Valorization of the aqueous phase produced from wet and dry thermochemical processing biomass: A review

Thermochemical processing technologies such as hydrothermal liquefaction/carbonization/gasification, pyrolysis, torrefaction, and gasification have been widely used for the production of oil, char, and gas fuels from biomass. However, aqueous phase (AP), a co-product or process wastewater with high concentrations of carbon, nitrogen, phosphorus, and other elements, is inevitably produced in massive amounts after thermochemical treatment. The treatment and utilization of AP is one of the bottlenecks for the industrialization of biomass processing technologies. In this review, the production and characteristics of AP from dry and wet thermochemical processes were compared. Various AP valorization pathways were comprehensively discussed, including (i) the direct utilization of AP as a solvent, reaction medium, or a culture nutrient source for the cultivation of organisms such as microalgae; (ii) the separation and purification of nutrients such as struvite or chemicals such as carboxylic acids from AP, and (iii) the conversion of AP components to energy and fuels such as bioelectricity, biohydrogen, biogas, syngas, bioethanol, or chemicals such as polyhydroxyalkanoates (PHAs) through fermentation, anaerobic digestion, microbial electrolysis cells, microbial fuel cells, supercritical water gasification, and catalytic reforming. Finally, various strategies have been proposed to promote future research, including matching AP with preferable valorization technology, tuning AP compositions and properties (e.g., detoxification of AP), and process optimization and integration. This review could also be conducive to promoting the utilization of organic wastewater from other sources.