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This paper critically reviews and discusses the utilization of ultrasound as an eco-friendly approach for enhancing biomass valorization efficiency. In contrast to other methods, this process requires no additional chemicals, no post-treatment (such as wastewater treatment), and helps achieve industrial electrification goals. Ultrasound irradiation has been employed as a powerful tool in biomass pretreatment and biorefining, as well as a green extraction method for recovering bioactive molecules. By partially degrading lignin structures, ultrasound irradiation significantly improves the biological conversion of waste materials and lignocellulose, facilitating the exposure of valuable macromolecules, i.e., cellulose and hemicelluloses. This research begins with an introduction to sonication technology, and subsequently presents comprehensive discussions focusing on the application of ultrasound in the pretreatment of sludge and lignocellulosic materials for anaerobic digestion. Furthermore, various facets of ultrasound usage in bioethanol production, including substrate pretreatment, enzymatic and acid hydrolyzes, and fermentation techniques, are also examined. Additionally, the benefits of employing ultrasound technology to recover high-value, heat-sensitive bioactive compounds at temperatures below their degrading points, thereby preserving their functionality, are explored. Looking ahead, this review explores current trends in ultrasound technology adoption and its potential for scaling up and commercialization, introducing pathways for a more sustainable and efficient approach for biomass valorization.

Biochemical conversion of lignocellulosic feedstocks to advanced biofuels and other bio-based commodities typically includes physical diminution, hydrothermal pretreatment, enzymatic saccharification, and valorization of sugars and hydrolysis lignin. This approach is also known as a sugar-platform process. The goal of the pretreatment is to facilitate the ensuing enzymatic saccharification of cellulose, which is otherwise impractical due to the recalcitrance of lignocellulosic feedstocks. This review focuses on hydrothermal pretreatment in comparison to alternative pretreatment methods, biomass properties and recalcitrance, reaction conditions and chemistry of hydrothermal pretreatment, methodology for characterization of pretreatment processes and pretreated materials, and how pretreatment affects subsequent process steps, such as enzymatic saccharification and microbial fermentation. Biochemical conversion based on hydrothermal pretreatment of lignocellulosic feedstocks has emerged as a technology of high industrial relevance and as an area where advances in modern industrial biotechnology become useful for reducing environmental problems and the dependence on fossil resources.

Hydrolysis lignin, i.e., the hydrolysis residue of cellulosic ethanol plants, was extracted with the green solvent γ-valerolactone (GVL). Treatments at 170–210 °C were performed with either non-acidified GVL/water mixtures (NA-GVL) or with mixtures containing sulfuric acid (SA-GVL). SA-GVL treatment at 210 °C resulted in the highest lignin solubilization (64% (w/w) of initial content), and 76% of the solubilized mass was regenerated by water-induced precipitation. Regenerated lignins were characterized through compositional analysis with sulfuric acid, as well as using pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS), high-performance size-exclusion chromatography (HPSEC), solid-state cross-polarization/magic angle spinning ¹³C nuclear magnetic resonance (CP/MAS ¹³C NMR) spectroscopy, ¹H^–13C heteronuclear single-quantum coherence NMR (HSQC NMR), and Fourier-transform infrared (FTIR) spectroscopy. The characterization revealed that the main difference between regenerated lignins was their molecular weight. Molecular weight averages increased with treatment temperature, and they were higher and had broader distribution for SA-GVL lignins than for NA-GVL lignins.