PULSE process: recovery of phosphorus from dried sewage sludge and removal of metals by solvent extraction.

Phosphorus (P) is an indispensable nutrient for agriculture. Recovery and recycling of phosphorus from waste streams is necessary to ensure a circular P economy and reduce dependence on disproportionately distributed mineral P resources. In this study, a new process called 'PULSE' is presented for the recovery of P from sewage sludge, which can handle high metal contents. The process involves drying of sludge prior to acidic leaching to overcome the challenge of solid-liquid separation at low pH and to reduce the overall material flows. Another key point of the process is the removal of metals using reactive extraction to obtain a high-quality product with good plant availability. Laboratory experiments were conducted to evaluate and select the best process options. A chemical equilibrium tool was developed to simulate the unit operations of the process for optimization. Dissolution of P from sludge depends on leaching pH and the fraction of inorganic P in the sludge. The maximum P leaching efficiency for the sludge used in the study was between 65 and 70%. Under the tested conditions, Fe, Cd, Cu, Hg, Pb, and Zn were successfully removed from the sludge leach liquor by reactive extraction. The recovered product has a nutrient mass fraction of about 51% that includes Ca, PO43-, Mg, and K. Pot trials confirmed that the agronomical efficiency of the product is comparable to that of triple superphosphate.