Nanobubbles (NBs), gaseous voids often less than 200 nm in size, have surfaced as a potential improvement to traditional dissolved-air flotation (DAF) methods. In contrast to traditional microbubbles (10–100 μm), NBs demonstrate an exceptionally high surface area, gradual ascent rates, prolonged stability, and charged surfaces (often with a zeta potential of –20 to –60 mV).
Nanoparticles quickly adhere to tiny particulates and pollutant colloids, promoting the clumping and flotation of hydrophobic and colloidal impurities. Recent laboratory and field investigations indicate substantial performance improvements in DAF with the use of NBs or amalgamated micro–nanobubble (MNB) streams.
Principal performance determinants encompass nanoparticle dimensions (~100 nm) and concentration (>10^8 bubbles/mL), coagulant composition to improve floc flotation, and refinement of saturation pressure and recycling ratio.
Nanobubbles significantly improve the adhesion of bubbles to particles and the kinetics of flotation, allowing the more effective elimination of fine and hydrophobic contaminants (such as oil, fine solids, algae, microplastics, and even heavy-metal precipitates). The synergy of NB-coagulants can enhance flocculation and the buoyancy of flocs.
1.0 NANOBUBBLE FUNDAMENTALS:
Nanobubbles (NBs) are typically defined as gas-filled cavities <1 μm in diameter, often subdivided into surface NBs and bulk NBs. In practice, NBs in water are generally <200 nm. By contrast, conventional DAF microbubbles range ~10–100 μm (mean ≈40 μm). The leading explanation is that NBs carry a strong negative surface charge (zeta potential typically –20 to –60 mV at neutral pH).
Typical NB diameters reported range from ~50 to 500 nm. Nanokriti’s nanobubble generators produce~80 nm mean nanobubble diameter, and >1×10^8 Nanobubbles/ml concentration with 75–80% Gas Transfer Efficiency.
Attachment Mechanisms: Nanoparticles readily adhere to solid or liquid particles via interfacial interactions; their extensive surface area and negative charge enhance the adsorption of neutral or positively charged pollutants. On hydrophobic particles (such as oils and microplastics), NBs can create surface coatings that enhance perceived hydrophobicity. In minerals, nanoparticles on particle surfaces have been demonstrated to enhance collector adsorption and improve recovery (fine phosphate ore research).
Essentially, NBs function as nanoscale flotation centres that adhere to minute particles, creating buoyant clusters. The use of coagulants (such as alum and polymers) often results in the formation of larger, more buoyant flocs through NB–particle interactions.
2.0 EFFECTS ON FLOTATION PERFORMANCE:
Bubble–Particle Attachment: The fundamental benefit of NBs is enhanced collision and attachment with fine particles. Smaller bubbles have far higher surface area per volume, increasing collision probability. NBs (especially charged NBs) can also attach to hydrophobic particle surfaces even without surfactant, forming a robust bubble–particle aggregate. Studies in mineral flotation and water treatment find NB presence sharply improves fine particle recovery. In DAF, micro- and nanobubbles work synergistically; MBs provide lift, NBs promote flocculation.
Rise Velocity and Coalescence: According to Stokes’ law, the rise of NB is minimal (affected by Brownian motion). Nevertheless, the aggregates they produce are far bigger, the ultimate flotation depends on the connected flocs ascending with the entrained microbubbles. Diminutive aggregates laden with NB ascend at a reduced rate compared to larger air bubbles, thereby facilitating extended contact duration (as posited in mineral flotation theory).
Flotation Kinetics: Empirical studies report accelerated flotation kinetics with NBs. In coal and mineral tests, nanobubble streaming reduced induction times (time required for a bubble to attach to a particle) and increased attachment rates. In water DAF, NB presence shortened the time to clarity in pilot tests.
