Phosphorus recovery from sewage sludge by thermochemical conversion: Separation from the gas phase and final product treatment

This study investigates the phosphorus release through thermochemical conversion of sewage sludge coke and wet-chemical recovery of phosphorus from the gas phase. The phosphorus release experiments were performed in a high-temperature fixed-bed reactor under N 2 and N 2 /H 2 atmospheres using different sewage sludge coke feedstocks. Despite compositional differences, all experiments achieved phosphorus release ratios above 65%. The release ratio is primarily governed by the feedstock composition, showing a positive correlation with aluminum and a negative correlation with iron. The gaseous phosphorus was captured in a copper sulfate (CuSO 4 ) scrubber, yielding a solid copper-phosphorus-rich precipitate (Cu 3 P). A systematic analysis of the influence of process parameters such as temperature, hydrogen concentration, and copper sulfate concentration was performed resulting in a set of optimal process conditions capturing up to 99% of the released phosphorus. Subsequent dissolution and ion exchange experiments demonstrated that copper and phosphorus can be effectively separated, thereby confirming the technical feasibility of the proposed recovery and purification concept. While further optimization is required for quantitative phosphorus recovery, the results provide a practical proof of concept for an integrated process chain enabling both phosphorus recycling and copper regeneration.