Aqueous byproducts from biomass wet thermochemical processing: Valorization into fuels, chemicals, fertilizers, and biomaterials

Thermochemical pretreatments are employed prior to energy, chemicals, and fuels production from biomass. Wet thermochemical processes (WTCP) are treatments used to modify biomass properties in water as the primary solvent, with or without added reactants/catalysts. WTCP includes hot water extraction, steam explosion, hydrothermal liquefaction, hydrothermal carbonization (HTC), and supercritical water gasification. WTCP also includes processes that add chemicals to reduce reaction time and improve efficiency, i.e., organosolv, alkali, and low acid pretreatment. Operational parameters in WTCP are usually selected to optimize the yields of sugars after enzymatic hydrolysis of the resulting solids and biogas from the pretreated solids, or to ensure that hydrochar (e.g., from HTC) performs adequately in environmental applications. However, a key byproduct from WTCPs is an aqueous fraction (rich in nutrients, hemicellulose-derived sugars, and chemicals) often disposed of as waste. The necessity of resource conservation and proper management and the need to make WTCP-based biorefineries economically and environmentally sound require using all the byproducts of biomass processing. Options for downstream conversion of the WTCPs' aqueous byproducts are dispersed in the literature. Thus, this paper aims to put together works that report the parameters of WTCPs that allowed removing hemicellulose-derived fractions and nutrients from biomass (either partially or almost entirely), the yields and properties of this aqueous byproduct, methods of characterization, current and expected uses, and the challenges for scaling up WTCPs and using the aqueous stream. The paper focuses on expected and existing methods that allow the valorization of the aqueous fraction and reduce wastes within a circular bioeconomy framework.