Anaerobic digestion (AD) serves as an important technology for the resource utilization of biomass energy, especially when processing agricultural waste with an energy content of 16 MJ/kg. Efficiently converts it into biogas for combustion or power generation. (Rahmani et al., 2023). Currently, AD is widely applied in the treatment of sewage sludge, livestock manure, municipal organic waste, and agricultural waste, promoting environmental protection, greenhouse gas emission reduction, and achieving renewable energy production (Laiq et al., 2019). Nevertheless, large-scale application of AD technology is still hampered by several limitations. For example, the hydrolysis step serves as a crucial step in the AD process, but the reaction rate is often slow when dealing with complex substrates, resulting in low hydrolysis efficiency. As a result, the hydrolysis step is widely recognized as the rate-limiting step in the AD process of lignocellulose and high solid organic wastes ( Tamang et al., 2023 ). On the other hand, methane production is a multifactorial process involving the synergistic effects of multiple neutrophils, so various factors (oxygen content, pH, temperature, organic load, etc.) can influence the methanation process. Yes (Li et al., 2019; Liu et al., 2021). Researchers are currently working to improve AD performance through pretreatment, additive supplementation (e.g., biological additives, micronutrients, carbon-based materials, metal-based materials), and reactor optimization. (Liu et al., 2021; Meegoda et al., 2018; Nguyen et al., 2021). However, difficult to maintain operating conditions or addition of expensive chemicals, along with subsequent processing of some additives, comes with increased costs. For example, external addition of micronutrients in large-scale Alzheimer's plants, which often relies on expensive and specialized chemicals, faces challenges due to the lack of standardized addition schemes and has limited environmental benefits. operating costs may increase (Bardi et al., 2023). . Furthermore, biochar as an environmentally friendly additive in AD systems also faces unresolved issues since its effectiveness is closely related to its inherent properties (e.g., raw materials, synthesis methods, process parameters, etc.). (Nie et al., 2024). Therefore, new strategies to increase methane production by combining cost and pollution reduction are urgently needed.
Nanobubbles (NB) refer to nanoscale gases (usually composed of air, O).2Colorado2,N2, and other gases) cavities in aqueous solutions. It is defined by the International Standards Organization (ISO) as having a volume-equivalent diameter of less than 1 μm and exhibiting the ability to alter the standard properties of an aqueous solution (Jia et al., 2023). More recently, NB technology has attracted increasing attention due to its unique properties and lack of cross-contamination, and researchers have investigated properties such as extended residence time, high mass transfer efficiency, and negative surface charge. We are exploring its use to improve AD performance. , and decays to produce free radicals such as 
OH radical (Atkinson et al., 2019; Khan et al., 2020).
Unlike coarse gas bubbles (CBs), NBs are hardly affected by buoyancy in solution, undergo free Brownian motion, and rise slowly, allowing them to remain in solution for more than 2 weeks (Lyu et al. ., 2019). Generally, the solubility of CB in water is always influenced by environmental pressure and limited by the saturation solubility, so it is difficult for CB to reach above the saturation solubility under normal environmental conditions. In contrast, according to Henry's law, the NB gradually contracts during its slow rise, and the surface tension of the NB increases with gas compression and internal pressure increase (Koshoridze and Levin, 2018; Lohse and Zhang, 2015). Therefore, the internal pressure of NB is higher than the external atmospheric pressure, which allows them to break the solubility limit under atmospheric pressure and obtain higher solubility (Craig, 2011). It has been proven that under the same volume gas transport conditions in water, the gas transport coefficient of NB can be increased by 11 times compared to CB (Atkinson et al., 2019). In addition, NBs are characterized by having a negatively charged surface and a high zeta potential, mainly due to the adsorption of hydroxyl ions on the interfacial surface of NBs, which also makes NBs more stable in solutions. This is one of the reasons why. The negative surface charge and high zeta potential of NBs are important factors that influence cation mass transfer, hydrophobicity, and water mobility in solution. For example, 1) it efficiently controls cation mass transfer processes by attracting and immobilizing cations through electrostatic interactions; Significantly increases the absorption efficiency of local nutrients. 2) It affects the hydrophobicity by changing the arrangement of surrounding water molecules and indirectly enhances particle attachment ( Chuenchart et al., 2021 ). Additionally, NBs can generate reactive species upon decay (Atkinson et al., 2019; Ghadimkhani et al., 2016). OH radicals are the main reactive species continuously generated at the gas-liquid interface when NBs decay in aqueous solution, which can destroy the stable structure of complex substrates and accelerate their decomposition. (Agarwal et al., 2011; Atkinson et al., 2019). .
In addition to unique physical and chemical properties, NB also has non-toxic and environmentally friendly properties. Therefore, the application of NB technology in environmental protection, biomedicine, food processing, and other fields does not generate secondary pollution and is consistent with sustainable development goals (Phan et al., 2020; Tekile et al. ., 2017; Xiong et al., 2021). Based on these properties, researchers have recently applied NB technology to improve energy recovery from AD systems, and nanobubble water (NBW) promotes substrate degradation and improves microbial activity. and has been shown to have the potential to increase hydrogen and methane production. Table 1 summarizes specific application examples.The result is that N2-NBW can contribute to establishing a strict anaerobic environment and promote the degradation of lignocellulose for methane production to increase cellulase activity (Wang et al., 2020a). In comparison, aviation NBW and O2-NBW can create a trace oxygen environment, which appears to be in contrast to the strictly anaerobic environment required for AD systems. What is interesting is the addition of air-NBW or O.2-NBW in the AD system enhanced methane production efficiency instead of inhibiting methane production (Hou et al., 2022; Wang et al., 2020c). The predicted mechanisms are related to enhanced degradation of substrates, accelerated production and consumption of soluble nutrients, and improved activity of facultative anaerobic bacteria (Fu et al., 2016; Zhu et al., 2022) .
It can be observed that different gas types of NBW show tremendous potential in AD processes of organic wastes such as lignocellulosic biomass, food waste (FW), and livestock manure (Fan et al., 2021a; Hou et al., 2021a; Wang et al., 2020a). However, the interaction mechanisms between NBWs and various substrates in AD, especially the enhancement of the mass transfer properties of substrates by NBWs, are unclear. Moreover, the NB production method and conditions directly affect the properties of NBs (e.g., particle size, bubble concentration, zeta potential), and therefore the performance of AD (Wang et al., 2019; Wang et al., 2021). Additionally, a clear understanding of the operating principle of the NB generator is required to design and optimize the operation of the NB generator according to the requirements of various organic solid waste treatment AD systems.
This review focused on the optimization of the structural design of venturi-type NB generators and discussed the working principles and characteristics of several NB generators. Meanwhile, the mechanisms related to the enhancement of AD performance and the effects of NB on the mass transfer properties of digestate were discussed in the context of his NB technology advancements in AD of various organic solid waste systems. Finally, we prospected the potential and future prospects of NBW in substrate and inoculum pretreatment.
