In membrane separation technology, membrane distillation (MD) has gradually become an important method in the field of water treatment due to its high separation precision and adaptability to high-salinity water sources. However, the long-term operation of this process is inevitably constrained by membrane scaling. The crystallization and deposition of inorganic salts (such as CaSO₄ and CaCO₃), as well as the accumulation of organic matter (such as humic acid complexes), can clog the membrane surface or pores, thereby reducing water flux and shortening membrane lifespan. Traditional scaling control relies heavily on chemical pretreatment and cleaning agents, which, although effective, are associated with large chemical consumption, high operating costs, and possible side effects such as membrane pore wetting. In recent years, microbubbles (MBs) have been introduced into the MD process because of their unique physicochemical properties, showing remarkable advantages in scaling inhibition and membrane cleaning enhancement.
Firstly, the role of MBs in inhibiting scaling in MD is mainly reflected in two aspects: interfacial isolation and nucleation interference. When microbubbles enter the membrane channel, they form tiny gas-liquid interfaces on the membrane surface, acting as a barrier that partially blocks the direct deposition of Ca²⁺ ions and reduces the likelihood of inorganic salt crystals adhering to the membrane. At the same time, MBs themselves can serve as nucleation sites for crystallization, leading salts to preferentially nucleate and grow in the liquid phase rather than on the membrane surface. This mechanism significantly alleviates the rate of scaling. Studies have shown that after the introduction of MBs into simulated high-salinity feedwater, the system's Critical Concentration Factor (CCF) increased from 1.2 to 1.95, indicating a delayed scaling risk. Although the anti-scaling effect of MBs decreases with higher feedwater temperatures, compared with control groups without MBs, the system stability is still significantly improved.
Secondly, the introduction of MBs can also alleviate organic fouling in MD. Humic acid (HA) and its complexes with cations often cause severe fouling during the process. MBs attach to HA particles, altering their charge distribution and ζ-potential, thereby suppressing HA–cation interactions and reducing the probability of combined organic-inorganic fouling. Experimental data showed that after adding MBs, the normalized water flux improved from 19.7% to 37.0%, clearly demonstrating the potential of MBs in mitigating organic fouling.
In addition to anti-scaling, MBs also exhibit outstanding effects in cleaning fouled membranes. The rupture of bubbles in the fluid generates instantaneous micro-jets and local turbulence. This mechanical disturbance helps dislodge fouling layers already formed on the membrane surface. Particularly under moderate feed flow rates (e.g., 0.4–0.8 L/min), the turbulence induced by MBs is more pronounced, making it easier for fouling substances to be removed. Although this cleaning effect weakens at higher flow rates (e.g., 1.2 L/min), MBs still remain effective in removing inorganic crystals and organic deposits. As this physical cleaning method does not rely on additional chemicals, it offers advantages in reducing both operational costs and environmental impacts.
Looking ahead, MB technology holds broad application prospects in MD. On one hand, combined with MD process optimization, it is possible to further determine the optimal injection conditions for MBs, such as bubble concentration, size distribution, and feed flow rate, thereby achieving dual enhancement in scaling inhibition and cleaning. On the other hand, MBs may be coupled with low-dose antiscalants or temperature control strategies to form a "physical + chemical" hybrid anti-scaling system, balancing efficiency and cost-effectiveness. It should be noted, however, that despite the promising results, the stability of MBs under high-temperature conditions and their potential long-term effects on membrane material structures still require further investigation.
In conclusion, microbubbles in membrane distillation can effectively mitigate inorganic salt crystallization and organic fouling through interfacial effects and turbulence, while also demonstrating unique physical reinforcement in membrane cleaning. They not only contribute to improved water flux and extended membrane lifespan but also offer a feasible pathway for MD to evolve toward greener and more chemical-saving processes. With continued research, MBs are expected to become a key component in the scaling prevention system of MD, injecting new momentum into the sustainable development of the water treatment industry.
