Feasibility of hydrothermal liquefaction in phosphorus-recovery from wastewater sludges

A new process train is proposed to stabilize wastewater sludges and to recover calcium phosphate (CaP) from them via hydrothermal liquefaction (HTL). The HTL process concentrates most of the P-content of wastewater sludges into a solid residue referred to as HTL-char. Acid leaching of HTL-char to elute P followed by a 3-step precipitation process is demonstrated for recovering high-quality CaP. Leaching HTL-char with 0.4 M HCl at a liquid:solid ratio of 20 for 0.5 h at 60.C enabled >99 % elution of P. The 3-step precipitation process resulted in 60-89 % recovery of the P-content of HTL-char as CaP, yielding 15-26 g of CaP/kg dry sludge. Yield of CaP could be increased further if the raw sludge mix is dewatered to increase its solid content to similar to 5 wt% prior to HTL. Inductively coupled plasma optical emission spectrophotometer results confirmed that the 9 regulated heavy metals in the recovered CaP were below the limits defined by the U.S. Environmental Protection Agency and the European Environmental Agency. Resource and energy costs associated with the proposed process are found to be comparable to those reported for the five patented processes for recovering P from anaerobically digested sludge (17.9 vs 21.2 +/- 12.7 (sic)/kg P recovered).

Automated summary

A new process is proposed to stabilize wastewater sludges and recover calcium phosphate from them. This process concentrates most of the phosphorus content of the sludges into a solid residue called HTL-char. Acid leaching and a 3-step precipitation process are used to recover high-quality calcium phosphate. The results show that this process can efficiently recover phosphorus and the recovered calcium phosphate meets environmental standards.