquafin's focus isn't just on producing bio-based fertilisers from recycled nutrients, but also on making wastewater treatment more sustainable. For instance, recovering nitrogen means less of it ends up in the main wastewater treatment process, so the system doesn't need as much aeration. That cuts down on energy use. So, nitrogen recovery can help cut nitrous oxide emissions by creating better treatment conditions in the main wastewater process. This fits with the new European Urban Wastewater Treatment Directive, which pushes for more energy-efficient and climate-friendly wastewater treatment.

Any other way Aquafin is helping achieve energy neutrality?

Well, for example, we're cutting down on energy use and shifting to renewable sources like solar and wind. We're also phasing out the company's fossil fuel-powered service cars and using heat from nearby industries. Life cycle assessments guide us in making sustainable choices, even when they're not the cheapest.

Would you say wastewater's perception has changed over time?

For sure! It used to be seen as something of a useless product, but now it's considered a resource. For example, Aquafin produces biomethane as part of sludge treatment and is part of a research project where CO₂ is converted into acetic acid. The company will also invest in phosphorus recovery. But it doesn't end there. We also recover heat from sewer systems. It's about always aiming for more sustainable solutions.

Has the project in Aartselaar engaged with the local communities?

Not directly, but Ghent University, which is a partner in the WalNUT project, is doing some field trials where farmers can see firsthand how bio-based fertilisers work. Also, Aquafin regularly hosts industry and academic visits at Aartselaar, helping raise awareness about the potential of wastewater treatment for resource recovery.

Are micropollutants a concern in bio-based fertilisers from wastewater?

Some micropollutants do end up in fertilisers or activated sludge, but most just stay in the wastewater and eventually get discharged into rivers and streams—just like in conventional treatment systems. That's why full-scale treatment plants need extra steps, like quaternary treatment, to remove them. This is where advanced methods like ozone treatment or activated carbon come in. That said, the WalNUT project isn't about eliminating micropollutants—it's all about recovering nutrients from wastewater. But the issue is important to us.

Are EU institutions addressing the issue of micropollutants in wastewater treatment?

Definitely. The new EU Urban Wastewater Treatment Directive also aims to remove at least 80% of key micropollutants from wastewater before it is discharged into the environment. The aforementioned quaternary treatment was also introduced under the same EU legislation.

What are the future perspectives of the project here in Belgium?

Looking ahead, if we want to move forward with things like nitrogen recovery, we'll need to focus on sidestream treatment rather than mainstream treatment since it's more realistic, given current tech and market conditions. As for implementing more energy-neutral wastewater treatment, we will use life-cycle assessments and virtual CO₂ taxes to make sure we're prioritising long-term environmental benefits over short-term costs—even though sustainable options can be pricier.

Contacts:

Project coordinator:

Francisco Corona Encinas, Ph.D. FUNDACIÓN CARTIF—fraenc@cartif.es

Communication Manager:

Francesco Agresta, ICONS—info@walnutproject.eu

Project website: https://walnutproject.eu/

LinkedIn: WALNUT

Twitter: @walnut_project

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