Chemical and biological solutions to the thermodynamic challenges posed by hydrothermal liquefaction process water

Abstract Its ability to upconvert myriad wet carbonaceous wastes into biofuels and platform chemicals makes hydrothermal liquefaction (HTL) an attractive process to incorporate into a future bioeconomy. However, while HTL is well suited to process feedstocks with high moisture content, it generates a carbon-laden process water (PW). There is considerable research on the state-of-the-field of HTL; the impact of feedstocks and process conditions on products is well established, as are methods to upgrade recovered biocrudes (BCs). However, methods to efficiently separate, recover, and utilize the fugitive carbon in PW are less well understood. We believe this is because of the intrinsic thermodynamic limitations imposed by the PW; PW is a solutropic solution for which liquid–liquid extraction is, depending on the solvent, of minimal utility. Aqueous phase processing and electrocatalytic oxidation could produce high-value products like H 2 for BC upgrading, though issues of catalyst stability and electrode fouling, along with selectivity and efficiency, plague these nascent technologies. The literature is replete with conflicting opinions on the potential to recycle PW in the reactor (some authors find enhancement of hydrochar or BC yield, others no change or a negative impact). The current Edisonian approach to biological treatment (e.g. grow one bacteria on one PW) leaves the field without a clear understanding of the HTL PW compounds that inhibit or promote growth beyond broad classifications. Through this review, we hope to encourage the HTL field to move beyond the current norm of processing singular feedstocks to assess the BC produced and consider the carbon balance of the entire system to develop recovery and valorization pathways for the carbon present in HTL PW.