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Causes and Solutions for the “Seven Major Phenomena” of Reverse Osmosis Membranes
Reverse osmosis membranes operate by using pressure as the driving force to separate solvents from solutions.They have proven highly effective in industries.
Mar 12th,2026
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Reverse osmosis emerged as a rapidly developing water treatment process in the 1960s. Today, it is widely applied in municipal water supply, boiler feedwater, power plant boiler feedwater, industrial wastewater treatment, seawater desalination, and the separation of solutes in various solutions. Reverse osmosis membranes operate by using pressure as the driving force to separate solvents from solutions. They have proven highly effective in industries such as food processing, electronics, pharmaceuticals, and textile dyeing. However, prolonged use inevitably leads to certain issues. How do these “phenomena” manifest during reverse osmosis membrane operation?
01 Scaling Phenomenon
Users occasionally encounter problems with purified water equipment, such as reverse osmosis membrane scaling or leaks at equipment joints. Initially, these may seem minor, but over time, they can escalate significantly. Let's explore the causes of reverse osmosis membrane scaling. First, during low-pressure flushing in purified water systems, freshwater is produced. This naturally increases the concentration of water on both sides while also intensifying salt concentration. Since salts contain substantial precipitable substances, prolonged exposure inevitably leads to scaling. Second, uneven chemical dosing is another major cause of reverse osmosis membrane scaling. If the scale inhibitor system leaks severely, it can significantly affect the dosage of the inhibitor. If operators fail to pay sufficient attention during operation—such as neglecting timely flushing during shutdowns—it often leads to reverse osmosis membrane scaling.
Methods to Prevent Reverse Osmosis Membrane Fouling
1. Maintain appropriate operating pressure during reverse osmosis system operation. Generally, increasing pressure boosts water production, but excessive pressure may compact the membrane.
2. Maintain turbulent flow on the brine side during operation to reduce concentration polarization on the membrane surface and prevent precipitation of certain insoluble salts.
3. When water production from the reverse osmosis system significantly decreases, indicating membrane scaling or fouling, perform chemical cleaning.
4. Conduct regular raw water quality analysis. Adjust the dosage of scale inhibitors based on water quality conditions and the dosing schedule provided by the technical support department.
5. Immediately shut down the reverse osmosis system when abnormalities occur to prevent escalation of the issue.
6. Strictly prevent leaks, spills, drips, or overflows from dosing equipment. Ensure chemicals are added to the reverse osmosis feed water to effectively prevent scaling.
7. It is recommended to switch the post-shutdown flushing method for reverse osmosis equipment to deionized water flushing.
8. Ensure thorough pretreatment of raw water, with particular attention to meeting pollution index standards. Simultaneously perform disinfection to prevent microbial growth within the equipment, while being mindful of chloride ion damage to membranes.
9. During reverse osmosis equipment shutdowns, perform chemical flushing for short-term storage and add formaldehyde for long-term protection.
02 Oxidation Phenomena
Oxidation of reverse osmosis membranes is another common occurrence during operation, inevitably impairing performance. Several prevalent oxidation phenomena include: 1. Residual Chlorine Attack: Chlorine-based disinfectants added to the feedwater, not fully consumed during pretreatment, enter the reverse osmosis system and cause oxidation. 2. Catalytic oxidation reactions between trace residual chlorine and heavy metal ions in the feedwater within the desalination layer. 3. Use of oxidizing disinfectants other than residual chlorine during water treatment, such as chlorine dioxide, potassium permanganate, ozone, or hydrogen peroxide, which can all cause oxidation.
Preventive Measures Against Oxidation
1. Ensure reverse osmosis feedwater contains no residual chlorine.
a. Install online ORP (Oxidation-Reduction Potential) or residual chlorine detection instruments in the reverse osmosis feed pipeline. Continuously monitor and ensure feedwater is chlorine-free by dosing reducing agents like sodium bisulfite.
b. For systems using treated wastewater as feedwater with ultrafiltration (UF) as pretreatment, chlorination is typically employed to control microbial contamination in UF. Under such operating conditions, combine online monitoring with periodic offline testing to measure residual chlorine and ORP in the water.
2. Separate the reverse osmosis (RO) cleaning system from the ultrafiltration (UF) cleaning system to prevent residual chlorine leakage from the UF system into the RO system.
3. Select non-oxidizing biocides such as isothiazolinone or DBNPA for the reverse osmosis system's disinfection process.
03 Rupture Phenomenon
Backpressure on the product water side of reverse osmosis membranes can cause membrane detachment or damage. To prevent this, a bursting disc can be installed on the product water side. If backpressure rises to the bursting disc's rupture pressure due to clogged pipelines or closed valves, the disc will rupture. Simply replace it with a new one.
