Unique in Europe: A combination of sewer heat and geothermal energy heats and cools a residential neighborhood in Mechelen

The Mechelen Keerdok site now has Belgium's largest district heating network powered by wastewater

Three buildings containing offices, residential units, meeting rooms, and five commercial spaces at the Keerdok site in Mechelen are now heated and cooled via a district heating network that intelligently combines two sustainable energy sources : both heat and cooling from wastewater and the ground. This combination of wastewater heat and geothermal energy makes the system unique in Europe. The sustainable energy development company Noven implemented the heat network in collaboration with the Flemish wastewater management company Aquafin and project developers Kairos and Montreal. At the same time, the Keerdok site is the largest site in Belgium where wastewater is used as the primary heat source.

How does sewer heat recovery work?

Wastewater from showers, washing machines, dishwashers, and other household uses flows continuously through our sewer system at a fairly constant temperature of about 12 to 18 degrees Celsius. Even during the winter, that temperature remains surprisingly stable.

At the Keerdok site, that wastewater is collected in a large collector at a depth of about eight meters. A heat exchanger approximately 140 meters long extracts heat from it and transports it to the buildings via a heat transfer fluid. There, a heat pump adjusts the temperature to the desired level for heating or cooling. In this way, the energy that would normally be lost into the sewer system is put to maximum use.

“Sewage heat recovery is a simple technology that uses a particularly stable energy source,” says Maarten Raemdonck, New Business Manager at Aquafin.

“Wastewater is always present, and the basic infrastructure is already right beneath our feet. Aside from a heat pump and a few pipes, the system requires very little additional space. Moreover, with an average investment cost of between 700 and 1,000 euros per kW of capacity and high energy efficiency, sewer heat recovery is also an economically attractive solution.”

A unique combination of sewer heat recovery and geothermal energy

Although sewer heat recovery is becoming increasingly common internationally, its use in Belgium has remained limited until now. Aquafin uses it to heat and cool its own offices, and in Sint-Niklaas, the technology is being used for a swimming pool and sports complex. The Keerdok site, however, is the first Belgian heat network to use sewer heat recovery as its primary heat source on this scale.

The real innovation, however, lies in the combination with geothermal energy. Noven developed an energy system in which sewer heat and geothermal energy complement each other. For most of the year, the sewer water provides sufficient energy. Only when the temperature of the wastewater drops during severe winter periods do the BEO fields (Borehole Energy Storage) take over as a supplementary heat source.

“This combination of sewer heat recovery and geothermal energy on this scale is unique in Europe,” says Menno Janssens of Noven. “The BEO fields consist of boreholes drilled to a depth of approximately 150 meters and serve primarily as a backup during freezing temperatures. This allows us to guarantee residents and users a comfortable and sustainable heating and cooling supply at any time of the year.”

According to Noven, the project demonstrates how different renewable energy sources can reinforce each other within a collective heating network. To this end, the company brings together public and private partners and translates complex energy technologies into practical, feasible solutions for residential and urban development.

Fully climate-neutral

For developers Kairos and Montreal, the sustainable energy system is a key part of the vision behind the project.

Wim Straetmans, Managing Director of Kairos: “Here, we’re using 100% renewable energy from a waste stream that’s literally located beneath the square. Furthermore, by powering the heat pumps with locally generated solar energy, the system is completely CO₂-neutral. This demonstrates that sustainable urban development can go hand in hand with innovative energy technology today.”

 

Bart Langens, managing director of Mont-Real: “The residential neighborhood of the future is defined not only by its architecture, but also by the way it manages energy. By integrating this unique combination of sewer heat recovery and geothermal energy into the project from the very beginning, we are demonstrating that sustainable energy is not just an added feature, but a fundamental part of high-quality urban development.”

 

Residents and office users were able to experience firsthand this summer, during the first trial days, that the system works in practice. That was when they first got to experience the comfortable and energy-efficient cooling provided by the district heating network on hot summer days.