Nanobubbles (NBs) are extremely small (typically 70–150 nm) gas cavities in liquid that exhibit unusual stability, high internal pressure, large surface-area-to-volume ratio, and strong surface charge. These unique properties greatly enhance gas–liquid mass transfer, NBs can dissolve far more gas in water than would be predicted by Henry’s Law, and under certain gas and operating conditions, NB systems have also been associated with reactive oxygen species generation, which may contribute to substrate disintegration.
In the context of anaerobic digestion (AD), adding NB-enriched water (with gases like O₂, CO₂, H₂, etc.) has been shown in laboratory studies to boost methane yields roughly up to 10–30% on average, improve COD/VS destruction, reduce VFA (Volatile Fatty Acids) inhibition, and enhance digestate dewaterability. These performance gains are attributed to mechanisms such as greatly improved gas mass transfer, mild micro-aeration (from O₂-NBs) that stimulates hydrolytic bacteria, additional reducing substrates (from H₂-NBs) for methanogens, ROS-mediated disintegration of organics, and electrostatic effects that alter floc structure.
1.0 Nanobubbles Definitions and Physics:
Nanobubbles are gaseous voids in a liquid, usually measuring less than 200 nanometers in diameter. In contrast to bigger bubbles, NBs possess very elevated internal pressure (attributable to Laplace pressure) and a significant surface charge (often negative) that inhibits coalescence. These elements render nanobubbles metastable, allowing them to remain suspended for hours or days rather than promptly ascending to the surface. Nanobubbles possess a substantial surface-area-to-volume ratio and are capable of transporting dissolved gas far above the solubility limits established by Henry’s Law.
Essential characteristics (Fig. 1) encompass elevated zeta potential (about –20 to –50 mV), neutral buoyancy, and the capacity to emit reactive oxygen species (such as hydroxyl radicals) upon collapse.
Figure1: Key physicochemical properties of nanobubbles: negative surface charge (zeta potential), neutral buoyancy, and reactive oxygen species (ROS) generation.
In practical terms, injecting nanobubbles into Anaerobic Digesters can dramatically increase gas transfer rates (e.g. dissolved O₂ or CO₂) and create novel chemical/biological interactions not seen with coarse aeration.
2.0 Interactions with Anaerobic Digester Microbiology and Biochemistry:
Nanobubbles influence Anaerobic Digesters through both physical and biochemical effects on the microbial community and substrates:
1.Enhanced hydrolysis: NBs can accelerate the breakdown of proteins, carbohydrates and lipids. This is attributed to
(a) Micro-cavitation during NB collapse, which mechanically disrupts solids and increases enzyme-substrate contact.
(b) ROS generation (hydroxyl radicals, etc.) that oxidize complex organics.
(c) Improved transport of organics into solution.
2. Microbial stimulation: The adverse surface charge and chemical properties of NBs can influence microbial behaviour. Numerous investigations noted increased activity and a variety of hydrolytic bacteria and methanogens in the presence of NBW. For example, NBW has been documented to enhance the functionality of microbial electron-transport mechanisms and improve microbial resilience to stressors (such as salts or inhibitors). Air- or O₂-NBs provide minuscule micro-aerobic environments that can enhance facultative bacteria and ATP production. This frequently results in expedited acidogenesis and acetogenesis, hence diminishing VFA buildup. Moreover, NB oxygen or ozone might inhibit H₂S-generating bacteria, hence reducing sulfide concentrations.
Figure 2: Interaction of Nanobubbles with Anaerobic Digestion (AD) Microbiology and Biochemistry
3. Additional substrates: Nanobubbles can carry extra gaseous substrates into the digester. H₂-NBs dissolve hydrogen more efficiently than sparging, directly feeding hydrogenotrophic methanogens. Studies have shown H₂-NB supplementation can raise methane production. Similarly, CO₂-NBs increase dissolved CO₂, which can buffer pH and may be converted back to CH₄ by homoacetogens or hydrogenotrophs.
4. Biochemical effects: Nanobubbles may enhance enzyme activities. For example, Fan et al. noted that NBW’s higher spin–spin relaxation time (from NMR measurements) correlated with increased enzymatic activity and hydrolysis. Increased electrical conductivity in NB-treated digesters suggests greater numbers of charged species and improved nutrient diffusion. NBW has also been reported to increase the abundance of key archaeal methanogens, thereby contributing to more robust methane production.
5. Digestate dewatering: By altering sludge floc structure, NBs can improve dewaterability. Wang et al. (2024) reported that NBW addition to sludge AD reduced capillary suction time (CST) and specific resistance to filtration (SRF). This was attributed to NBs fragmenting flocs (smaller particle size) and increasing zeta potential, making the sludge cake more compact. In practice, NB-treated digestate often contains finer solids and releases water more readily during centrifugation.
