Abstract
The management of the aqueous phase generated from hydrothermal liquefaction (HTL) has always remained a challenging issue. Over the years, various strategies have been adopted for the valorization of the aqueous phase (AP), including anaerobic digestion, gasification, and biomass cultivation. However, AP recirculation (APR) within the HTL system has recently gained significant attention, as it enhances bio-crude yield and reduces the economic and environmental costs associated with aqueous waste disposal. The existing literature primarily consists of the integration of HTL with other technologies or APR within HTL, but with a limited range of feedstocks. Since numerous feedstocks have now been tested, there is a significant need for a critical, and in–depth review to deepen the understanding of the behavior of different biomass during APR. This review critically evaluates the role of APR in the HTL, covering the latest state–of–the–art of lignocellulosic and protein–containing biomass with an emphasis on the underlying mechanisms that contribute to bio-crude production and other HTL products (solid, AP, and gas). The effects of key process parameters, including temperature, residence time, catalyst, co–solvent, and mixing ratios, on the performance of APR are systematically analyzed. Moreover, the challenges encountered during the APR are addressed with suitable recommendations in the final part of the study.