Nanobubbles (NBs), gas-filled cavities <1 μm diameter, have unique physical and chemical properties (exceptional stability, high surface area and negative charge) that can intensify conventional wastewater processes. In sewage systems, NBs (air, O₂, ozone or CO₂ gas) enhance oxygen transfer (higher kₗₐ and %O₂ transfer), accelerate organics and nutrient removal, and aid flotation/dewatering via coagulation or oxidative mechanisms.
Performance gains reported include up to 3–8× increases in volumetric oxygen transfer coefficient (kₗₐ), up to ~10–60% faster BOD/COD removal and nitrification rates, and strong reduction in inhibitory compounds (surfactants, sulfides) that improves treatment stability. Full-scale demonstrations reported by nanobubble aeration technology providers have indicated up to 45–60% improvement in oxygen transfer and 30–45% reduction in blower energy consumption compared with conventional aeration systems, performance depends strongly on wastewater characteristics and reactor configuration. In membrane bioreactors, NB-assisted backwash dramatically improves membrane cleaning. In anaerobic digesters, CO₂- or air-NBs enhanced hydrolysis and CH₄ yield.
In this technical note, we will review the NB definition and properties, action mechanisms, specific applications (primary, secondary, tertiary stages) and quantitative impacts (DO, BOD/COD removal, nitrification, sludge). We also summarize case studies and identify research gaps and pilot-test recommendations.
Nanobubbles: Definitions and Properties
Nanobubbles are defined by ISO as gaseous cavities with volume-equivalent diameters <1 μm. In practice, the most functional “bulk” NBs used in treatment are often tens to a few hundred nanometers across (e.g. ~100–300 nm). Unlike larger bubbles, NBs rise very slowly (negligible buoyancy) and remain suspended for hours to months due to stabilizing surface forces. Key NB properties include extremely high surface-area-to-volume ratio, low buoyancy, and persistent surface charge. The NB interface in water carries a high negative zeta potential (e.g. –10 to –60 mV, depending on pH and gas). This charge creates an electric double layer that counterbalances Laplace pressure, blocking rapid gas diffusion and giving NBs exceptional longevity. The ion-shielding effect explains how NBs defy classical dissolution predictions. The combination of small size and negative zeta potential makes NBs behave like colloidal particles, resisting coalescence and staying dispersed.
NB gas content can be air, oxygen, ozone, CO₂, or inert gases. Different gases affect NB behavior: oxygen- and ozone-filled NBs carry the most negative surface charge, while nitrogen NBs are least charged. Gas type also influences reactivity: e.g. ozone-NBs carry oxidizing power.
NB water exhibits altered chemistry: experimental work has shown that collapsing NBs generate reactive oxygen species (hydroxyl radicals). This gives NBs a chemical effect in addition to their physical aeration role, e.g. nanobubble ozonation
delivers strong oxidation and disinfection without added chemicals. These unique NB properties, stability, high charge, radical generation, underpin their ability to intensify treatment processes.
Mechanisms of Action in Sewage Treatment
Nanobubbles affect treatment processes via several synergistic mechanisms:
1. Enhanced Oxygen Transfer: Due to their tiny size and high surface area, NB dramatically improves gas-liquid mass transfer. Studies report nanobubbles yielding higher kₗₐ values than conventional coarse bubbles. The result is faster DO uptake, and higher BOD, COD removal. In practice, NB aeration often allows lower blower speeds for the same DO, or higher DO at fixed blower power.
2. Micro-mixing and Shear: NBs entrained in liquid induce micro-mixing. As NBs collapse, they create localized turbulence and even micro-jetting at the bubble interface. This shear action can help clean biofilms (in MBR backwash) or break flocs.
3. Oxidation and Disinfection (ROS generation): Collapsing NBs (especially O₂- or O₃-filled) generate reactive oxygen species (OH•, O₂•–). This catalytic effect can partially oxidize dissolved organics and kill pathogens without added chemicals. Reportedly, NB ozonation can match some advanced oxidation processes in micropollutant removal. Enhanced biological oxidation has been observed as well: NB treatments have inactivated surfactants and inhibitory organics that normally poison microbes.
4. Flotation and Coagulation: NBs act like tiny coagulants. The large population of NBs can attach to fine particles and colloids, carrying them to the surface (dissolved air flotation) or helping them settle faster. This improves solids separation. Lab studies also show NB-assisted FOG and TSS removal: NB attracts hydrophobic fats/surfactants, causing them to aggregate and float out.
5. Biofilm and Microbial Interactions: NBs can enhance biofilm performance indirectly. Higher DO and lower inhibitors both boost nitrifiers (bacteria that remove ammonia nitrogen) and heterotrophs (bacteria that consume organic matter). The Microbubble/Nanobubble SBR study found NB aeration selectively favored ammonia-oxidizers (AOB) over nitrite-oxidizers (NOB), yielding rapid partial nitrification. Improved oxygen distribution and surfactant removal reduce “dead zones” in biomass. In anaerobic digestion, specialized NBs (O₂, H₂ or CO₂) can drive different stages.
6. Mass Transfer of Other Gases: Besides oxygen, NB systems can deliver O₃, H₂, CO₂ etc. For instance, injecting ozone-NBs can disinfect effluent, while hydrogen-NBs could supply anoxic zones. The net effect is a higher dissolved gas concentration per unit supply and efficient utilization.
Specific WWTP Applications by Process Stage
Raw Influent/Headworks: NB injection upstream can pre-treat and strip inhibitory compounds. In lagoons or raw sewers, oxygen NBs have oxidized H₂S and VOCs to control odor. In commercial installations, NBs injected at a headworks basin removed surfactants, instantly improving downstream aeration performance. NBs also can float out debris and grease early in the plant.
