The ANCHOR project explores how source-separating wastewater systems can help cities become more circular, resilient, and climate-neutral. During the ANCHOR lunch talk on September 25, 2025, the focus was on one often-overlooked opportunity within these systems: recovering heat from greywater. Drawing on research results and hands-on experiences from several ANCHOR demo sites, the session highlighted both the potential and the practical realities of greywater heat recovery across different scales.
Why Greywater Is a Valuable Heat Source
Greywater contains significant amounts of low-temperature heat originating from showers, washing machines, and sinks. Compared to air or groundwater, greywater has a higher and more stable temperature, which makes it an attractive source for renewable heating systems. Based on work conducted within the ANCHOR project, KWR estimates that greywater heat recovery could replace 6–15% of the annual heating demand of a typical Dutch household.
Ruben van den Berg (KWR) explained that heat recovery can be implemented at three different scales: building level, local (area) level, and central level. Each scale comes with its own trade-offs between temperature, continuity, maintenance needs, and spatial integration.
Building-Level Heat Recovery: Close to the Source
At building level, heat can be recovered passively, for example directly from shower drains or via heat exchangers on greywater pipes. The main advantage of these systems is that heat is captured right at the source, minimizing transport losses. Literature shows that up to 64% of the heat used for showers can be recovered in this way.
Measurements from the H+ demo site in Helsingborg showed greywater temperatures between 16 and 39°C inside buildings. However, ventilation of sewer pipes can cause cooling due to the so-called chimney effect, leading to lower temperatures further downstream. Practical experience indicates that vertical heat exchangers are preferable, as they are less prone to fouling and easier to maintain than horizontal systems.
Local Systems: Balancing Continuity and Losses
At a local or district scale, greywater is collected, treated, and then used as a heat source for heat pumps. While some heat is lost during transport, these systems benefit from more continuous heat availability and lower maintenance risks because heat recovery takes place after treatment.
In Helsingborg, greywater from the H+ district is treated at RecoLab, where heat is recovered after several treatment steps. Interestingly, temperatures after treatment were higher than at inflow, reaching around 22.9°C, due to heat generated during treatment. The recovered heat is used on-site to warm anaerobic digesters, and the installed heat pump has operated for four years without maintenance.
A similar approach is used in De Nieuwe Dokken in Ghent. There, treated greywater reaches temperatures of about 24.5°C, and heat recovery supplies the local district heating network. Measurements show that up to 15°C of heat can be extracted by the heat pump, covering up to 25% of the apartment heating demand—a higher share than average due to the relatively low energy demand of the buildings.
Central Recovery: Scale Versus Proximity
Heat recovery is also possible at centralized wastewater treatment plants, where large volumes of wastewater are available. However, transport losses reduce temperatures, and the recovered heat can only be used if there is a nearby heat demand.
Comparisons between the Superlocal demo site in Kerkrade and the central wastewater treatment plant showed that greywater at the local level was about 3°C warmer than at the treatment plant, underlining the importance of proximity when aiming for efficient heat recovery.
Lessons From Practice and Design
Reflections from the demo sites emphasized the value of recovering heat after treatment to reduce maintenance issues, a lesson learned from earlier projects that attempted to extract heat from raw wastewater. Experiences from Stockholm further showed that, with optimized system design and increased heat transfer area, up to 67% of domestic hot water demand could technically be recovered from greywater—although regulatory constraints may limit this in practice.
Key design considerations include:
- matching heat exchanger type to greywater quality
- insulating pipes to reduce transport losses
- minimizing chimney effects in sewer ventilation
- carefully balancing heat recovery potential with system constraints
What ANCHOR Takes Forward
The lunch talk confirmed that greywater is a valuable complementary heat source for renewable heating systems. Heat recovery can be applied at multiple scales, with local systems offering a strong balance between efficiency and operational reliability. Real-world experiences from H+ and De Nieuwe Dokken show that greywater heat recovery can already make a measurable contribution to urban heating demand today.
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