Sewage heat recovery: wastewater as a circular heat source for sustainable cities
In Amsterdam, the pioneering Buikslotermeer project is showing how cities can unlock new forms of renewable energy from existing water infrastructure. From 2026 onwards, more than 600 social housing apartments will be heated with sustainable warmth sourced from wastewater. This is the first large-scale application of sewage heat recovery – or riothermia – in existing buildings in the Netherlands. A breakthrough in the transition away from natural gas and towards low-carbon urban heating.
Innovation in circular water technology
Riothermia is a prime example of circular water technology. Wastewater flowing through sewers contains residual heat ranging from 10 to 25 degrees Celsius, depending on the season. This heat is normally discharged unused. In the Buikslotermeer project, however, the heat is recovered through a heat exchanger and upgraded to the required temperature with collective heat pumps. The project uses a so-called very low temperature (ZLT) district heating network, which distributes the warmth to homes. This provides residents with heating and hot water without fossil fuels.
According to Harry de Brauw, project leader and energy transition adviser at Waternet:
“Project Buikslotermeer is the first in Amsterdam where riothermia is being applied. Internationally, it is a leading project, as heat is being recovered from untreated wastewater in a pressure pipe to warm existing housing – something that, to our knowledge, has never been done before.”
A best practice in public–private collaboration
The project is built on strong public–private cooperation. Housing corporation Lieven de Key, water utility Waternet, energy network specialist Firan, and the Municipality of Amsterdam each play a crucial role. Waternet constructs and manages the sewer heat exchanger. Firan develops and operates the heat network. Lieven de Key ensures that the buildings are technically ready for connection. Lastly, the municipality facilitates permits and contributes financially through subsidies.
“The success of this project comes from the determination and persistence of the partners,” explains de Brauw. “Back in 2018, De Key asked how their housing stock in Amsterdam could best be made gas-free. Waternet and Firan took up the challenge. Soon, it became clear that the municipal transport sewer offered the greatest opportunity.”
That was the moment the three partners committed to developing the concept together.
“We consistently maintained trust in one another, which is remarkable given the many challenges we faced,” notes de Brauw. “Developing a viable business case and arranging legal and organisational agreements proved particularly demanding. Here, the role of an independent project manager was crucial. By taking a neutral position above the partners, he helped to resolve difficult issues in a broadly supported way.”
Equally important was the involvement of residents. From the early stages, tenants acted as ambassadors and were closely engaged in decision-making. This resulted in strong community support – with more than 70 per cent of households approving the transition.
Affordability has also been a guiding principle. Households will not pay more for sustainable heat than they currently do for gas. Moreover, the heat price is decoupled from volatile gas markets, shielding residents from energy price shocks. This ensures that the transition remains fair and inclusive – a critical aspect when scaling up sustainable energy systems in urban areas.
Feasibility, impact and sustainability
Financial feasibility depends on scale: “Riothermia becomes viable in larger sewers that transport wastewater to treatment plants,” explains de Brauw. “These contain so much recoverable heat that many homes can be warmed, making it possible to cover the investment in the required equipment.”
He adds that timing is just as important: “The greatest opportunity arises when sewers are due for replacement. Installing heat recovery infrastructure at that moment only marginally increases costs, while the roadworks also create opportunities to lay a distribution network at limited expense.”
Linking technical feasibility with the right timing, projects like Buikslotermeer prove that riothermia can be scaled with tangible results. Once operational, the project will reduce CO₂ emissions by more than 500 tonnes annually. This directly contributes to national and municipal climate goals, while also validating technical solutions that can be replicated elsewhere.
The Netherlands Enterprise Agency (RVO) estimates that the Buikslotermeer project could pave the way for 100 to 150 similar initiatives – each covering around 2,000 homes. On a national scale, this would represent a substantial contribution to emission reduction targets and create a model for scaling up renewable heat technologies across the housing sector.
International relevance and inspiration
Internationally, riothermia has already been applied for decades. Switzerland, Germany, France and Norway implemented the first projects 30 years ago. However, the Buikslotermeer initiative is unique because it recovers heat from untreated wastewater in a pressure sewer pipe to warm existing housing – something not seen before.
“Its potential is significant,” stresses de Brauw. “Today, riothermia is mainly used to make swimming pools and new apartment blocks more sustainable, but with the Netherlands aiming for a gas-free built environment, existing neighbourhoods are now also being considered.”
As urban centres worldwide work to cut carbon emissions, increase resilience, and keep heating affordable, the Buikslotermeer project offers a practical model showing how existing water infrastructure can be transformed into a renewable resource for cities. By combining technical innovation, strong partnerships, and community involvement, riothermia delivers solutions that are both environmentally and socially sustainable.
