Phosphorus recycling from sewage sludge to close regional nutrient cycles


© DreiSATS
DreiSATS concept for decentralized thermal sewage sludge treatment.
© Fraunhofer IKTS
Hot gas filtration for the separation of heavy metal from sewage sludge. The nutrients contained in the ash are then converted into fertilizer.

With the Sewage Sludge Ordinance amended in 2017, phosphorus recovery from sewage sludge will be mandatory in the future. As early as 2023, sewage treatment plant operators must therefore demonstrate how they will comply with this recovery obligation. Therefore, it is necessary that new disposal strategies consider both sustainable and economical sewage sludge recycling and the production and marketing of approved phosphorus fertilizer products. Local sewage sludge utilization represents a promising approach. This allows regional nutrient cycles to be closed with greatly reduced transport costs, and conventional fertilizers to be adequately reduced.

In the joint project "DreiSATS", a process chain for decentralized phosphorus recycling from sewage sludge is currently being developed and demonstrated for the "central German tri-border area of Saxony-Anhalt, Thuringia and Saxony". The region is characterized by both dense urban and rural areas and is one of the most intensively farmed agricultural regions in Germany. Analyses show that the current phosphate demand in the "DreiSATS" model region can be covered to approx. 60 % with the regionally produced sewage sludge or the correspondingly processed fertilizers.

In process development for phosphorus recycling, a major focus is on process-integrated heavy metal separation during incineration. This is because the limit values for lead, copper, cadmium and nickel specified in the Fertilizer Ordinance are frequently exceeded. Fraunhofer IKTS is developing resistant ceramic hot gas filter systems to separate the ashes produced during incineration. The corrosive atmosphere created during the process, combined with temperatures of up to 1000 °C, requires the use of ceramic filters that are particularly resistant to temperature and chemicals. This allows heavy metals to be separated reliably and efficiently. In addition, there is extensive know-how on how to optimize the gas-phase conversion of heavy metals in sewage sludge by adding additives. This makes it possible for the first time to economically recycle sewage sludge thermally in much smaller incineration plants, thus enabling a decentralized recycling approach.

Another goal is to improve the phosphorus availability to the plant and to convert the nutrients contained in the ashes into standardized fertilizers for use in regional agriculture. To this end, Fraunhofer IKTS is developing methods for evaluating the intermediate and end products in terms of toxicity and their nutrient properties. Nutrient availability can be evaluated at IKTS with the help of plant test stands.

In addition, the researchers are engaged in stakeholder analysis and life cycle assessment, which considers all consumption of resources and energy as well as emissions and waste.

Project name: DreiSATS Technology Demonstration for the Combination of Dust Combustion and Acid Digestion Granulation with Integrated Heavy Metal Separation for Regional Phosphorus Recycling in the "Central German Tri-Border Area of Saxony-Anhalt, Thuringia and Saxony"

Funding: BMBF-FONA (funding reference: 02WPR1544)

Project period: July 2020 to June 2023 (Phase I)

Project partners: Veolia Klärschlammverwertung Deutschland GmbH, Carbotechnik GmbH, LTC Lufttechnik Crimmitschau GmbH, Pontes Pabuli GmbH, MFPA – Materials Research and Testing Institute at Bauhaus University Weimar