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Microfiltration is a process of sieve-like separation under pressure difference, concentrating insoluble substances. It is commonly used to filter suspended particles between 0.02-10 μm in liquid mixtures. This technology is widely applied in pharmaceutical filtration and sterilization, high-purity water preparation in the electronics industry, food processing, drinking water production, and municipal wastewater treatment. For example, it removes insoluble proteins, polysaccharides, colloids, and bacteria from wine, thereby improving clarity, preserving color, aroma, and flavor, and achieving heatless sterilization. In beer and other alcoholic beverage brewing, it is used to remove microorganisms and off-flavors.
Due to its advantages of no phase change, low energy consumption, and a clean system, microfiltration technology is currently the most widely used and economically valuable membrane technology. However, membrane fouling occurs during microfiltration, altering membrane performance such as permeate flux and rejection rate, shortening membrane lifespan, and significantly impacting the practical application of microfiltration technology. Therefore, analyzing the causes of membrane fouling and implementing corresponding cleaning and prevention measures to partially or completely restore membrane performance is essential. In fact, research on membrane fouling cleaning has become a hot topic in membrane separation technology research.
1. Definition of Membrane Fouling
Membrane fouling refers to the phenomenon where particulates, colloidal particles, or large solute molecules in the treated material adsorb and deposit on the membrane surface or within the membrane pores due to physicochemical or mechanical interactions with the membrane, causing a reduction in membrane pore size or blockage, resulting in irreversible changes in permeate flow and separation characteristics. It should be said that membrane fouling begins once the feed solution comes into contact with the membrane. Membrane fouling commonly occurs in three situations: concentration polarization, adsorption of large solutes, and polymerization of the adsorbate layer.
The degree of membrane fouling is related to the membrane material, the concentration and properties of the solvent and large molecular solutes in the retention solution, the pH value, ionic strength, charge composition, temperature, and operating pressure of the solution. Severe fouling can reduce membrane flux by more than 80%. The obvious characteristics of membrane fouling are: a gradual decrease in the water migration rate per unit area (decreased membrane flux); a gradual increase in the pressure across the membrane and the pressure difference across the membrane (gradual increase in feed pressure and ΔP); and a gradual increase in the membrane's permeability to substances dissolved in water (decreased mineral rejection rate).
2. Factors Affecting Membrane Fouling
Factors affecting membrane fouling include: the relationship between particle or solute size and membrane pore size; membrane structure; interactions between the membrane, solute, and solvent; membrane surface roughness, porosity, and other physical properties; protein concentration; solution pH and ionic strength; temperature; and feed flow rate.
3. Methods for Preventing Membrane Fouling
Controlling the factors affecting membrane fouling can significantly reduce membrane fouling, extend the effective operating time of the membrane, reduce cleaning frequency, and improve production capacity and efficiency. The following measures can be taken to mitigate membrane fouling during use: pre-treating the feed solution before membrane filtration to remove larger particles; adjusting the pH value away from the isoelectric point of proteins to reduce adsorption; changing the properties of the membrane material or membrane surface; improving the structure of the membrane module and membrane system; and controlling solution temperature, flow rate, flow state, and pressure.
