Aachen flushes heat out of the sewage canal

In Aachen, heat from sewage water is now being used to keep over 300 households warm during the winter. In the framework of the EU-GUGLE Smart Cities Initiative, an innovative district heating system based on sewage water heat recovery has been installed in the Northern district of the city. The system supplies the area and their inhabitants with approximately 800,000 kWh of low-emission heat each year.

Since the early 1990s, Aachen’s energy provider STAWAG has always been at the forefront of innovation for integrating renewable energy sources in the city’s electricity mix. STAWAG installed the first solar façade back in 1991, and since then has developed the production of various renewable energy sources in the region, including biogas, wind and hydropower.

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The idea of using waste heat from sewage water came when STAWAG experts realised that the temperature of the waste water in the canal throughout the year was between 12-15°C, and sometimes up to 20°C. In line with the city’s objectives in terms of renewable energy use, Aachen decided to tap the potential of sewage water and establish a local heating network to supply part of the EU-GUGLE pilot buildings in the Aachen North district.

To this end, a new energy plant with two 220kW brine-to-water heat pumps has been installed to capture the warmth of wastewater through a heat exchanger located in the canal, and to transfer it to different district areas via a local heat pump network. Of course, the process is operated as a closed-loop system, meaning that the dirty water never touches the clean water. To ensure process efficiency and sustainability, all the heat pumps are supplied with green electricity produced locally by STAWAG and the buildings are also equipped with exhaust air recovery systems.

Different solutions have been implemented in the district to fully exploit the capacities of this new system. In one location, four building blocks have been equipped with their own energy substation and combine heat recovered from waste water with heat from the buildings’ exhaust air through 12kW air-to-water heat pumps. To ensure continuous supply of heat and hot water throughout the day, dedicated thermal energy storage systems have also been installed. At another building in the district, heat from the sewage water heat recovery system is complemented by a natural gas boiler, in the absence of energy storage capacities. In this same district, 19 other buildings use both heat from sewage water through the local heat pump system and heat from the district heating network supplied by the city’s cogeneration plant.

It has taken three years to set up the system, as work to install the heat exchanger in the canal was only possible during the night. The efficient operation is possible because of the good condition of the canal, and sufficient year-round availability of heat. “Sewage heat recovery is a sustainable and cost-efficient energy source for the future, with high potential for primary energy reduction” says Rosa Hemmers, Consultant and Owner of the SynergyKomm Agency for Sustainability and Innovation. In Aachen, the total cost of the investment amounts to €780,000 but will enable the city to save 264 tonnes of CO2 emissions each year, while diversifying STAWAG’s renewable energy mix.


Photo: STAWAG.