Distribution of nutrients and phosphorus recovery in hydrothermal liquefaction of waste streams

Hydrothermal liquefaction (HTL) of phosphorus (P) rich wastes is a promising technology to recover P in solid form. For the applicability as fertilizer, the HTL solids should contain nutrients, in particular P, in plant available form and low concentrations of heavy metals. In the present study, the fate of P and other nutrients has been studied by performing HTL of four P-rich wastes. The effect of temperature on the distribution of the HTL products and nutrients has been investigated. It was found that up to 100% of the total P can be accumulated in the solid phase. However, the bioavailability of P decreased to 0.6–1% and the concentrations of heavy metals exceeded threshold values for fertilizers. Due to the limited P bioavailability and high contents of heavy metals, the reclamation of P as struvite is also calculated. Based on these estimations, between 42 and 93% of P can be recovered as struvite.