Sustainable fertiliser made out of sewage sludge ash
Freiburg, 19/12/2025
An interdisciplinary research team led by the University of Freiburg is investigating the effectiveness and acceptance of phosphorus recycling fertilisers. The aim is to strengthen organic farming. The project is funded by the Baden-Württemberg Ministry of Science, Research and Arts (MWK).
How can the plant nutrient phosphorus be recovered from sewage sludge ash and used sustainably in regional organic farming? This is being analysed by the research project ‘PHÖNIX – Phosphorus recycling fertilisers in organic farming: Sustainable integration and circular economy with P-XTRACT’. The project is led by Dr Peter Hajek, plant ecologist at the Institute of Biology II/III, and Prof. Philipp Kurz from the Institute of Inorganic and Analytical Chemistry at the University of Freiburg. The Freiburg Materials Research Centre (FMF) is responsible for project administration and special investigation methods.
Scientists from the University of Hohenheim and the Augustenberg Agricultural Technology Centre (LTZ) are also involved, as is the joint wastewater association Staufener Bucht, which has been operating the P-XTRACT plant for the incineration of sewage sludge at its Breisach-Grezhausen site near Freiburg since 2024. The project is being funded over three years with a total of around €500,000 from the Baden-Württemberg Ministry of Science, Research and Arts (MWK). The funds come from the research programme “Mit Innovationen den Ökologischen Landbau gemeinsam stärken“ (“Strengthening organic farming together through innovation”), with which the MWK aims to support sustainable and practical solutions.
“We are directly involving organic farmers in our project and making a direct contribution to the circular economy and our independence from raw materials.“
Dr Peter Hajek
Plant Ecologist at the Institute of Biology II/III, University of Freiburg
Impact and acceptance
The aim of the project is to investigate the fertilising effect, environmental impact and social acceptance of recycled phosphorus fertilisers – known as P-recyclates – in the context of organic farming and to test legally compliant applications. To this end, innovative processes for the recovery and regional recycling of phosphorus from sewage sludge ash are being analysed in the laboratory and also in field trials. In addition, detailed surveys are being conducted. “We are directly involving organic farmers in our project and making a direct contribution to the circular economy and our independence from raw materials,” says Hajek.
Dr Peter Hajek researches the effects of global change on ecosystems. He focuses on the complex interactions between soil, plants and the environment, with a particular emphasis on plant nutrition, phosphorus recycling and circular economy. Prof. Dr Philipp Kurz conducts research in the field of bioinorganic chemistry and, from 2020 to 2024, headed the scientific support for the construction of the P-XTRACT® plant, which was funded by state and EU funds.