Direct hydrothermal liquefaction involves converting high-moisture biomass to an oily liquid. Depending on the biomass used, the resulting bio-oil can have a heating value com­parable to bunker crude oil (30-40 MJ/kg). The resulting oil can be burned in boilers or upgraded and refined into higher value fuel or chemical compounds. Direct hydrothermal liquefaction works by contacting biomass with water at elevated temperatures (300-350°C) and sufficient pressure to maintain the water in the liquid phase (12-20 MPa) [21]. Addi­tionally, alkali catalysts may be added to promote organic conversion. In the process, water acts as a necessary reaction medium, therefore eliminating the need to dry down biomass and, thus, reducing the total energy footprint. Hydrothermal liquefaction processes have the potential to become an important group of technologies for converting wet biomass or organic waste into bio-oil for fuel or other applications. The hydrothermal liquefaction process holds significant potential, particularly for producing specific fuels targeted for the heavy transport sector, combustion purposes, and as a raw material for further chemical processing [22].

The robust reaction conditions and aqueous environment make hydrothermal liquefaction well suited for the conversion of low-lipid, fast-growing algae that proliferate in wastewa­ter treatment facilities [23]. Integrating algae cultivation into a wastewater treatment plant offers the synergetic benefit of providing nutrient remediation because algae capture and use dissolved nitrogen and phosphorous present in wastewater to support growth [23]. These plentiful nutrients would otherwise be released into the environment, creating harmful eutrophication of natural systems. By converting nutrient waste into a resource, environ­mental pollution will be reduced, as energy is created and water resources are preserved [24].