[Hongjie Water Service] Methods for Extending the Service Life of Reverse Osmosis Membranes

After the reverse osmosis equipment was tested, we used two methods to protect the membrane to extend its service life. Test run the equipment for two days (15-24 hours), and then maintain it with a 2% formaldehyde solution; Or after running for 2-6 hours, use a 1% NaHSO3 aqueous solution for maintenance (the air in the equipment pipeline should be drained to ensure that the equipment is not leaking, and all inlet and outlet valves should be closed)

1. Prevent damage to membrane performance

New reverse osmosis membrane components are usually soaked in 1% NaHSO3 and 18% glycerol aqueous solution and stored in sealed plastic bags. Even if the plastic bag is not broken and stored for about 1 year, it will not affect its lifespan and performance. When the plastic bag is opened, it should be used as soon as possible to avoid adverse effects on the components due to the oxidation of NaHSO3 in the air. Therefore, the membrane should be opened as much as possible before use

After the trial operation of the reverse osmosis equipment, we used two methods to protect the membrane. Test run the equipment for two days (15-24 hours), and then maintain it with a 2% formaldehyde solution; Or after running for 2-6 hours, use a 1% NaHSO3 aqueous solution for maintenance (the air in the equipment pipeline should be drained to ensure that the equipment does not leak, and all inlet and outlet valves should be closed). Both methods can achieve satisfactory results. The first method is more expensive and can be used for long periods of idle time, while the second method can be used for shorter periods of idle time.

Improper operation of reverse osmosis equipment causing damage to membrane performance

2.1 Residual gas in reverse osmosis equipment operating under high pressure can cause air hammer damage to the membrane

There are often two situations that occur: A. After the equipment is emptied, when it is restarted, the gas is not completely discharged and the pressure is rapidly increased. The remaining air should be discharged at a pressure of 2-4 bar before gradually increasing the pressure for operation. B. When the joint between the pre-treatment equipment and the high-pressure pump is poorly sealed or leaks water (especially the microfilter and its subsequent pipelines), and the pre-treatment water supply is not sufficient, if the microfilter is blocked, some air will be sucked in due to vacuum in the poorly sealed area. The microfilter should be cleaned or replaced to ensure that the pipeline is not leaking. In summary, the flow meter should be gradually pressurized and operated without any bubbles. If bubbles are found during operation, they should be gradually depressurized and the cause checked.

2.2 Incorrect method for shutting down reverse osmosis equipment

A. Rapid depressurization during shutdown without thorough flushing. Due to the higher concentration of inorganic salts on the concentrated water side of the membrane compared to the raw water, it is easy to scale and contaminate the membrane.

B. Rinse with pre-treatment water containing added chemical reagents. Water containing chemical reagents may cause membrane fouling during equipment shutdown.

When preparing to shut down the reverse osmosis equipment, the addition of chemical reagents should be stopped, and the pressure should be gradually reduced to about 3 bar. Rinse with pre treated water for 10 minutes until the TDS of the concentrated water is very close to that of the raw water.

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

This is a common problem in the use of composite polyamide films, as polyamide films have poor residual chlorine resistance and are not properly disinfected with chlorine or other disinfectants during use. In addition, users do not pay enough attention to microbial prevention, which can easily lead to microbial contamination. At present, many manufacturers produce pure water with excessive microorganisms, which is caused by inadequate disinfection and maintenance.

The main manifestation is that the RO equipment was not maintained with disinfectant when leaving the factory; Failure to disinfect the entire pipeline and pre-treatment equipment after installation; Intermittent operation without taking disinfection and maintenance measures; Failure to regularly disinfect pre-treatment equipment and reverse osmosis equipment; Maintenance fluid failure or insufficient concentration.

2.4 Insufficient residual chlorine monitoring in reverse osmosis equipment

If the pump for adding NaHSO3 fails or the solution fails, or if the membrane is damaged due to residual chlorine when the activated carbon is saturated.

Damage to membrane performance caused by untimely cleaning and incorrect cleaning methods

During the use of equipment, in addition to normal performance degradation, the degradation of equipment performance caused by pollution is more severe. The common pollutants include chemical scale, organic and colloidal pollution, microbial pollution, etc. The symptoms exhibited by different types of pollution are different. The symptoms of membrane fouling proposed by different membrane companies also vary to a certain extent.

In engineering, we found that the duration of pollution varies, and its symptoms also vary. For example, when calcium carbonate fouling occurs on the membrane and the fouling time is one week, the main manifestation is a rapid decrease in desalination rate, a slow increase in pressure difference, and no significant change in water production. Cleaning with citric acid can fully restore performance. The pollution time was one year (for a certain pure water machine), and the salt flux increased from the initial 2mg/L to 37mg/L (for raw water ranging from 140mg/L to 160mg/L). The water production decreased from 230L/h to 50L/h. After cleaning with citric acid, the salt flux decreased to 7mg/L, and the water production increased to 210L/h.

Furthermore, pollution is often not singular, and its symptoms vary, making it more difficult to identify pollution.

To identify the type of pollution, it is necessary to comprehensively evalsuate the raw water quality, design parameters, pollution index, operation records, equipment performance changes, and microbial indicators

(1) Colloidal pollution: When colloidal pollution occurs, it is usually accompanied by the following two characteristics: A. The microfilter is blocked quickly in the pre-treatment, especially the pressure difference increases quickly, B. The SDI value is usually above 2.5.

(2) Microbial contamination: When microbial contamination occurs, the total number of bacteria in the permeate and concentrated water of the RO equipment is relatively high, and maintenance and disinfection are not carried out as required.

(3) Calcium scale: can be judged based on the quality of raw water and design parameters. For carbonate water, if the recovery rate is 75% and scale inhibitors are added during the design, the LSI of the concentrated solution should be less than 1; When no scale inhibitor is added, the LSI of the concentrated solution should be less than zero, and generally no calcium scale will be generated.

(4) 1/4 inch PVC plastic pipes can be inserted into the component to test the performance changes of different parts of the component for judgment.

(5) Determine the type of pollution based on changes in equipment performance.

(6) Acid washing (such as citric acid, dilute HNO3) can be used to determine calcium deposits based on the cleaning effect and cleaning solution, and further confirmation can be obtained through analysis of the cleaning solution composition.

(7) Chemical analysis of cleaning solution: Take three samples for analysis: raw water, cleaning solution, and cleaning solution.

After determining the type of pollution, it can be cleaned according to the methods in Table 1 and then disinfected for use. When the type of pollution cannot be determined, a cleaning (3)+disinfection+0.1% HCl (pH 3) step is usually used for cleaning. The author's use of cleaning solution (1)+cleaning solution (3)+disinfection steps also has a good effect.

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