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The shocking truth about membrane fouling: How to improve water treatment technology through science?
Membrane fouling is an important technical challenge that plagues the implementation of many water treatment technologies. Membrane fouling occurs when solutions or particles are deposited on the surface or in the pores of the membrane, resulting in a decrease in membrane performance. This process can occur in technologies such as membrane bioreactors, reverse osmosis, front osmosis, membrane distillation, ultrafiltration, microfiltration or nanofiltration, and has become a major obstacle to the popularization and application of these technologies.
Membrane fouling can cause a sudden drop in flow, which can affect the quality of the water produced.
There are many factors for membrane fouling, including colloids (such as clay), microorganisms (such as bacteria and fungi), organic matter (such as grease and polyelectrolytes) and scaling (mineral precipitates). Membrane fouling can be divided into reversible and irreversible types based on the strength of particle attachment to the membrane surface. Reversible contamination can be removed by strong shear force or backwashing, while irreversible contamination occurs when particles are strongly attached and cannot be removed by physical cleaning methods.
Factors affecting membrane fouling
There are many factors that can change membrane fouling, such as system fluid dynamics, operating conditions, membrane properties, and solute material properties. Research shows that at low pressure, low feed concentration and high feed rate, the concentration polarization effect is minimal and the flow rate is almost proportional to the transmembrane pressure difference. However, in the high-pressure range, the flow rate is almost independent of the applied pressure, and this deviation arises from concentration polarization. At low inflow rates or high feed concentrations, flow restriction can be observed even at relatively low pressures.
Measurement of membrane fouling
Flow rate, transmembrane pressure (TMP), permeability and resistance are the best indicators for measuring membrane fouling. In constant flow operation, TMP increases to compensate for membrane fouling, while in constant pressure operation, the flow rate decreases due to membrane fouling. In some technologies such as membrane distillation, fouling reduces the rejection capacity of the membrane, so the degree of fouling can be measured through permeate water quality (e.g. conductivity measurement).
Control of membrane fouling
Although membrane fouling is an inevitable phenomenon in the membrane filtration process, it can be minimized through strategies such as cleaning, selection of appropriate membrane materials, and selection of operating conditions. Membrane cleaning can be performed by physical, chemical or biological methods. Physical cleaning methods include the use of bubbles, sponges, water jets or backwashing, while biological cleaning utilizes biological agents to remove all active microorganisms. Chemical cleaning involves the use of acids and alkalis to remove contaminants and impurities. In addition, researchers are also exploring the impact of different coatings on the wear resistance of membranes.
Selecting membrane materials suitable for specific operations, such as hydrophilic membranes for water filtration and hydrophobic membranes for membrane distillation, is critical to controlling membrane fouling.
The impact of membrane fouling on membrane mechanical properties
Membrane fouling can cause mechanical damage to membrane performance, leading to unnecessary pressure and flow gradients. Not only will the physical properties of the membrane be directly affected, but there may also be indirect effects due to antifouling strategies. For example, the backwashing process exerts strong shear forces on the membrane. This frequent cleaning of dirt causes the membrane to suffer fatigue damage, leading to failure. Studies have shown that long-term backwashing can cause cracks and physical structural damage, and may even shorten the service life of the membrane.
Most membrane materials are polymers, such as polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), etc. These materials usually have good elasticity and strength, but their physical properties will deteriorate under the accumulation of pollutants. destroyed.
On how to effectively control membrane fouling, continued in-depth research is needed to seek more optimized membrane technology and operating methods. As water becomes increasingly scarce, improving water treatment technology becomes increasingly important. How will the scientific community respond to this challenge and provide ongoing solutions in the future?
