The process of recycling phosphorus from phosphate-rich residues and waste materials is becoming increasingly important. The pilot project P-XTRACT® shows the best way of recovering valuable phosphorus from sewage sludge in fluidised bed incineration cycles. The project showcases a new and promising technology based on a special method of extracting the useful phosphorus from the sewage sludge.
The process consists of the modified fluidised bed incineration of sewage sludge and the addition of certain alkali and alkaline earth components. These additives significantly increase the plant availability of the phosphorus as compared to conventional ashes from incineration. The incineration conditions and additive dosage can be adjusted in such a way as to allow a depletion of heavy metals during the incineration process. The ash containing the phosphorus is carried in the flue gas as fly ash and is then separated from the flue gas at high temperatures of > 700 °C. A valuable and virtually pollutant-free material containing phosphorus is extracted from the ash which can then be processed into fertiliser.
The research scientists at the ZSW have demonstrated the basic technical feasibility of sewage sludge incineration with additives in their own fluidised bed test facility on a pilot scale of 10 kWth in Stuttgart. The ZSW has a broad base of expertise and many years of experience in the field of integrated phosphorus recovery and fluidised bed technology – including the special analysis facilities required for this.
The P-XTRACT® process is now set to be tested for the first time in real conditions in a 1-megawatt pilot plant in Breisach-Grezhausen at the Staufener Bucht sewage treatment association. Construction work on the pilot plant is set to begin in September 2021, with Wehrle-Werk AG in charge of the build. Regular operation of the pilot plant is scheduled from July 2023. More than 80 per cent of the phosphorus present in the sewage sludge could be recycled with the system.
The ZSW will be involved in the first large-scale pilot and will conduct validation tests on the additive infeed on the relevant scale. The Institute of Inorganic and Analytical Chemistry at the University of Freiburg will be responsible for the selection of additives and the evaluation of the ash as regards its potential for use as a fertiliser.