Sewage sludge from urban or industrial Wastewater Treatment Plants is a very serious problem and their volume is increasing every day. According to the European regulations on sludge management, sludge landfilling shall be forbidden in 2020 and must be 100% used. Given the serious amount of phosphorus contained in sludge and following European and world trends, any treatment should be guaranteed to extract phosphorous from sludge, where the most efficient and economically viable way is to extract phosphorus from the ash formed as a result of burning sludge