Reverse osmosis (RO) is one of the most important water treatment technologies today. With a filtration precision at the 0.0001 μm level, it is widely applied in seawater desalination, industrial wastewater reuse, and high-purity water production. However, RO membranes are highly susceptible to fouling during operation, particularly by inorganic scaling (e.g., CaCO₃ and CaSO₄), which reduces water flux, increases transmembrane pressure (TMP), and accelerates membrane aging. Conventional mitigation strategies rely on antiscalants and acid/alkaline cleaning. While effective, these methods increase chemical costs, create risks of secondary pollution, and may leave fouling only partially removed while causing membrane damage. As a result, non-chemical, low-energy fouling control and cleaning approaches have become a priority for the industry.
Air microbubbles (MBs), with diameters in the micrometer range, bring new possibilities to RO systems. Their surfaces are charged, and their rupture is accompanied by micro-jets and localized turbulence. These unique features enable MBs to play a reinforcing role in both fouling inhibition and membrane cleaning.
In terms of fouling inhibition, MBs provide multiple effects:
- Electrostatic effect – MB surfaces carry negative charges, which attract positively charged ions and molecules in solution, thereby preventing them from depositing directly on the membrane surface.
- Nucleation interference – MBs act as alternative nucleation sites, promoting salt crystal growth in the bulk solution instead of on the membrane surface, effectively diverting scaling away from the interface.
- Turbulent disturbance – The motion and collapse of MBs disrupt the concentration polarization layer, reducing solute accumulation near the membrane.
Experimental evidence is clear: under conditions with both CaCO₃ and CaSO₄, continuous injection of MBs for four days increased flux retention from 55–63% to 83–86%. Remarkably, the performance not only surpassed the control group but also exceeded that of commercial antiscalants, showing that MBs can function as a green, chemical-free scaling control method.
For cleaning, MBs demonstrate equally strong potential. Unlike traditional chemical cleaning, MBs rely on physical disturbance:
- Brownian motion – MBs move randomly in solution, disrupting the concentration polarization layer and enhancing solute transport.
- Surface adsorption – With their large specific surface area, MBs adsorb surfactants and other active molecules, altering membrane wettability and surface tension, which makes fouling layers easier to detach.
- Interfacial collapse – Bubble collapse releases localized bursts of energy, physically dislodging fouling deposits.
In situ cleaning experiments have shown that MBs can restore RO membrane flux to 100% while increasing solute rejection by 0.8%. Compared with acid or alkaline cleaning, MB-assisted cleaning is milder, reduces chemical corrosion of the membrane, and lowers chemical discharge.
Operating conditions are critical to MB performance. At lower temperatures and pressures, MBs survive longer and function more effectively; appropriate airflow rates help increase bubble density at the membrane boundary layer, enhancing turbulence. By contrast, at higher temperatures and pressures, bubble collapse occurs more quickly, reducing their effectiveness. Therefore, optimal industrial application of MBs requires integration with process parameter optimization.
Looking forward, several trends are emerging. As the water treatment industry shifts toward greener and lower-chemical consumption operations, MBs are likely to be used in combination with conventional antiscalants or intelligent monitoring systems. Low-dose chemical dosing plus MBs could achieve effective fouling control at reduced cost, while real-time monitoring of flux and TMP could enable adaptive MB injection strategies for precise prevention. Long-term research is also needed to evaluate whether free radicals generated during bubble collapse may affect membrane material stability.
In summary, MBs enhance RO performance by inhibiting fouling through electrostatic adsorption, nucleation interference, and turbulence, while also reinforcing cleaning through Brownian motion, adsorption, and interfacial collapse. They not only stabilize flux and TMP but also reduce reliance on chemical agents, offering a greener and more cost-effective option for RO operation. With further research and the maturation of equipment, MBs are poised to become a key technology for fouling inhibition and cleaning in RO membranes, advancing the water treatment industry toward higher efficiency, lower energy use, and greater sustainability.
