The positive connotations of sewage sludge may be few and far between, but RWE sees things differently. The “Definition project to establish a virtual innovation and technology centre (ITZ) for the use of sustainable carbon sources to produce materials (carbon conversion/CC) in North Rhine-Westphalia”, in short ITZ-CC, has set itself the goal of making carbon and phosphorus found in sewage sludge reusable. In an initial sub-project, preparatory work for the construction and authorisation of a plant, which encompasses the entire conversion chain from the introduction of the sewage sludge and other raw materials containing carbon into a gasifier through to the production of phosphorus products and synthesis gas, will continue until the end of 2019. The plant itself is expected to be completed in a second project phase starting in 2020. The costs of the initial phase amount to approximately €2 million and 50% of this is being funded by the Ministry for Economic Affairs, Innovation, Digitalisation and Energy of the state of North Rhine-Westphalia. Detailed background information on the process and the use of sewage sludge as a raw material can be found here.
Sewage sludge: valuable raw material in the supply of phosphorus and carbon
Sewage sludge is generally regarded as a problematic residual material or waste, rather than a valuable raw material. Conventional disposal routes are agriculture and thermal utilisation in power plants or so-called mono-incinerators, which exclusively use sewage sludge as a fuel. However, if we take a closer look at the components of sewage sludge, it is clear that it contains valuable raw materials. In addition to carbon which can be found in almost all everyday objects, sewage sludge contains a significant amount of phosphorus – in its dry state, it can account for up to 5% of its weight.
As phosphorus is essential for cell metabolism, it is a raw material that all life depends on. It enters sewage sludge via the human food chain. Natural phosphorus resources are finite and increasingly contain harmful trace elements. As a result, the EU Commission regards phosphorus, or phosphate, as a critical raw material and the recycling of phosphorus from sewage sludge will be significantly expanded in future. Following the amendment of the Sewage Sludge Ordinance at the end of 2017, German operators of wastewater treatment plants will be required to recycle at least 50% of the phosphorus found in sewage sludge as of 2029 or 2032, depending on the size of the waste water treatment plant. The implementation of this ordinance requires smart processes that focus on the efficient utilisation of sewage sludge as a primary energy source and carbon carrier, in addition to recycling phosphorus as an added bonus.
With funding from the state government of North Rhine-Westphalia, a RWE project is driving the development of recycling processes
The gasification of sewage sludge, in other words incineration under oxygen deficiency conditions , offers up exciting possibilities. Phosphorus is found in sewage sludge as a phosphate, i.e. a stable molecule containing oxygen. This oxygen can be separated using high temperatures, oxygen deficiency conditions and the presence of a reducing agent such as carbon, thus creating pure phosphorus or phosphorus suboxides. This can then be separated and used to produce phosphoric acid and then, for example, fertilisers. The gasification process encompasses high temperatures and oxygen deficiency conditions. Carbon acts as a reducing agent in the sewage sludge, which may need to be supplemented with additional carbon, e.g. from biomass, pre-treated residual materials or coal.
Gasification thus provides the technical foundation for extracting the phosphorus from the sewage sludge and is also an important part of developing carbon cycles. Gasification results in a gaseous mixture that mainly contains 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 washing process. The residual gas is purified until a synthesis gas (CO + H2) is created that can be used to produce primary energy sources, fuels or important raw materials for the chemical industry.
In contrast to other disposal methods, the advantages of gasification are manifold. Sewage sludge can be thermally treated and phosphorus and carbon recycled as part of a single process step. Problematic trace elements, such as medicine residues, heavy metals or micro-plastics that cause problems in agriculture when sewage sludge is disposed of, can be safely treated in the gasification process.
Significant research and development work is needed to turn this method of recycling reusable materials from sewage sludge into a reality.
How the fuel (mixture of sewage sludge and a carbon carrier) required in the gasifier can be introduced into it must first be clarified. Processes for introducing fuel into a gasifier do already exist. However, these need to be qualified and tested to determine their suitability for the fuel combinations used in this project. Moreover, suitable processes and process conditions for the extraction of phosphorus from the producer gas of the gasifier must be developed. Last but not least, the necessary operating parameters in the core components of the process chain, i.e. gasification, need to be determined on the basis of experiments. Only then efficient gasification and extraction of the phosphorus can be facilitated.
The required R&D phases are being pressed ahead within a project by RWE Power. To this end, all permits required for the construction and operation of the plant will be gathered and a technical concept for the fuel input system drawn up, put into practice in a testing facility and tested. In addition, a detailed engineering is being prepared for the whole plant to ensure that procurement, assembly and construction of the whole plant can be started at at the beginning of the next project phase in 2020. A technical concept for the washer to extract phosphorus from the raw synthesis gas will be developed and tested by the end of 2019.