Sewer heat recovery: unlocking urban energy potential

Urban areas face unique challenges in the energy transition: limited space for solar panels or solar thermal systems, shading from dense buildings, and strict preservation regulations often restrict the use of conventional renewable energy sources. Yet, a largely untapped resource lies directly beneath our feet-sewer heat recovery. More use cases for waste heat utilization can be found on the feature page waste heat recovery.

Why sewer heat recovery?

In cities, households, businesses, and industries continuously discharge warm to hot wastewater into the sewer system. This energy is usually lost, flowing unused to treatment plants. Modern sewer heat exchangers now make it possible to efficiently and economically tap into this energy source.

Sewer heat recovery is not only climate-friendly but also already competitive and officially recognized as a renewable energy source. With wastewater temperatures ranging from 10–12°C in winter to 17–20°C in summer, the sewer system provides an attractive and stable temperature level for heating and cooling via a wastewater heat pump.

The existing infrastructure is extensive, with hundreds of thousands of kilometers of sewer pipes. In Germany, for example, estimates suggest that up to 15% of the building sector’s heating demand could be supplied by sewer heat recovery-enough to provide heating for 4 to 12 million people.

Key components of a sewer heat recovery system

A typical sewer heat recovery system consists of three main components:

• Sewer heat exchanger: Extracts heat from the wastewater during winter for heating, or injects excess heat into the wastewater during summer for cooling.
• Wastewater heat pump: Brine-to-water heat pumps are especially suitable, as the brine heated by the sewer heat exchanger can be used directly as a heat source. These systems are highly efficient because wastewater temperatures remain relatively constant year-round.
• Connection and access: The distance between the sewer heat exchanger and the heat pump can be several hundred meters. This distance often determines the economic feasibility of a project.

Why is heat recovery in the sewer network often more efficient than at the treatment plant?

Heat can be recovered either directly from the sewer or at the outflow of the wastewater treatment plant. Both approaches have their advantages, but for decentralized supply in cities, using the sewer network is often more efficient:

• Proximity to heat consumers: Sewers usually run directly beneath or near the buildings to be supplied, minimizing heat losses and piping costs. Treatment plants are typically located on the outskirts, requiring longer transport routes.
• Higher and more stable temperatures: Wastewater in the sewer is generally warmer than after treatment and is constantly replenished. After heat extraction, the temperature recovers quickly, enabling efficient and repeated use within the heating network.
• Flexibility and scalability: Multiple smaller, decentralized systems can be installed in the sewer network, making it easy to adapt to local demand. After each extraction point, the wastewater is replenished and reheated by new inflows and ambient heat.

Sewer heat exchanger: function and installation types

Installing a sewer heat exchanger is a proven technology for waste water heat recovery, with some systems operating for over 15 years. The heat exchangers are integrated directly into the sewer and extract thermal energy without impairing wastewater flow. The recovered heat is then used via a wastewater heat pump for heating and hot water. This is an attractive solution, especially in densely populated areas where other renewable technologies reach their limits.

Installation types include:

• Retrofitted sewer heat exchangers: Installed in existing sewers, requiring coordination with network operators and authorities as they reduce the flow cross-section.
• Factory-integrated heat exchangers in new sewer pipes: Only possible for new builds or complete pipe replacements, with performance determined by proper sizing.

Where are sewer heat exchangers used?

Due to the higher volume of wastewater, sewer heat exchangers are mainly used in public sewer networks and less frequently in industrial sewers. Typical consumers include district heating networks producing district energy and buildings with varying usage profiles, such as commercial properties, schools, hospitals, residential complexes, and large multi-use buildings.

Larger projects are often implemented via energy contracting models, typically by municipal utility companies or regional energy suppliers. Smaller projects, such as single buildings or family homes, are usually self-managed.

Sewer heat recovery is used for heating, cooling, or both. Economically, wastewater heat pump systems are most viable for heating capacities from 50 kW up to 1 MW. For smaller installations, planning and approval costs can outweigh the benefits, though some small systems are already in operation.

Impact of temperature and wastewater flow

Wastewater temperatures fluctuate less than air but still show daily and seasonal variations, especially after rainfall events that cool the wastewater with larger volumes of mixed water. Increased flow during rain can negatively affect heat exchanger performance due to lower temperatures, but it also improves heat transfer and helps clean the exchanger surface.

Construction and installation of Uhrig sewer heat exchangers

The system is modular, with each module made of stainless steel plates or pipes with internal flow channels. A partial flow is directed through the modules, and the surface is bathed by the wastewater.

Modules can be rectangular or circular, depending on the sewer cross-section.

Supply and return lines, as well as monitoring conduits, are laid in trenches 0.8 to 1.2 meters deep.

Planning and simulation with Polysun

Planners can use Polysun simulation software to precisely design heat pump systems and district heating networks utilizing sewer heat exchangers. This provides contractors and utilities with a solid planning basis for projects. The sewer heat exchanger component in Polysun allows for detailed simulation, ensuring that return temperatures meet requirements and the optimal system is identified for the required heating and cooling loads.

Polysun offers templates for:

• Brine-to-water heat pump systems with buffer storage, hot water preparation, underfloor heating, and cooling using a sewer heat exchanger.
• Cold local heating networks use wastewater heat with cascaded heat pumps, where the first stage provides an initial temperature lift and a booster heat pump efficiently supplies 60°C hot water.

Conclusion

Sewer heat recovery is a forward-thinking solution for urban energy transition. With innovative sewer heat exchangers, intelligent simulation and planning, and proven systems, cities, and utilities can make a significant contribution to climate protection while securing their heat supply. Polysun simulation provides a reliable basis for subsidy applications and the development of funding schemes across Europe via a wastewater heat pump for heating and hot water. This is especially attractive in densely populated areas where other renewable technologies reach their limits.