Sewage sludge is generally perceived more as a problematic residue or waste rather than a valuable raw material. Previous methods of disposal have included agriculture and combustion in power stations or monocombustion plants, which use only sewage sludge as fuel. But a closer examination of the components of sewage sludge reveals that it contains valuable raw materials. In addition to carbon, which is present in almost all objects in our everyday lives, sewage sludge also contains significant proportions of phosphorus – up to 5 percent by weight in dry state.

Phosphorus is essential for cell metabolism, and is therefore an essential primary material for all life. It finds its way into sewage sludge via the human food chain. Natural phosphorus resources are finite and increasingly contain harmful trace substances. The European Commission therefore considers phosphate or phosphate ore as a critical raw material, and much more recycling of phosphorus from sewage sludge will be required in the future. Germany’s Sewage Sludge Ordinance (Klärschlammverordnung), amended at the end of 2017, obliges German treatment plant operators to recycle at least 50 percent of the phosphorus in sewage sludge from 2029 or 2032 onwards, depending on the size of their facilities. To have the technical capability to put this ordinance into practice, they will require smart processes that go beyond the recycling of phosphorus to focus on efficiently using sewage sludge as a source of energy and carbon.

This is an area where converting sewage sludge at a high temperature, in other words, oxygen-deficient combustion or gasification, offers some interesting options. Phosphorus is present in sewage sludge in the form of phosphate, i.e. in combination with oxygen. This oxygen can be partially or fully removed again at a high temperature, with oxygen-deficient combustion and in the presence of a reducing agent such as carbon, leaving either pure phosphorus or phosphorus suboxides. These can then be separated off and used in the production of phosphoric acid, for instance, and, ultimately, products such as fertilisers. The gasification process includes high temperatures and an oxygen deficiency. The carbon in the sewage sludge can serve as the reducing agent, although in some cases it will have to be supplemented with further carbon, e.g. from biomass, pre-treated residues like municipal waste.

Gasification therefore provides the technical foundation for mobilising the phosphate from sewage sludge, and is also an important component in the creation of carbon cycles. The resulting product is a gas mixture that contains mainly carbon monoxide (CO), hydrogen (H₂), water (H₂O), carbon dioxide (CO₂) and phosphorus (P or PxOy). The phosphorus can be separated from this gas using a water-based scrubbing process. The remaining gas is further purified until a synthesis gas (CO + H₂) develops, which can be used as a feedstock for energy sources, fuels or important base materials for the chemical industry.

There are many benefits of high-temperature conversion compared to other methods of disposal. Thermal treatment of sewage sludge, and phosphorus and carbon recycling, can take place in a single process stage. With these methods, problematic trace substances such as drug residues, heavy metals or microplastics, which can cause problems if the sewage sludge is disposed of in an agricultural environment, are safely destroyed or isolated.

A significant amount of R&D is needed to be able to turn this method of recycling materials found in sewage sludge into reality.

The first step is to establish how the solid fuel – a mixture of sewage sludge and a carbon carrier – can be fed into the reactor. In principle, suitable processes are already in existence, but they still have to be tested and verified to ensure their suitability for the fuel mixtures used in this project. In addition, the necessary operating parameters in the key element in the process chain, the gasification process, must also be determined through experiment. That’s the only way to mobilise the phosphorus efficiently. And lastly, appropriate procedures and procedural conditions must be developed for recovering phosphorus from the product gas.

An additional partner in our project is the Ruhr University of Bochum.

The Ruhr University of Bochum supports our research with detailed investigations concerning the necessary environment for conversion during the release of phosphorus from sewage sludge.