Recent advances and challenges of inter-disciplinary biomass valorization by integrating hydrothermal and biological techniques

Biomass valorization for the production of various value-added biochemicals and biofuels plays a significant role in modern biorefineries/bioenergy towards a climate neutrality future. Among the developed valorization techniques for biorefining, hydrothermal and biological treatments have been demonstrated to valorize raw biomass materials or upgrade biorefinery intermediate products to afford respectably desired product yields. From the perspective of both green chemistry and circular bioeconomy, whether the inter-disciplinary approach could achieve complete biomass valorization with balanced energy and extra profits remains questionable. This review is presented to provide an overview of the opportunities combining both hydrothermal and biological techniques for biomass valorization. Combinations including (1) hydrothermal pretreatment followed by biological valorization (such as enzymatic hydrolysis, fermentation, anaerobic digestion, or composting) and (2) hydrothermal valorization of substrates generated from biological techniques (such as enzymatic hydrolysis pretreated food waste, or anaerobic digestion produced digestate) for the production of biochar, biocrude or syngas are comprehensively reviewed. The recent advances regarding treatment conditions, synergies between hydrothermal and biological techniques, and optimal performances are summarized and compared. Assessment of the energy balance and economic feasibility of different integrated options reported in previous studies is also compared. Finally, challenges and perspectives for advancing integrated hydrothermal and biological techniques toward complete biomass valorization are concluded.