*4.2. Phosphorous Recovery*

Waste biomass as OFMSW and sewage and agro-industrial sludge contain large amounts of phosphorous. Phosphorous (P) is a "critical raw material" for Europe [143] meaning that this element is of strategical importance but reserves of P rock are limited and mostly concentrated in Morocco, USA and China. As stated above, European legislation is pushing and financing for an increase in recovery of P from waste streams [144]. Hydrothermal carbonation of waste biomass, and in particular of sewage sludge containing up to 4% by weight on a dry basis, has demonstrated the possibility of recovering up to 95% of the total phosphorous content [70,145–147]. It has been demonstrated that during HTC, the phosphorous element segregates into the solid phase and treatment of the recovered hydrochar by an acidic leaching leads to the removal of P which moves to the aqueous phase. Phosphorous element in sewage sludge could be found in different forms depending on the reaction conditions [148]. P can be then precipitated by alkalization of the aqueous solution by adding CaO [145] or sodium hydroxide solution[146]. The addition of magnesium chloride and ammonium to the acidic phosphorous solution prior to alkalization led to the precipitation of struvite directly usable as solid fertilizer [146].
