QualiSûre: nature-based solutions to protect the Sûre Lake's water quality

If you’ve ever walked around the Sûre Lake, you know how peaceful it looks: clear water, quiet bays, green hills. What many don’t realise is that this lake provides around 45% of Luxembourg’s drinking water. It powers a hydroelectric station, supports tourism and fish farms, and is one of the country’s most important natural assets.

But as populations grow and new types of contaminants—from pharmaceuticals to industrial chemicals—find their way into rivers, the lake faces increasing pressure. Protecting it has become both a scientific and political priority.

This is where QualiSûre comes in. Launched in 2024 as a three-year INTERREG project, it unites researchers, public administrations and water operators from Luxembourg, Wallonia and Rhein-Palatinate to create a long-term plan to preserve the lake’s water quality. At the University of Luxembourg, Dr. Silvia Venditti, leads the work on nature-based solutions, an approach that uses soil, plants and microorganisms to naturally remove contaminants and preserve the lake for the future.

From Emisûre to Qualisûre

The EmiSûre project (2017–2021) was one of the first in Europe to test nature-based solutions for removing pesticides and pharmaceuticals from wastewater treatment plant effluent. It generated essential knowledge, models and practical experience that QualiSûre now directly reuses.

The shift, however, is significant: instead of looking only at wastewater, QualiSûre examines all emission points within the Sûre basin. This broader approach is necessary because the lake is fed by tributaries with origins well beyond Luxembourg’s borders, as two-thirds of the catchment lie in Wallonia.

If emissions from upstream in Belgium aren’t addressed, local actions downstream won’t be enough. This makes the basin an ideal “living laboratory” for testing how to monitor, treat and prevent micropollutants in a shared, cross-border ecosystem.

The area also brings together all the challenges modern water management must deal with:

micropollutants from daily life (pharmaceuticals, industrial chemicals, additives),
nutrients like nitrogen and phosphorus that can cause summer blooms,
microorganisms relevant for potential agricultural reuse,
and ageing infrastructure struggling with heavier rain events.

Protecting the lake means working across borders, systems and disciplines; precisely what QualiSûre is designed to do.

Nature-based solutions: the role of soils and plants

Before designing solutions, the team identifies “emission hotspots”: places where pollution has the greatest impact. These often include wastewater treatment plants and combined sewer overflows that release untreated water during heavy rain events. Uni.lu and its partners analyse these sites to understand where additional treatment—or new nature-based infrastructure—would bring the most benefit.

Once researchers identify hotspots, they can test nature-based solutions in these areas. QualiSûre introduces a central innovation: locally produced soil filters. Water trickles through layers of soil and plants, and microorganisms living in the soil and roots filter, retain or biodegrade contaminants. ‘’Using local materials reduces transport impacts and aligns with INTERREG’s sustainability goals,’’ explains Silvia.

After laboratory tests using synthetic contaminated water, Silvia and her team selected the most promising materials. Teams are now constructing six pilot filters—three in Luxembourg and three in Wallonia—for one year of real-life testing.

‟ It’s not easy to imagine that a plant can be the treatment. But sometimes, the most effective solutions are the ones nature already offers.”

Dr. Silvia Venditti

An intelligent tool for water operators

To manage overflows and treatment capacity, QualiSûre is also developing an AI-powered decision-support tool. Designed by RTC4Water, it will help operators decide whether excess water should be diverted to natural filters, stored, or sent to treatment plants that still have capacity. Researchers are already running simulations, and they will refine the tool once pilot results arrive.

By the project’s end, QualiSûre aims to deliver the first joint cross-border Master Plan to protect the Sûre Lake—a strategic tool co-developed with administrations from both countries. The filters, models and monitoring methods developed here can serve as examples for other European regions facing similar challenges.

For communities, the benefits are concrete: cleaner water, more resilient systems, and greater transparency in how water is protected.

For Silvia, the motivation is both scientific and personal. After years of research, she hopes to see a Sûre where operators minimise untreated overflows and the lake regenerates naturally. “It already looks clean,” she says, “but in ten years, I want to see a system that we protect, that stays resilient and that we use at its best.”