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Oxygen Transfer Efficiency of Nanobubbles Versus Microbubbles
Nanobubbles, typically less than 200 nm in diameter, exhibit superior OTE due to their high surface area-to-volume ratio, extended residence time, and enhanced mass transfer rates compared to microbubbles.
The cost savings associated by using nanobubbles compared to microbubbles of oxygen is significant.
Based on the OTE it is estimated to be in excess of a multiple of 15 times less oxygen for the same oxygenation result.
These properties enable effective oxygenation at depths up to 40 meters, where pressure and buoyancy challenges limit conventional methods.
Recent studies, highlights why nanobubbles offer a promising solution for hypoxia remediation in deep aquatic systems.
Introduction
Oxygenation of water bodies is critical for maintaining ecological balance, particularly in environments like Macquarie Harbour, where hypoxia has been a persistent issue affecting aquaculture and biodiversity.
Traditional aeration methods using macro-bubbles suffer from low OTE, often below 10%, due to rapid bubble rise and limited dissolution.
Bubble Size and Properties
Bubbles are classified by size: microbubbles range from 1 to 100 μm, while nanobubbles are typically smaller than 1 μm (often <100 nm).
Key physical properties influencing OTE include:
• Surface Area-to-Volume Ratio: Smaller bubbles have a higher ratio, facilitating greater gas-liquid
interaction. For the same gas volume, nanobubbles provide orders of magnitude more surface area
than microbubbles.
• Rise Velocity: Governed by Stokes' law, rise velocity decreases with bubble size. Microbubbles rise
at 0.1-1 mm/s, while nanobubbles exhibit near-zero buoyancy, remaining suspended for hours or
days.
• Stability and Internal Pressure: Nanobubbles have high Laplace pressure (up to several
atmospheres), making them resistant to collapse and enabling supersaturated oxygen levels (up to
30-40 mg/L, compared to ~10 mg/L for saturation).
These attributes result in nanobubbles outperforming microbubbles in mass transfer efficiency.
Performance at Depths Up to 40 Meters
At greater depths, hydrostatic pressure (increasing by ~1 atm per 10 m) compresses bubbles, affecting their behaviour.
Conventional macro-bubbles rise quickly, limiting oxygen delivery to surface layers. Microbubbles perform better but still buoy upward at moderate speeds.
Nanobubbles excel in deep environments due to:
• Neutral Buoyancy and Sinking: Their minimal rise velocity allows them to remain suspended or even
sink under certain conditions, delivering oxygen throughout the water column.
This enables effective transfer at depths where macro-bubbles fail.
Pressure Resistance: High internal pressure and surface charge (zeta potential) stabilise nanobubbles against compression, preventing premature collapse. They can implode at depth, releasing oxygen precisely where needed, such as hypoxic bottom layers.
Enhanced Dissolution: Depth increases gas solubility (Henry's law), and nanobubbles' large interfacial area amplifies this effect. Systems can achieve supersaturated DO levels (up to 40 times conventional aeration) even at 40 m, where pressure is ~5 atm.
• In applications like aquaculture ponds or harbors, nanobubbles maintain DO gradients, supporting
aerobic processes at the sediment-water interface.
For Macquarie Harbour, with depths up to 50 m and hypoxic zones below 20 m, nanobubbles could provide targeted oxygenation, outperforming the microbubbles likely used by the previously employed technology.
Conclusion
Nanobubbles surpass microbubbles in OTE through superior physical properties, achieving higher SOTR, (Standard Oxygen Transfer Rate) and energy efficiency. Their stability and low buoyancy make them ideal for deep-water oxygenation up to 40 meters, addressing challenges in pressured environments like Macquarie Harbour.
Implementing nanobubble technology will enhance remediation efforts, reduce operational costs, and improve ecological outcomes.