Pulverized Sewage Sludge Combustion with Potassium Chloride Addition─Fate of Phosphorus, Potassium, Sulfur, and Heavy Metals

Sewage sludge is a sink for heavy metals and plant nutrients. Thermal treatment of sewage sludge can remove heavy metals, providing a potential raw material for fertilizer production. Stationary fluidized bed combustion is a common thermal treatment for sewage sludge, with its process-related maximum temperature of 850 °C. This temperature sometimes does not sufficiently remove heavy metals. As an alternative, we demonstrated pulverized combustion in an entrained flow reactor at 850 and 1100 °C, including the addition of KCl. Thermodynamic equilibrium calculations allowed for the assessment of reaction conditions and the effect of additives on the evaporation of heavy metals. Pulverized combustion at 850 °C enabled the partial removal of Hg, Tl, Pb, As, and Zn from the bottom ash. Pulverized combustion at 1100 °C combined with the addition of KCl evaporated 51% of Cu, 55% of Zn, 60% of As, 70% of Pb, 72% of Cd, 83% of Tl, and 91% of Hg. Due to the enhanced evaporation of Cd and Cu, this ash complied with all heavy metal limit values of the German Fertilizer Ordinance, except for Ni. Overall, subsequent ash separation at high temperatures enabled the valorization of sewage sludge contaminated with heavy metals. P and K remained in the ash, showing the potential of this thermal treatment method for nutrient recovery.