Insights Into Valuing The Aqueous Phase Derived From Hydrothermal Liquefaction

Hydrothermal liquefaction (HTL) process is a wet-thermochemical conversion technology that was used to convert biomass into bio-crude oil. Commercialization of the HTL technology faces drawbacks due to product stability, by-products generation and working area. In order to overcome these challenges, this review especially focuses on possible pathways to valorise and recover nutrients from post-hydrothermal liquefaction wastewater (PHWW) that was obtained during the HTL process. Numerous studies were reported on bio-oil production from biomasses like algae, forest and agriculture residues, etc at a temperature range of 240-320 degrees C at time of 30-60 min. Apart from bio-oil, nearly 25 to 50 wt% of aqueous phase was generated, disposal of this aqueous phase is hectic since it comprised of low molecular weight acid compounds. In this study, the composition of PHWW and possible routes (biological and thermochemical pathways) to valorise it were discussed in detail. In addition, recycle and reutilization of PHWW were reviewed with recent findings. From the review, the use of anaerobic digestion as a detoxify step prior to microalgae cultivation resulted in decreasing the fresh water dilution from 20 x to 4 x and improved the energy output from 3.44 to 20.7 kJ g-1 COD. This review will provide new insights towards closed circular approach opportunities in thermochemical pathways during sustainable energy production.