District heating from sewage gas

24.04.2026

Source: E & M powernews

The "Alte Weberei" in Tübingen was built on the site of the former terry towel manufacturer Egeria. Today, the district is supplied with heat from the Tübingen sewage treatment plant.

The "Alte Weberei" is a new development on the site of the former Egeria company in the south of the Lustnau district, between the Neckar and Ammer estuary. From the 1920s, Egeria produced terry towelling there and was at times the largest industrial employer in the area. The company ceased operations in 1990. The site lay fallow for many years until the city of Tübingen decided to develop it into a residential and working quarter in 2009. Today, around 800 people live and work there.

Due to its proximity to the sewage treatment plant, the district uses surplus heat from sewage gas power generation all year round. Surplus renewable heat from sewage gas cogeneration plants is fed into the district heating network of Stadtwerke Tübingen. To further increase efficiency, a third CHP plant and an associated heat pump were installed last year.

Sewage treatment plants are suitable locations for district heating centers, as they can provide both waste heat from sewage gas power generation and purified wastewater as a potential heat source for heat pumps.

Extended use of sewage gas

A new CHP module has been at the heart of the plant since November 2025. The Jenbacher engine comes from the INNIO Group and achieves an electrical output of around 470 kW and a thermal output of 550 kW. Three exhaust gas heat exchangers were connected in series to the CHP module. The components utilize the heat contained in the exhaust gas in several stages. The first heat exchanger operates in the high-temperature range. Another heat exchanger uses the heating water return and the third uses the source circuit of the heat pump. According to the CHP supplier Energas, the Jenbacher engine has an operating time of around 7,000 hours per year.

Two older compact modules from the manufacturer Senergie, each with around 170 kW electrical and 200 kW thermal, are available as reserve and off-peak machines. They achieve around 1,500 operating hours per year. These modules were completely overhauled in 2024. They operate in the sewage gas low load and in the event of a malfunction or maintenance of the CHP unit. These two modules were also each equipped with an exhaust gas condensing heat exchanger with around 25 kW.

The sewage gas volume is around 7.9 million kWh per year. The heat generated by the sewage treatment plant is around 5.2 million kWh per year. The annual heat consumption of the sewage treatment plant is around 2.8 million. The operator feeds any surplus into the district heating network of Stadtwerke Tübingen. The electricity consumption of the Tübingen sewage treatment plant is around 3.4 million kWh, around two thirds of which is generated in the plant's own facilities.

To enable efficient operation of the heat pump, the heat supply to the sewage treatment plant was switched to low-temperature operation. The heating water flow temperature is set to a maximum of 65 degrees Celsius on a weather-controlled basis. By cascading several heating circuits, the return temperature was reduced to around 45 degrees Celius. This favors the use of the calorific value and improves the extraction from the exhaust gas.

The high-temperature heat of around 90 degrees Celius is primarily used to feed into the "Alte Weberei" district heating network. The low-temperature heat is mainly used for process heat to heat the digestion towers (around 60 degrees Celsius). It is also used to heat rooms and domestic hot water for the plant buildings.
At the sewage treatment plant, electricity and heat are generated from digester gas, which is produced during anaerobic sludge stabilization. To do this, the operators pump primary sludge from the primary treatment stage and excess sludge from the biological stage into closed digestion towers. Under anaerobic conditions, microorganisms break down the organic substances. This produces methane-rich digester gas. After intermediate storage in a gas tank, it is fed to the CHP modules as required and used there to generate energy.

Heat pump raises residual heat to a usable level

The Jenbacher gas engine is already highly efficient in itself thanks to four-valve technology, compression specially adapted to sewage gas and mixture cooling integrated on the heating water side. In addition, the planning office, IBS Ingenieurgesellschaft mbH, has further optimized the heat extraction by installing a heat pump for additional use of the low-temperature heat sources: The heat pump uses the low-temperature heat from the cooled exhaust gas and from the engine room cooling.

On the source side, it operates at 25/20 degrees Celius, on the sink side at 55/65 degrees Celius. The heat output is around 190 kW and the COP is around 4. The heat pump for the CHP unit only makes economic sense because the additional heat can be used all year round. Stadtwerke Tübingen initiated the heat pump. The heat pump primarily supplies the sewage treatment plant's low-temperature consumers.

There is a maintenance contract with Energas BHKW GmbH for the new combined heat and power module. The contract covers a term of 80,000 operating hours. There are separate service agreements for the two older CHP modules. Maintenance is carried out by Schick Motorenservice. Maintenance of the heat pump is the responsibility of the system supplier during the warranty period. Alois Müller GmbH is responsible for this.

A 20 cubic meter heat storage tank serves as a control storage tank. The operators do not need a large-volume storage tank, as the grid continuously absorbs the heat generated.

Prospects for the grid and generation

Stadtwerke Tübingen has expanded the "Alte Weberei" network in 2025 to include the neighboring Queck site. They are also planning an interconnection with other grids, initially with Südstadt.

The players are also planning a photovoltaic system on the sewage treatment plant site. A large heat pump that uses treated wastewater as a heat source is planned for 2030. The amount of sewage gas limits the additional heat generation from the existing CHP units. Further feed-in potential will therefore arise primarily from new heat pump capacities.

Author: Heidi Roider