Ultrafiltration (UF) membranes, with their excellent separation precision (0.001–0.1 μm), are widely applied in drinking water production and wastewater treatment. However, natural organic matter (NOM), proteins, polysaccharides, and other macromolecules tend to accumulate on the membrane surface and within its pores, leading to flux decline and increased transmembrane pressure (TMP). This fouling phenomenon remains a critical bottleneck that restricts the sustainable operation of UF technology. Conventional strategies such as backwashing, chemical cleaning, and pretreatment can mitigate fouling in the short term, but frequent cleaning, shortened membrane lifespan, and higher operational energy consumption are often unavoidable. As a result, exploring novel physical reinforcement methods to alleviate UF fouling has become a research hotspot in both academia and industry.
The application of air microbubbles (MBs) offers a promising new pathway for fouling inhibition in UF membranes. With diameters typically in the micrometer range, MBs can be uniformly dispersed in water, generating turbulence and interfacial effects that provide unique advantages in membrane processes. Studies have demonstrated that introducing MBs into UF feedwater can significantly reduce the aggregation and deposition of foulants on the membrane surface, thereby improving operational stability.
Firstly, MBs exert a fouling inhibition effect through dispersion and isolation mechanisms. Once introduced into the UF system, MBs act as tiny "separators," dispersing foulants and weakening their interactions. Natural organic matter that would otherwise aggregate into clusters becomes more evenly distributed under the influence of MBs. This dispersion not only reduces the likelihood of foulant deposition on the membrane surface but also decreases the compactness of the cake layer, making it more porous and looser, thus facilitating water permeation. Experiments have shown that the presence of MBs can markedly reduce the apparent viscosity of feedwater, which is a key factor in enhancing flux performance.
Secondly, MBs can modify the interactions between organics and the membrane surface. Humic acid (HA), a major component of organic fouling in UF membranes, is particularly affected. MBs attach to HA particles in solution, altering their ζ-potential distribution and reducing the tendency of charged particles to aggregate. This means MBs effectively prevent the formation of dense organic layers on the membrane surface. Studies indicate that in UF experiments with HA-containing feedwater, the introduction of MBs significantly increased normalized flux under both dead-end and cross-flow filtration modes, with maximum improvements reaching up to 139%. This highlights the critical role of MBs in controlling organic fouling.
Thirdly, MBs positively influence TMP and flux stability. Under conventional UF operation, flux steadily declines while TMP rises with prolonged use. With MBs, however, the looser fouling layer structure and weakened adhesion slow down flux decline and suppress TMP increases. This effect extends the effective operating time of the membrane and reduces the need for frequent cleaning and downtime.
In addition, MBs offer energy-saving and environmental benefits. By alleviating fouling, MBs reduce the frequency of backwashing and chemical cleaning, thereby conserving large amounts of water and chemicals and lowering both costs and environmental burdens. Moreover, reduced reliance on harsh cleaning agents minimizes chemical damage to the membrane material, helping to extend membrane lifespan and further reduce long-term replacement expenses.
It is worth noting that the effectiveness of MBs in fouling inhibition is influenced by several operational parameters. For instance, feedwater pH significantly affects MB performance. Studies have shown that when the pH is close to neutral (around 6), MBs exhibit the strongest inhibition against HA fouling. This is because HA molecules exhibit charge distributions most favorable to MB adsorption and dispersion under these conditions. Other factors such as MB concentration, bubble size, feedwater temperature, and pressure also play important roles. In general, lower temperature and pressure, moderate MB concentration, and higher airflow rates are beneficial for maximizing MB fouling inhibition performance.
Looking ahead, MBs hold significant potential for UF fouling control. On one hand, combining MBs with online monitoring systems could allow for real-time adjustment of MB dosage based on flux and TMP signals, achieving precise and adaptive fouling prevention. On the other hand, MBs could be integrated with adsorbent particles, eco-friendly chemical additives, or hybrid treatment strategies to form a "physical + chemical + material" multi-mechanism fouling control system, further enhancing performance. Additionally, future studies should address the long-term stability of MBs and their potential microscopic effects on membrane materials to ensure safety and reliability in large-scale industrial applications.
In summary, MBs in UF membranes effectively inhibit fouling caused by natural organic matter and other macromolecules through dispersion, interfacial regulation, and turbulence effects. They significantly slow flux decline and stabilize TMP. With their green and low-energy characteristics, MBs align with the sustainable development goals of water treatment technologies and provide strong support for broader adoption. As the technology matures and application strategies expand, MBs are expected to become a vital component of UF fouling control systems, delivering more efficient and environmentally friendly solutions for the water treatment industry.
