Reverse osmosis (RO) is widely used for its stable and efficient desalination capacity, and its performance directly affects the stable operation of subsequent water-using equipment. However, membrane fouling often occurs during the daily operation of the system, leading to reduced water production, increased energy consumption, frequent cleaning, and shortened service life of membrane elements. Therefore, identifying the causes of membrane fouling and formulating reasonable cleaning methods are of great significance for maintaining the stable operation of RO systems.
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Inorganic scaling: Deposition of insoluble salts (such as calcium carbonate, magnesium carbonate, calcium sulfate, and calcium fluoride) on the membrane surface.
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Metal oxide fouling: Colloidal particles formed by iron, aluminum, manganese and other ions that block membrane pores.
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Organic fouling: Adhesion of natural organic matter, oil and grease to membrane pores, reducing the membrane's water permeability.
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Biological fouling: Formation of biofilms by bacteria and algae on the membrane surface, along with metabolic products clogging flow channels and accelerating membrane aging.
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Reduced water production efficiency: Decreased membrane flux leads to lower water output per unit time.
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Declined salt rejection rate: Membrane pore blockage and frequent cleaning reduce solute retention capacity, deteriorating product water quality.
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Increased operating pressure: To maintain water production, feed water pressure must be raised, accelerating fatigue damage to membrane modules and increasing energy consumption.
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Shortened membrane element service life: Frequent fouling reduces the replacement cycle of membrane elements, increasing operating costs.
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Standard permeate flow rate decreases by 10% to 15%.
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Standard system pressure difference increases by 10% to 15%.
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Standard system salt rejection rate decreases by 3% to 5% or there is a significant increase in permeate salinity.
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Membrane surface scaling is confirmed.
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Before long-term shutdown of the membrane system.
The above reference standards should be based on the system's operational performance during the initial 48 hours of operation.
Scaling is the most common type of membrane fouling in chemical water treatment, mainly including carbonate, sulfate, silica, and iron scales. Its main formation mechanism is that the solubility product of some salts exceeds the limit during concentration polarization, causing solutes to precipitate and deposit on the membrane surface. After scaling, the data is mainly characterized by increased pressure difference in the second stage and decreased salt rejection rate, especially in the second stage. When inspecting membrane elements, the last membrane element in the system shows the most obvious weight gain, with obvious powder precipitation on the end face after drying.
For scaling, the following cleaning schemes can be adopted for conventional acid-base cleaning:
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Carbonate scale: 0.2% HCl / 2% citric acid (pH 2, maximum temperature 45℃).
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Sulfate scale: 0.1% NaOH + 1% Na₄EDTA (pH 12, maximum temperature 30℃).
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Silica scale: 0.1% NaOH (pH 12, maximum temperature 30℃).
Colloidal fouling is mostly composed of fine suspended solids and high molecular polymers not fully removed from raw water, such as silica colloids and iron colloids. These substances are difficult to completely remove through conventional pretreatment methods. After entering the RO system, they adhere to the membrane surface to form "sludge", increasing the pressure difference. Colloids and particulates generally manifest as increased pressure difference in the first stage, decreased water production, and no significant change in salt rejection rate. Systems that add flocculants usually have colloidal fouling.
For colloids, the following schemes can be adopted for conventional acid-base cleaning:
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Iron and aluminum colloids: 2% citric acid (pH 2, maximum temperature 30℃) / 1% sodium dithionite (pH 4-5, maximum temperature 30℃).
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Silica colloids: 0.1% NaOH (pH 12, maximum temperature 30℃).
Organic pollutants mainly come from natural organic matter in raw water (such as humic acid, algae metabolites), residual organic flocculants, and surfactants. Their fouling mechanism usually involves the formation of a dense fouling layer through electrostatic and hydrogen bonding interactions with the membrane surface, affecting the membrane's selective permeability to water molecules. Organic fouling generally leads to increased pressure difference in both the first and second stages, decreased water production, and no significant change in salt rejection rate. There is obvious slime on the membrane housing and RO end faces. Organic fouling is usually accompanied by biological fouling.
For organic fouling, the following scheme can be adopted for conventional acid-base cleaning:
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First, clean with 0.1% NaOH + 0.025% sodium dodecylbenzene sulfonate (pH 12, maximum temperature 30℃).
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Then, clean with 0.2% HCl (pH 2, maximum temperature 35℃).
Note: For severe fouling, repeated cleaning is recommended. Special attention should be paid to cleaning flow rate and temperature during the process.
Biological fouling of RO membranes is one of the common problems in systems with long operation cycles. Microorganisms such as bacteria and fungi carried in raw water multiply on the membrane surface and form biofilms. Biofilms not only affect membrane water flux but also cause membrane structure damage by producing viscous metabolic products. Biological fouling generally results in increased pressure difference in both the first and second stages and decreased water production. There is obvious slime on the membrane housing and RO end faces. Organic fouling and biological fouling often occur simultaneously. After biological fouling occurs, a distinct fishy odor will be present when opening the RO end cover.
For biological slime, the formula used for conventional acid-base cleaning is the same as that for organic fouling. However, biological fouling generally requires two-step cleaning, and at least two cleaning cycles are needed.
During the daily operation of RO systems, membrane elements are prone to be affected by pollutants such as inorganic scales, microorganisms, colloids, and insoluble organic matter, leading to performance degradation. To ensure the stable operation of the membrane system and extend its service life, scientific and efficient cleaning strategies should be adopted. Combining chemical cleaning, combined processes, and physical auxiliary technologies can effectively restore membrane performance.
For different pollutants, Ochemate provides a variety of professional cleaning solutions. Compared with the traditional simple "acid + alkali" cleaning method, our solutions offer higher cleaning efficiency, safer and more controllable processes, and more reliable service guarantees. Choosing Oumei New Materials means choosing professional care for your membrane system.
