Detailed analysis of various reasons for shortened lifespan of reverse osmosis membranes-Hezhong Environment (Beijing) Co., Ltd. was established in 2003

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Detailed analysis of various reasons for shortened lifespan of reverse osmosis membranes

Published Time:

2023-03-03

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RO reverse osmosis membranes have pores as small as nanometers (1 nanometer = 10*-9 meters). Under certain pressure, water molecules can pass through the RO membrane, while impurities in the source water such as inorganic salts, heavy metal ions, organic matter, colloids, bacteria, and viruses cannot pass through the RO membrane, thus strictly separating the pure water that can pass through from the concentrated water that cannot.

In the civilian sector, RO membranes are most commonly used in water purifiers as one of the filtration devices for purification. They play a crucial role in the filtration process from tap water to purified water. RO membranes effectively remove calcium, magnesium, bacteria, organic matter, inorganic matter, metal ions, and radioactive substances from water. The water purified by this device is crystal clear and sweet. This device is suitable for household use and drinking water purification in hotels, hospitals, and other enterprises and institutions.

The conductivity of pure water after filtration by a typical RO membrane is 5μs/cm (RO membrane filtered water conductivity = influent conductivity × desalination rate. Generally, imported reverse osmosis membranes can achieve a desalination rate of over 99%, and can maintain over 97% for 5 years. For applications with higher requirements for effluent conductivity, a two-stage reverse osmosis system can be used, followed by simple treatment to achieve a water conductivity of less than 1μs/cm), meeting the national standard for Grade 3 laboratory water. After further filtration through an atomic-level ion exchange column, the resistivity of the effluent can reach 18.2MΩ.cm, exceeding the national standard for Grade 1 laboratory water (GB 6682—92).

Reasons for Reduced RO Lifespan

Improper operation of the reverse osmosis equipment causing damage to the membrane performance

Residual gas in the reverse osmosis equipment operating under high pressure, forming water hammer that can damage the reverse osmosis membrane.

Two situations often occur:

A After the equipment is drained, when restarting, the air is not completely exhausted before quickly increasing the pressure. The remaining air should be exhausted at a pressure of 2-4 bar before gradually increasing the pressure.

B If the joint between the pretreatment equipment and the high-pressure pump is not well sealed or leaks (especially if the microfiltration unit and subsequent pipelines leak), when the pretreatment water supply is insufficient, such as microfiltration blockage, some air may be sucked in due to vacuum at the poorly sealed location. The microfiltration unit should be cleaned or replaced, and the pipeline should be ensured to be leak-free. In short, the pressure should be gradually increased when there are no bubbles in the flow meter, and if bubbles are found during operation, the pressure should be gradually reduced to check the cause.

Incorrect shutdown method of the reverse osmosis equipment

A Rapid depressurization during shutdown without thorough rinsing. Since the concentration of inorganic salts on the concentrate side of the membrane is higher than that of the raw water, it is easy to scale and pollute the membrane.

B Rinsing with pretreated water containing added chemical reagents. Water containing chemical reagents may cause membrane fouling during equipment shutdown.

C When the reverse osmosis equipment is ready to be shut down, the addition of chemical reagents should be stopped, and the pressure should be gradually reduced to about 3 bar, and the system should be rinsed with pretreated water for 10 minutes until the TDS of the concentrate is very close to that of the raw water.

Poor disinfection and maintenance of the reverse osmosis equipment leading to microbial contamination

This is a common problem in the use of composite polyamide membranes, because polyamide membranes have poor resistance to residual chlorine. If disinfectants such as chlorine are not added correctly during use, and users do not pay enough attention to microbial prevention, microbial contamination is likely to occur. Currently, many manufacturers' pure water has exceeded the microbial standard, which is caused by poor disinfection and maintenance.

This mainly manifests as: the RO equipment is not maintained with disinfectant at the time of leaving the factory; the entire pipeline and pretreatment equipment are not disinfected after the equipment is installed; disinfection and maintenance measures are not taken during intermittent operation; the pretreatment equipment and reverse osmosis equipment are not disinfected regularly; the maintenance solution is invalid or the concentration is insufficient.

Preventing Membrane Damage and Maintenance

Preventing Membrane Damage

New reverse osmosis membrane elements are usually immersed in a 1% NaHSO3 and 18% glycerol aqueous solution and stored in sealed plastic bags. If the plastic bag is not broken, storage for about one year will not affect its lifespan and performance. Once the plastic bag is opened, it should be used as soon as possible to avoid adverse effects on the element due to the oxidation of NaHSO3 in the air. Therefore, the membrane should be opened as close to the time of use as possible.

After the reverse osmosis equipment is tested, we have used two methods to protect the membrane. The equipment is tested and run for two days (15-24 hours), and then maintained with a 2% formaldehyde solution; or after running for 2-6 hours, it is maintained with a 1% NaHSO3 aqueous solution (the air in the equipment pipeline should be exhausted, ensuring that the equipment does not leak, and closing all inlet and outlet valves). Both methods can achieve satisfactory results. The first method is more expensive and is used when the idle time is long, while the second method is used when the idle time is shorter.

Maintenance

After a period of normal operation, the membrane elements may be contaminated by suspended solids or insoluble substances that may be present in the feed water. Under standard conditions, when the system performance drops by 10%, or obvious scaling or fouling occurs, cleaning should be carried out promptly. Regular water rinsing and chemical cleaning can restore the performance of the membrane elements and extend their service life.

During short-term shutdown of the system, the system should be rinsed every 5 days, and the valves should be closed after rinsing to prevent scaling and blockage.

For long-term shutdown of the system (more than 30 days), add a 1% sodium bisulfite solution to prevent bacterial growth.