In summary, nanobubbles act on AD at multiple levels: they physically mix and oxygenate the medium at a micro scale, supply extra gases (O₂, H₂) to specialized microbes, generate radicals that break down complex substrates, and chemically alter particle surfaces and charge. These changes together accelerate hydrolysis/acidogenesis and stabilize methanogenesis, leading to higher biogas yields and quality.
3.0 Reported Performance Impacts:
Published studies indicate that nanobubble (NB) treatment can improve several key performance parameters of anaerobic digestion. The extent of improvement depends on the substrate, NB gas type, operating conditions and reactor configuration. Reported benefits include:
Improved biogas and methane production:
NB treatment has generally resulted in higher methane yields than untreated controls, with improvements commonly reported in the range of 10–30%. The effect is attributed to improved hydrolysis, greater substrate availability and enhanced microbial activity.
Improved biogas quality: NBs can influence gas-liquid mass transfer and promote the removal or solubilisation of gases such as CO₂. This can result in a modest increase in methane concentration in the produced biogas.
Enhanced organic matter degradation: NB treatment has been associated with improved COD and volatile solids reduction, indicating more effective conversion of organic matter during digestion. This is particularly relevant for substrates with high solids content or slower hydrolysis rates.
Faster digestion kinetics: Several studies have reported earlier achievement of peak methane production under NB-assisted conditions. This indicates the potential for higher reactor productivity and, in some cases, reduced effective digestion time, although further work is required to establish practical HRT reductions.
Better VFA control and process stability: NB treatment can support the conversion of volatile fatty acids into methane, reducing excessive VFA accumulation. This contributes to better pH stability and more balanced digester operation, particularly under higher organic loading conditions.
Improved digestate dewaterability: NB treatment has also shown potential to improve downstream sludge handling by reducing resistance to dewatering and lowering cake moisture. This can simplify digestate management and reduce the energy or chemical requirement for subsequent processing.
Overall, the available research suggests that nanobubbles can act as a process-intensification approach for anaerobic digestion, improving methane recovery, organic matter conversion, digestion kinetics and digestate handling. Reported improvements in methane yield are typically in the 10–30% range, although performance varies considerably with feedstock and operating conditions.
4.0 Conclusion:
Nanobubbles offer a promising approach for intensifying anaerobic digestion by improving gas transfer, substrate breakdown and microbial activity within the digester. Research to date indicates potential improvements in methane production, organic matter degradation, process stability and digestate dewaterability, with methane yield improvements commonly reported in the range of 10–30%.
The overall benefit depends on the feedstock, gas type, NB characteristics and operating conditions. While laboratory studies provide encouraging results, further pilot- and full-scale studies are required to establish optimum operating conditions, energy efficiency and long-term performance. Nevertheless, nanobubble technology has strong potential as a retrofit or process-intensification option for improving the performance of existing anaerobic digestion systems.
References:
- Nirmalkar, N., Pacek, A. W., & Barigou, M. (2018). On the existence and stability of bulk nanobubbles. Langmuir, 34(37), 10964–10973. DOI: 10.1021/acs.langmuir.8b01163
- Sharma, H., & Nirmalkar, N. (2022). Enhanced gas-liquid mass transfer coefficient by bulk nanobubbles in water. Materials Today: Proceedings. DOI: 10.1016/j.matpr.2022.01.029
- Chuenchart, W., Karki, R., Shitanaka, T., Marcelino, K. R., Lu, H., & Khanal, S. K. (2021). Nanobubble technology in anaerobic digestion: A review. Bioresource Technology, 329, 124916.
- Fan, Y., Lei, Z., Guo, Z., Huang, W., Wang, D., Wang, X., Zhang, Z., & Shimizu, K. (2020). Enhanced solubilization of solid organics and methane production by anaerobic digestion of swine manure under nano-bubble water addition. Bioresource Technology, 299, 122512.
- Wang, D., Yang, X., Tian, C., Lei, Z., Kobayashi, N., Kobayashi, M., Adachi, Y., Shimizu, K., & Zhang, Z. (2019). Characteristics of ultra-fine bubble water and its trials on enhanced methane production from waste activated sludge. Bioresource Technology, 273, 63–69.
- Yang, X., Nie, J., Wang, D., Zhao, Z., Kobayashi, M., Adachi, Y., Shimizu, K., Lei, Z., & Zhang, Z. (2019). Enhanced hydrolysis of waste activated sludge for methane production via anaerobic digestion under N₂-nanobubble water addition. Science of the Total Environment, 693, 133524.
- Wang, X., Yuan, T., Lei, Z., Kobayashi, M., Adachi, Y., Shimizu, K., Lee, D.-J., & Zhang, Z. (2020). Supplementation of O₂-containing gas nanobubble water to enhance methane production from anaerobic digestion of cellulose. Chemical Engineering Journal, 398, 125652.
- Hou, T., Zhao, J., Lei, Z., Shimizu, K., & Zhang, Z. (2021). Enhanced energy recovery via separate hydrogen and methane production from two-stage anaerobic digestion of food waste with nanobubble water supplementation. Science of the Total Environment, 761, 143234.