Sewage sludge ash to phosphorus fertiliser: Variables influencing heavy metal removal during thermochemical treatment

The aim of this study was to improve the removal of heavy metals from sewage sludge ash by a thermochemical process. The resulting detoxified ash was intended for use as a raw material rich in phosphorus (P) for inorganic fertiliser production. The thermochemical treatment was performed in a rotary kiln where the evaporation of relevant heavy metals was enhanced by additives. The four variables investigated for process optimisation were treatment temperature, type of additive (KCl, MgCl2) and its amount, as well as type of reactor (directly or indirectly heated rotary kiln). The removal rates of Cd, Cr, Cu, Ni, Pb, Zn and of Ca, P and Cl were investigated.The best overall removal efficiency for Cd, Cu, Pb and Zn could be found for the indirectly heated system. The type of additive was critical, since MgCl2 favours Zn- over Cu-removal, while KCl acts conversely. The use of MgCl2 caused less particle abrasion from the pellets in the kiln than KCl. In the case of the additive KCl, liquid KCl – temporarily formed in the pellets – acted as a barrier to heavy metal evaporation as long as treatment temperatures were not sufficiently high to enhance its reaction or evaporation.