A DISCONNECTED BASILICA FOR
SEWAGE-FREE WATERWAYS

An icon leading the way for the rest of the city

Any idea how big the roof of the Basilica of Koekelberg is? It’s big, very big. So when it rains, a massive amount of water falls on it. And all that rainwater? It flows straight into the sewer system. Lost. Meanwhile, every summer, the grass around the Basilica turns golden and dry due to a lack of water. But more importantly, Brussels is dealing with too much rainwater in the sewers. Combined sewer overflows pollute the city’s waterways up to six times a month.

Fortunately, there is a solution, one that leads to fewer overflows, less flooding, more infiltration into the soil, more quality outdoor spaces and a reduction in the use of precious tap water: disconnecting the roofs from the sewer system. Disconnecting the roof of an iconic building such as the Basilica of Koekelberg, could serve as a long-term example for the rest of the city.

With this project, we want to show that it’s not just new or renovated buildings that need to be disconnected, existing roofs must be too. Only then can Brussels become a resilient and playful sponge city, with clean and healthy waterways.

Why do we deed to disconnect existing buildings?

Brussels historically has a one-pipe system, which means that clean rainwater, that could nourish the ground or feed urban ecosystems, gets mixed up with wastewater. The result? Nearly 10 million m³ of sewage water is discharged into our rivers and canals each year. On top of that, climate change is intensifying rain events and floods become more frequent. While at the same time, the city is sealing more of its surface with paved streets, roofs, and parking lots, leaving the water nowhere to go but the sewers. All this results in unhealthy waterways where these could on the contrary be places to turn to for refreshment in hot summers, recreation all year round and a connection with biodiversity in the middle of the city.

Growing surface impermeability

Over the past 50 years, the share of impervious surfaces in the Brussels Region has drastically increased (from 26% to 53,2%). As a result, every rainfall puts pressure on our sewer system and water treatment plants. The system wasn’t built for this much runoff, and it’s failing under the weight, literally flushing polluted water directly into our waterways.

New buildings must comply with stricter stormwater regulations, but Brussels only renews itself at about 2% per year. We can’t afford to wait. If we want meaningful change in this decade, we need to act on what’s already here: existing buildings, infrastructure, and public spaces. Only then will we be able to reduce the amount of sewage overflows to almost zero. Paris achieved that goal last year and now it’s possible to swim again in the river Seine after a ban of more than 100 years.

Maps from Brussels Environment

The Koekelberg Basilica: A beacon for change

What better place to start than one of the most iconic landmarks in Brussels? The Basilica of Koekelberg! The building is not only monumental in scale, but also surrounded by a vast open park making it a perfect site to rethink rainwater management in a bold, visible way.

Working with Kollektif Landscape, we reimagined the basilica and its surroundings as a place where water, community, and heritage meet. A public space that celebrates rain instead of hiding it. This is what an iconic rainwater disconnection looks like:

Visuals by Kollektif Lanscape

When it rains:
A landscape that lives

Imagine standing inside the Basilica as the rain starts pooring from the sky. Instead of vanishing into underground pipes, rainwater cascades gently into a visible and functional landscape.

A rainwater tank stores water for the community garden.
A rain garden filters and slows runoff.
An amphitheater becomes a temporary basin during storms, storing water before it soaks into the ground.
An infiltration gully winds through the park, creating micro-ecosystems.
A water playground turns rainfall into joy, with kids playing in gentle flows designed for fun and learning.

Rain is no longer hidden. It’s present, celebrated, and useful.

When the sun shines:
A community space emerges

On dry days, the space transforms into a vibrant social landscape:

The amphitheater becomes a stage for gatherings and storytelling.
The community garden thrives with harvested rainwater.
The playground encourages connection through water-themed play.
The entire area becomes a destination, not a transit zone: a place to meet, linger, and learn about the building and its role in the future of the city.

Today, this space is a flat, underused lawn. Why not turn it into a living park, where nature, people, and history coexist?

How does this disconnection translate into numbers?

The roof of the Koekelberg Basilica covers an impressive 7,800 m² (we didn’t take into account the large impermeable surface around the basilica). To fully disconnect the roof from the sewer system, the infiltration infrastructure must be able to handle a storm with a 100-year return period which is a significant challenge.

Luckily, 25,000 m² of open grassland surrounds the Basilica. According to the Brussels stormwater calculator, only 1,600 m² of infiltration area is needed to manage the runoff and that’s just 6% of the available green space. So if we remove the areas with the tunnels underneath the grass there is still much more space than the necessary 1,600 m2.

In other words: this seemingly massive disconnection is very feasible!

Brussels map by Robert Littleford, April 2016 issue

From one icon to many: A citywide vision

Koekelberg could just be the beginning. What if we disconnected other landmarks in Brussels?

  • The Royal Palace

  • The Brussels Courthouse

  • Our many churches and public squares

  • And the roofs of private houses 

All these roofs form a vast surface with the potential to capture, store, reuse and infiltrate water. By starting with the iconic, we can inspire the others. While for us clean and healthy waterways are the main goal, disconnecting rainwater from the sewers also leads to climate adaptation and could position Brussels as a European leader in sustainable urbanism.

Let’s make it happen

Such a project needs support: from citizens, local and regional governments, and from anyone who believes Brussels can lead the way. Disconnecting the basilica of Koekelberg is more than a technical fix. It’s a story. A symbol. A shift. Let’s make it real. And let us know if you support the idea.

A beautifull example - Copenhagen

“It was difficult, but it has been done”, with these words Lykke Leonardsen, Head Director Resilient and Sustainable City Solutions at the city of Copenhagen, describes how they developed the Cloudburst Management Plan together with all actors and citizens. It became an ambitious 1,6 billion plan for urban rainwater management and it was initiated after a huge flood of the city in 2011. 350 different projects throughout the whole city will protect the city against floods in the future. Several parcs become rain parcs, streets become rain streets and at the end of the line some tunnels will lead the water underneath the city center into the harbor. The beautiful part of the plan is that Copenhagen uses it as an opportunity to create new quality public spaces for the citizens on top of the extra green areas. In this video you can see Lykke present the plan at our conference in 2023.

Did you know?

The parc around the basilica was once a huge vegetable garden!