Primary Treatment:
Dissolved Air Flotation (DAF): Conventional DAF uses microscale bubbles; adding NB pretreatment (higher bubble density) boosts solids and P capture. Microbubble/NB flotation can enhance TSS and phosphorus removal from primary effluent.
Secondary (Activated Sludge):
Aeration basins: Here NB aeration is most-studied and implemented. Nanobubble diffusers or bubble generators in the aeration tank supply O₂ with high efficiency. Reported gains include up to 6–8× kₗₐ and up to 45-60% more O₂ transferred relative to diffusers. This often translates into higher nitrification rates and effluent quality.
Membrane Bioreactors (MBRs): NB-enhanced backwash and scouring is highly promising. In bench trials, nanobubble backwashing cleaned fouling layers far better than plain water, nearly matching chemical cleaning. NBs (∼150 nm) permeated membrane pores and disrupted cake layers, reducing reversible fouling.
Anaerobic Digestion: Nanobubbles can enhance anaerobic digestion by improving mass transfer and promoting microbial activity during the hydrolysis and acidogenesis stages. Depending on the gas used, NBs can improve the availability of substrates and dissolved gases, supporting the conversion of organic matter into volatile fatty acids (VFAs) and subsequently enhancing biogas production. Nanobubbles may also help improve process stability and reduce issues such as odour and foaming under suitable operating conditions.
Tertiary/Nutrient Removal: In polishing, NB-assisted flotation can remove fine particulates or precipitates. NBs with O₃ can disinfect effluent without chlorination. NBs can also help control algal blooms in tertiary ponds via oxygenation. NB-driven micro-bubble flocculation is a nascent method to strip residual N and P.
Sludge Dewatering: NBs promote sludge thickening and dewatering. By aggregating fine particles (acting like micro-coagulant) and by generating gentle shear, NB pretreatment can reduce capillary suction time (CST) and improve filterability. In practice, plants observe faster settling and lower sludge volume index when upstream NB treatment is applied.
Conclusion
Nanobubble technology offers a practical approach to intensify conventional sewage treatment processes without major process modifications. Their unique properties, including high gas transfer efficiency, long residence time, and reactive oxygen species generation, improve oxygen transfer, biological treatment, flotation, membrane cleaning, sludge handling, and nutrient removal across different stages of a wastewater treatment plant. Reported studies consistently demonstrate improvements in dissolved oxygen transfer, organic and nutrient removal, energy efficiency, membrane fouling control, and biogas production. These benefits translate into improved treatment efficiency, lower energy consumption, and enhanced overall plant performance.
The ability to integrate nanobubble systems into existing treatment units with minimal infrastructure modification makes them an attractive retrofit solution for both municipal and industrial wastewater treatment plants. As demonstrated across laboratory, pilot, and full-scale applications, nanobubbles offer an effective and sustainable approach for improving water quality, optimizing plant operation, and supporting the development of next-generation wastewater treatment systems.
References:
Nirmalkar, N., Pacek, A. W., & Barigou, M. (2018). On the existence and stability of bulk nanobubbles. Langmuir, 34(37), 10964–10973.
Sharma, H., Nirmalkar, N., & Zhang, W. (2024). Nanobubbles produced by nanopores to probe gas-liquid mass transfer characteristics. Journal of Colloid and Interface Science, 665, 274–285.
Nirmalkar, N., Pacek, A. W., & Barigou, M. (2018). Interpreting the interfacial and colloidal stability of bulk nanobubbles. Soft Matter, 14(47), 9643–9656.
Jia, M., Farid, M. U., Kharraz, J. A., Kumar, N. M., Chopra, S. S., Jang, A., Chew, J., Khanal, S. K., Chen, G., & An, A. K. (2023). Nanobubbles in water and wastewater treatment systems: Small bubbles making a big difference. Water Research, 245, 120613.
Gurung, A., Dahl, O., & Jansson, K. (2016). The fundamental phenomena of nanobubbles and their behavior in wastewater treatment technologies. Geosystem Engineering, 19(3), 133–142.
Bibliometric analysis and review of nanobubbles applications to wastewater treatments: Challenges after 25 years of research. (2026). Cleaner Engineering and Technology, 32, 101240.
Nanobubbles as an emerging technology for water and wastewater remediation: Current outlook and future trends. (2026). Journal of Environmental Chemical Engineering, 14(3), 122540.
Nanobubble technology for water treatment: Fundamentals, transformative opportunities, and challenges to full-scale applications. (2025). Chemical Engineering Journal.
Metcalf & Eddy, Inc. (2014). Wastewater Engineering: Treatment and Resource Recovery (5th ed.). McGraw-Hill Education.
Tchobanoglous, G., Burton, F. L., Stensel, H. D., Tsuchihashi, R., & Abu-Orf, M. (2014). Wastewater Engineering: Treatment and Resource Recovery. McGraw-Hill.
American Water Works Association (AWWA). (Latest edition). Water Quality & Treatment: A Handbook on Drinking Water.
U.S. Environmental Protection Agency (EPA). Wastewater Technology Fact Sheets and Nutrient Removal Guidance.
Water Environment Federation (WEF). Operation of Municipal Wastewater Treatment Plants (MOP-11).
International Water Association (IWA). Water Practice and Technology publications on advanced aeration and wastewater treatment.
World Health Organization (WHO). Guidelines for the Safe Use of Wastewater, Excreta and Greywater.
United Nations Environment Programme (UNEP). Reports on sustainable wastewater management.
Pramanik, B., et al. (2026). Enhanced microplastic removal from wastewater using microbubble–nanobubble flotation. ES&T Water (American Chemical Society).
Agarwal et al., 2011, “Principle and applications of microbubble and nanobubble technology for water treatment”, Chemosphere, 84, 1175–1180.