More than 9,000 public sewage treatment plants in Germany ensure that wastewater is biologically purified, thereby minimizing pollution of local water bodies. However, valuable nutrients such as nitrogen and phosphorus are also lost during conventional treatment. In the RoKKa joint project coordinated by the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB and Umwelttechnik BW, ten partners from research and industry have tested several processes over the past three years in order to sustainably recycle raw materials such as phosphorus and nitrogen from wastewater and generate new products for the chemical industry and agriculture.

New modules for nutrient recovery from wastewater treatment plants

The project was implemented at the wastewater treatment plant of the town of Erbach (Danube). Here, several innovative processes were piloted and tested by the various partners. A total of seven demonstration plants were operated, which, according to the researchers, can also be integrated into existing sewage treatment plants as independent modules and are now available for testing by interested parties.

The project not only focused on the recovery of phosphorus and nitrogen compounds for fertilizer production. The CO₂ from digester gas, which is produced during sewage sludge digestion through the decomposition of organic waste and wastewater, was also to be made usable as a raw material for new products and nitrous oxide emissions at the sewage treatment plant were to be reduced.

With high-load digestion, the researchers at the Erbach wastewater treatment plant had a process at their disposal that can convert the sewage sludge produced into digester gas faster and more efficiently than conventional processes. In high-load digestion, organic substances from the wastewater are fermented to produce biogas as a renewable energy source. In the project, the sludge was dewatered and the plant nutrients phosphorus and nitrogen were filtered out of the sludge water.

Phosphorus and nitrogen fertilizer extracted

Instead of feeding the nutrient-rich filtrate into the aeration tanks of the sewage treatment plant as usual, the filter water was fed into a plant in which phosphorus and nitrogen are precipitated electrochemically. This produced magnesium ammonium phosphate, also known as struvite, which can be used as a phosphorus fertilizer in agriculture. Two membrane processes were used to extract nitrogen from the sludge water, which can also be used as a regional fertilizer in the form of ammonium sulphate. Using large-scale measurements, the researchers were also able to show that these processes reduce nitrous oxide emissions during biological nitrogen elimination.

Sludge water used for microalgae cultivation

In addition, the nutrients were fed with the sludge water to a novel photobioreactor system for microalgae cultivation, which was developed at the Fraunhofer IGB. In addition to light and CO₂, microalgae also need phosphorus and nitrogen to grow. As the researchers write, the algae produced plant-stimulating polysaccharides, known as beta-glucans, in the photobioreactor during their growth. According to the researchers, these substances can support plants in their defense against fungal infections such as mildew and replace chemical pesticides, for example in viticulture. The project also used an electrosynthesis plant to convert CO₂ into formic acid, thereby producing an important raw material for the chemical industry.

Contribution to a climate-friendly circular economy

As a result, the RoKKa project was able to demonstrate how new processes for nutrient recycling and the implementation of processes for nitrogen recovery can make climate-damaging nitrous oxide emissions from municipal wastewater treatment plants more sustainable and thus make an effective contribution to a climate-friendly circular economy. “Developed into biorefineries, wastewater treatment plants make valuable contributions to raw material security and climate protection and thus contribute to resilience and national climate and sustainability goals,” summarizes project manager Jürgen Schmidtke from Umwelttechnik BW.

The RoKKa project was one of a total of five projects funded by the Baden-Württemberg Ministry of the Environment, Climate Protection and the Energy Sector with funds from the European Regional Development Fund (ERDF) as part of the “Bioeconomy - Biorefineries for the Recovery of Raw Materials from Waste and Wastewater - Bio-Ab-Cycling” program between 2021 and 2024. 

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