Transforming Wastewater: Towards Climate-Neutral Sewage Treatment

“The Teaching and Research Sewage Treatment Plant at the University of Stuttgart has an important objective: To extract valuable raw materials from wastewater and reintroduce them into the economic cycle. In this way, the organic carbon in the wastewater can be used sustainably. The research sewage treatment plant is also focused on reducing the high emissions of climate-damaging gases typically produced by conventional sewage treatment plants,” said Minister President Winfried Kretschmann. “But the innovation here is not just happening at a high level. The sewage treatment plant is also a training center for wastewater treatment plant personnel and supports companies in the development of plant technology. In this way, the sewage treatment plant facilitates the crucial transfer from science to practical application.”

Unlocking Value: Turning Wastewater into Resources

Our wastewater not only contains dirt and excrement, but also valuable raw materials such as nitrogen and organic carbon compounds. With the help of chemical, biological and physical processes, these raw materials can be recovered from wastewater to make products such as fertilizer, hydrogen and bioplastics. The KoalAplan project (“Municipal wastewater as a source of ammonium nitrogen, hydrogen and bioplastics – the Büsnau biorefinery”) has been researching this process since 2021.

Scientists from the DVGW Research Center at the Engler-Bunte Institute of the Karlsruhe Institute of Technology (KIT), the University of Stuttgart, the Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB), the Technical University of Hamburg, and the Clausthal University of Technology are working on the KoalAplan project together with the State Agency for Environmental Technology and Resource Efficiency in Baden-Württemberg (Umwelttechnik BW). The KoalAplan is funded by the Baden-Württemberg Ministry for the Environment, Climate Protection and the Energy Sector and the European Regional Development Fund (ERDF). It is part of the state strategy “Sustainable Bioeconomy for Baden-Württemberg”.

At the Büsnau teaching and research sewage treatment plant at the University of Stuttgart, researchers are testing under real conditions how the recovery of raw materials in sewage treatment plants can be realized on an industrial scale. A biorefinery was set up as a pilot plant for this purpose, which has been operating successfully since 2023. During his visit on June 6, Baden-Württemberg’s Minister President Winfried Kretschmann was briefed on the project’s findings to date.

Advanced Treatment Processes: Transforming Wastewater into Sustainable Resources

Peter Maurer, Head of the Teaching and Research Sewage Plant at the University of Stuttgart, and Prof. Dr. Harald Horn, Coordinator of the KoalAplan project and Professor of Water and Water Technology at the Karlsruhe Institute of Technology (KIT), gave Minister President Winfried Kretschmann and a group of media representatives a tour of the biorefinery and explained the process used. Normally, carbon dioxide is produced from the organic carbon when it passes through a sewage treatment plant.

“Our processes reduce the production of this climate-damaging gas. At the same time, we obtain raw materials that help to replace petroleum-based substances. With this two-pronged strategy, the sewage treatment plants of the future could make an important contribution to climate neutrality,” says Prof. Dr. Harald Horn. The wastewater treatment concept applied by the project group consists of chemical, physical and biological process steps. As part of the preliminary treatment, mechanical cleaning takes place first. Coarse matter is removed, and it is here that a third of the organic carbon is separated. A further third of the carbon is then separated from the main wastewater flow with the aid of ultra-fine micro-sieves.

“The microsieves are a key part of our concept. The carbon obtained following this process is so highly concentrated that it can be used in biotechnology,” says Professor Horn.

Advanced Treatment Processes: From Fertilizer to Bioplastic

Subsequently, ammonium nitrogen is removed from the main wastewater stream using an ion exchanger. The result is a product that can be used as a fertilizer. The filtered solids and the primary sludge are first converted into organic acids by acid hydrolysis in the side-stream process, which also produces biohydrogen and CO2. The hydrolysate is filtered and converted to hydrogen using microbial electrolysis.

Hydrogen has many applications in the chemical industry and is regarded as a future energy source. In a feasibility study, gas flows from microbial electrolysis and dark fermentation are utilized in a biotechnological process to produce valuable chemicals. Additionally, the carbon dioxide generated is re-fixed during this process. The organic carbon remaining in the process of the microbial electrolysis cell is then fermented to form a basic material for bioplastics.

Finally, the main stream wastewater follows the conventional treatment path: Remaining nitrogen and carbon are broken down in several stages, and any accumulating sludge ends up in the secondary clarifier, where methane is extracted.

Pioneering Research and Education: Advancing Wastewater Treatment at Büsnau

“Since last year, our biorefinery has demonstrated through continuous operation that the process chain is effective and could therefore also be implemented in other sewage treatment plants,” says Peter Maurer, head of the research sewage treatment plant. “We hope that we can use the pilot project to sensitize other companies to the potential of bioeconomic strategies. Our example shows that you can reduce the consumption of fossil raw materials and even save costs and market new products as a result.”

At the Büsnau Teaching and Research Sewage Treatment Plant, students, doctoral students and research groups have been researching and experimenting with innovative processes for wastewater treatment on a semi-industrial and industrial scale since 1965. The teaching and research sewage treatment plant is affiliated to the Institute for Sanitary Engineering, Water Quality and Solid Waste Management at the University of Stuttgart. It is not used exclusively for research, but also treats the wastewater of 8,500 inhabitants of a Stuttgart district. Part of the main flow of the municipal wastewater is fed through the project’s KoalAplan pilot plant.

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