How to Prevent Membrane Performance Degradation
New reverse osmosis membrane elements are typically stored in sealed plastic bags after being immersed in a solution of 1% NaHSO₃ and 18% glycerol. If the plastic bag remains intact, storage for approximately one year will not affect the membrane's lifespan or performance. Once the plastic bag is opened, the membrane should be used promptly to avoid adverse effects caused by the oxidation of NaHSO₃ in air. Therefore, membranes should ideally be opened just before use. After commissioning the reverse osmosis system, two methods can be employed to protect the membranes: 1. Run the system for two days (15–24 hours), then maintain with a 2% formaldehyde solution;
2. After 2–6 hours of operation, maintain with a 1% NaHSO₃ aqueous solution (ensure all air is purged from the equipment piping, confirm no leaks, and close all inlet/outlet valves). Both methods yield satisfactory results. The first method is more costly and suitable for extended downtime, while the second is appropriate for shorter periods of inactivity.
04 Contamination Phenomena
RO membranes are the core technology of reverse osmosis. To rapidly determine whether a reverse osmosis unit is contaminated, it is essential to understand the contamination phenomena and states associated with reverse osmosis. 1. Membrane Degradation: Degradation of Hydranautics RO membrane elements occurs due to hydrolysis of cellulose acetate membranes caused by excessively low or high pH levels, oxidation (e.g., from various oxidizing agents), and mechanical damage such as water-injection compression, membrane roll bulging, overheating, or abrasion from fine carbon particles or sand.
2. Precipitate Deposition: Insufficient or improperly implemented measures can lead to precipitate buildup, commonly including carbonate scale, sulfate scale, and silica scale.
3. Colloidal Deposition: Colloidal deposits are typically caused by metal oxides and various other colloidal substances.
4. Organic Deposits: Natural organic matter, oils, excessive scale inhibitors, iron precipitation, and excess cationic polymers (originating from pretreatment filters) are all sources of organic deposits.
5. Biological Contamination: Microorganisms form biofilms on the surface of Hydranautics reverse osmosis membrane elements, while bacteria can erode cellulose acetate membranes. These microorganisms include algae, fungi, and others.
Reverse Osmosis Membrane Cleaning Methods
Membrane cleaning methods fall into two categories: physical and chemical.
1. Physical Methods
Physical cleaning removes contaminants from the membrane surface mechanically. Methods include: forward flushing, alternating direction flushing, backflushing, vibration cleaning, venting and flushing, air jetting, carbon dioxide cleaning, and automated sponge ball cleaning. Among these, alternating direction flushing is the most effective physical cleaning method.
2. Chemical Methods
Chemical cleaning typically employs chemical agents such as dilute acids, dilute alkalis, enzymes, surfactants, and binding agents. Acidic cleaners dissolve and remove minerals and DNA. Chemicals like citric acid and EDTA are effective against scale and alkaline contaminants. Descaling agents such as Biz and Ultrasol efficiently eliminate biological fouling.
For specific contaminants or contamination scenarios, specialized chemical cleaning agents from RO chemical suppliers may be required. Always adhere to the product performance specifications and usage instructions provided by the chemical supplier during application. In special cases, contaminated membrane elements may be removed from the reverse osmosis unit for testing and pilot cleaning to determine suitable chemicals and cleaning protocols. When employing chemical cleaning, the selected chemicals must be compatible with the membrane material. Cleaning must strictly adhere to the conditions specified by the membrane manufacturer (pressure, temperature, pH, and flow rate) to prevent irreversible membrane damage.
3. Implement Preventive Cleaning
After prolonged operation, scaling substances may form on the membrane surface. When cleaning conditions are not yet reached, these deposits act as nucleation sites for crystal growth, accelerating scaling and causing rapid increases in pressure drop across reverse osmosis stages. Preventive cleaning maintains membrane surface cleanliness and mitigates concentration polarization effects.
05 Telescope Phenomenon
During visual inspection of reverse osmosis membranes, procedures primarily include visual examination and element weight measurement. Visual inspection involves observing and documenting contamination and wear on membrane element end caps, end faces, membrane coils, outer FRP wrapping, central tubes, sealing rings, etc., to determine if elements exhibit “telescope” phenomena.
At this point, many may wonder: How can a reverse osmosis membrane become a “telescope”? In reality, the “telescope effect” in reverse osmosis membranes represents mechanical damage. It occurs when the outer jacket of the membrane element becomes misaligned and shifts downstream, sometimes even overlapping onto the next membrane element. Mild cases of the telescope effect may not necessarily damage the membrane element, but severe instances can lead to rupture of the bonding line and membrane sheets.