Contaminant Removal, Oil & Fats: Oily waste exhibits favourable behaviour during floating. Nanobubble flotation, frequently utilized with microbubbles, proficiently eliminates emulsified oil. Wen et al. (2011) documented the near-total elimination of TSS, BOD, and COD from oily laundry and cleaning effluents using ozone/air microbubbles. NB systems need to likewise catch minute oils; NB stabilizes oil droplets and promotes their coalescence into buoyant flocs.
Chemistry & Dosing: The interaction between coagulants and flocculants is essential. Flotation experiments frequently combine alum or polymers with surfactants. Coagulants mitigate charge repulsion, facilitating the adhesion of NBs and forming larger flocs. Surfactants (collectors) can reduce bubble size and enhance adhesion. Monira et al. observed that dissolved organics (such as fats, oils, and grease) did not hinder NB-DAF; in fact, when combined with coagulants, they enhanced hydrophobicity and floc buoyancy.
3.0 CONCLUSION:
Nanobubble technology represents a significant advancement in dissolved air flotation by addressing one of the key limitations of conventional DAF, the efficient capture and flotation of fine and low-density particles. Their high surface area, long residence time, and stable charged interfaces enhance bubble–particle interactions, promote floc formation, and improve flotation kinetics. When combined with conventional microbubbles and appropriate coagulation chemistry, nanobubbles provide a synergistic effect that enhances the removal of suspended solids, oils, greases, algae, microplastics, and metal precipitates while improving overall process efficiency.
The ability to integrate nanobubbles into existing DAF systems with minimal process modification makes them an attractive retrofit technology for municipal and industrial wastewater treatment. By improving contaminant removal, reducing flotation time, and strengthening floc buoyancy, nanobubble-assisted DAF offers a practical approach to achieving higher treatment performance and more reliable operation across a wide range of water and wastewater applications.
4.0 REFERENCES:
- Nirmalkar, N., Pacek, A. W., & Barigou, M. (2018). On the existence and stability of bulk nanobubbles. Langmuir, 34(37), 10964–10973.
- Nirmalkar, N., Pacek, A. W., & Barigou, M. (2018). Interpreting the interfacial and colloidal stability of bulk nanobubbles. Soft Matter, 14, 9643–9656.
- Takahashi, M. (2005). ζ Potential of Microbubbles in Aqueous Solutions: Electrical Properties of the Gas–Water Interface. Journal of Physical Chemistry B, 109(46), 21858–21864.
- Agarwal, A., Ng, W. J., & Liu, Y. (2011). Principle and Applications of Microbubble and Nanobubble Technology for Water Treatment. Chemosphere, 84(9), 1175–1180.
- Temesgen, T., Bui, T. T., Han, M., Kim, T. I., & Park, H. (2017). Micro and Nanobubble Technologies as a New Horizon for Water Treatment Techniques: A Review. Advances in Colloid and Interface Science, 246, 40–51.
- Tsuge, H. (2014). Micro- and Nanobubbles: Fundamentals and Applications. Pan Stanford Publishing.
- Alheshibri, M., Qian, J., Jehannin, M., & Craig, V. S. J. (2016). A History of Nanobubbles. Langmuir, 32(43), 11086–11100.
- Edzwald, J. K. (2010). Dissolved Air Flotation and Me. Water Research, 44(7), 2077–2106.
- Edzwald, J. K. (Ed.). (2011). Water Quality & Treatment: A Handbook on Drinking Water (6th ed.). American Water Works Association (AWWA).
- Han, M., Kim, W., Dockko, S., & Park, J. (2002). Development of an Efficient Dissolved Air Flotation System with Microbubble Generation. Water Science and Technology.
- Rubio, J., Souza, M. L., & Smith, R. W. (2002). Overview of Flotation as a Wastewater Treatment Technique. Minerals Engineering, 15(3), 139–155.
- United States Environmental Protection Agency (EPA). Wastewater Technology Fact Sheet: Dissolved Air Flotation. U.S. EPA.
- American Water Works Association (AWWA). Water Quality & Treatment Manual. American Water Works Association.
- Relevant manufacturer technical literature and published research articles on nanobubble generation systems and Dissolved Air Flotation (DAF) technology.