Microfiltration (MF) membranes, as pressure-driven separation units, typically have pore sizes in the range of 0.1–1.0 μm, enabling them to effectively retain colloidal particles, suspended solids, macromolecular organics, bacteria, and some viruses. They are often used as a pretreatment barrier before ultrafiltration or reverse osmosis, providing stable water quality for downstream processes. However, in practical operation, MF membranes are highly susceptible to clogging by contaminants. In food-processing wastewater, oily effluents, and industrial wastewater with high organic content, deposits of fats, oils, and proteins can rapidly block membrane pores and cause severe flux decline. Such fouling not only reduces system efficiency but also increases the frequency of membrane cleaning and replacement, thereby raising operating costs. Conventional cleaning methods mainly rely on chemical agents such as alkalis, acids, and surfactants. While effective in the short term, they often involve high energy consumption, large chemical usage, environmental burden, and potential long-term damage to membrane materials. Therefore, developing greener and more efficient enhanced cleaning strategies has become a research priority.
The introduction of air microbubbles (MBs) provides a new approach to MF membrane cleaning. With diameters at the micrometer scale, MBs possess a high specific surface area, interfacial activity, and unique rupture effects. They can generate turbulence and micro-jets in fluids, thereby enhancing the detachment and dissolution of fouling layers. Compared with cleaning methods that rely solely on chemical reactions, MBs strengthen mass transfer through physical and mechanical effects, not only improving cleaning efficiency but also reducing chemical consumption.
Firstly, the core mechanism of MBs in MF cleaning lies in interfacial disturbance and foulant detachment. When MBs are dispersed in cleaning solutions, they attach to the membrane surface and foulant layers. During continuous rupture, they release instantaneous micro-jets, which can disrupt the adhesion structures of oil layers, protein gels, and other hydrophobic organics deposited on the membrane surface, making them easier to remove. At the same time, the presence of bubbles increases turbulence in the cleaning solution, accelerating the contact and reaction between cleaning liquid and foulants, and thus promoting faster dissolution or dispersion of surface deposits.
Secondly, MBs can enhance the action of chemical cleaning agents. In common MF cleaning, alkaline solutions (e.g., NaOH) are often used to remove oil-based foulants. However, due to the strong hydrophobicity and viscosity of oily substances, chemical cleaning alone often requires high concentrations and prolonged contact time. Studies have shown that when MBs with a diameter of about 4.5 μm and a concentration of around 10³/mL were introduced into NaOH cleaning solutions, the flux recovery rate improved by approximately 235% compared with NaOH alone. This is because MBs continuously agitate the foulant surface during cleaning, accelerating the emulsification and dissolution of oils, thereby allowing the cleaning solution to penetrate more deeply into the fouling layer. This "physical disturbance + chemical dissolution" synergy significantly shortens cleaning time and reduces the required dosage of cleaning agents.
In addition, MBs offer notable environmental and economic benefits. By reducing reliance on high-concentration chemical cleaners, MB-assisted cleaning lowers chemical consumption and discharge, minimizing secondary environmental pollution. From an energy perspective, MB cleaning mainly relies on bubble generation devices, which are more energy-efficient compared to high-intensity ultrasonic cleaning or extensive chemical backwashing. Moreover, lower chemical concentrations help extend membrane lifespan by reducing chemical corrosion, ultimately lowering replacement costs over the long term.
It should be emphasized that the effectiveness of MB-assisted cleaning is influenced by multiple parameters. Bubble size distribution, concentration, injection method, and the flow rate and temperature of the cleaning solution all directly affect cleaning performance. Generally, smaller bubbles, with their larger surface area, can interact with more foulants, while an appropriate concentration ensures sufficient turbulence without impeding fluid flow. Future research may further explore how to optimize bubble characteristics through generator design, enabling precise control and maximizing cleaning efficiency.
Looking ahead, the application of MBs in MF cleaning is expected to integrate with intelligent monitoring and adaptive cleaning strategies. For example, by monitoring changes in membrane flux and transmembrane pressure in real time, systems could automatically adjust the dosage and frequency of MB injection, allowing for preventive light cleaning before severe fouling occurs. In addition, MBs may be combined with low-concentration enzymatic agents or eco-friendly surfactants, forming a diversified and low-energy membrane cleaning system. In industries such as food processing, pharmaceutical wastewater treatment, and other high-fouling-load scenarios, the adoption of MB technology will further enhance the sustainability and cost-effectiveness of membrane separation processes.
In summary, the reinforcing effects of MBs on MF cleaning are mainly reflected in the physical disturbance and chemical synergy that significantly improve the efficiency and environmental compatibility of traditional cleaning processes. MBs not only effectively remove surface deposits and restore membrane flux but also reduce dependence on chemicals, extend membrane lifespan, and pave the way for smarter and greener water treatment practices. With further research and equipment optimization, MBs are expected to play an increasingly central role in industrial membrane systems, becoming an essential tool for membrane cleaning and maintenance.