The so-called “telescope phenomenon” occurs when the pressure differential between the feedwater side and concentrate side of the reverse osmosis system exceeds the limit value. This causes the diaphragms and diaphragms, as well as the diaphragms and central tube, within the reverse osmosis membrane element to slip, resulting in one end of the membrane element's diaphragm concaving inward and the other end protruding outward, resembling a telescope.
Causes of this phenomenon:
First, failure to follow installation or removal procedures, leading to relative displacement between membranes.
Second, insufficient structural rigidity of the membrane housing, causing deformation under pressure during operation.
A significant additional cause is excessive rapid increase in system inlet pressure during high-pressure pump startup. It is worth noting that 8-inch membrane elements are more prone to this phenomenon due to their larger membrane cross-sectional area. It is essential to ensure that stress rings are installed within the membrane pressure vessel to support the outer jacket of 8-inch membrane elements. Smaller diameter membrane elements are supported by their product water pipes and their respective stress rings to prevent slippage of the outer jacket. Once the telescope effect occurs, the damaged membrane elements should be replaced with new ones, and the underlying causes should be addressed.
06 Compaction Phenomenon
When membranes become compacted, they typically exhibit reduced permeate flow and increased salt rejection. Under normal operation, membrane compaction is rare. However, significant compaction tendencies may occur under the following conditions: excessively high feed pressure, high temperatures, water hammer effects, or starting the high-pressure pump when air is present in the reverse osmosis system. Rapid pressure increases in the high-pressure pump can generate strong impact forces on the RO membranes, causing water hammer effects that may damage the reverse osmosis membranes.
Specific Solutions
Once membrane elements become compacted, damaged elements must be replaced promptly, or additional membrane elements should be added downstream in the system.
07 Concentration Polarization
During reverse osmosis, as water continuously passes through the membrane, a concentration gradient forms between the brine near the membrane surface and the incoming feedwater. The solution near the membrane surface has a higher concentration, known as concentration polarization. As water flows extensively through the membrane, the surface solution becomes highly concentrated and supersaturated. Certain salts with low solubility, such as CaSO₄ and MgSO₄, gradually precipitate as crystals. Initially, these are minute single crystals lacking nucleation sites, preventing growth. They can only deposit on the membrane surface or reach dissolution equilibrium within the solution. As the concentration at the membrane surface continues to rise and the water flow reaches a certain equilibrium state, crystal nuclei form and begin to grow, gradually developing into plate-like or spiral structures. If the external temperature is suitable and there is minimal dissolution of substances, the crystals will gradually grow larger. A solid scale forms on the membrane surface, clogging the membrane and significantly reducing water production efficiency.
Measures to Eliminate Concentration Polarization in Reverse Osmosis Equipment
1. Increased Flow Rate Method: First, employ agitation techniques commonly used in chemical engineering. This involves increasing the linear velocity of fluid flow across the membrane surface. Reducing residence time and accelerating fluid velocity within the water purification system minimizes solute adsorption time. Higher flow rates inhibit solute adsorption.
2. Packing Method: Incorporate 29–100 μm spheres into the feed solution. These particles flow through the reverse osmosis system alongside the liquid, reducing the membrane boundary layer thickness and thereby increasing permeate flux. The pellets can be made of glass or methyl methacrylate. Additionally, for tubular reverse osmosis systems, micro-sponge balls can be added to the feed solution. However, this method is unsuitable for plate-and-frame or spiral-wound membrane modules, primarily due to the risk of clogging the flow channels.
3. Pulsing Method: Incorporating a pulse generator into the reverse osmosis pure water system. Varying pulse amplitude and frequency generally correlates with increased permeate flow rate—higher amplitude or frequency yields greater flow. Agitators are widely employed across test setups, with empirical evidence showing mass transfer coefficients linearly correlate with agitator rotational speed.
4. Turbulence Enhancers: These refer to various obstacles that intensify flow patterns. For tubular modules, internal spiral baffles are installed. For plate or spiral-wound membrane modules, mesh liners or similar structures can be added to promote turbulence. Such turbulence enhancers demonstrate excellent effectiveness.
5. Adding dispersant scale inhibitors: To prevent membrane scaling in reverse osmosis (RO) systems, sulfuric or hydrochloric acid is typically added to adjust pH. However, acid system corrosion and leakage pose operational challenges. Therefore, dispersant scale inhibitors are generally added to maintain normal water treatment system operation.
Huamo Environmental Protection advises users that concentration polarization can cause actual permeate flux and salt rejection rates to fall below theoretical estimates. Preventing concentration polarization primarily involves controlling recovery rate. The concentration polarization coefficient for a single membrane element is typically maintained below 1.2, meaning recovery rate is kept under 18%. This applies to individual elements; recovery rates increase for systems with longer process flows.
