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In my country, reverse osmosis technology has been widely used in desalination processes for various types of water. Currently, most of the low-pressure polyamide composite membranes used in pure water projects are imported. The assembly level and process of reverse osmosis equipment are relatively mature and perfect. However, during the use of the equipment, the service life and performance degradation of the membrane are relatively serious, often failing to reach the expected design level (such as a three-year warranty period). The main problems are improper use and maintenance of the membrane and membrane fouling [1]. This article discusses the above problems.
1. Preventing damage to membrane performance
New reverse osmosis membrane elements are usually soaked in a 1% NaHSO3 and 18% glycerol aqueous solution and then stored in a sealed plastic bag. If the plastic bag is not broken, it will not affect its service life and performance after about one year of storage. Once the plastic bag is opened, it should be used as soon as possible to avoid adverse effects on the elements due to the oxidation of NaHSO3 in the air. Therefore, the membrane should be opened before use as much as possible.
After the equipment is tested, we have used two methods to protect the membrane. The equipment should be run for two days (15-24 hours) for trial operation, followed by maintenance with a 2% formaldehyde solution; or after 2-6 hours of operation, maintenance should be performed with a 1% NaHSO3 aqueous solution (air in the equipment pipeline should be completely purged, leak-free, and all inlet and outlet valves closed). Both methods can achieve satisfactory results. The first method is more expensive and is used when idle time is long, while the second method is used when idle time is short.
2. Damage to membrane performance caused by improper equipment operation
2.1 Residual gas in the equipment operating under high pressure can form air hammer, which can damage the membrane.
Two situations often occur: A. After purging the equipment, if the gas is not completely purged before restarting, the pressure is rapidly increased. The remaining air should be purged at a pressure of 2-4 bar before gradually increasing the pressure. B. When the joint between the pretreatment equipment and the high-pressure pump is poorly sealed or leaks (especially leaks in the microfilter and downstream pipelines), if the pretreatment water supply is insufficient, and if the microfilter becomes clogged, some air will be drawn in due to vacuum at the poorly sealed areas. The microfilter should be cleaned or replaced to ensure the pipeline is leak-free. In short, the pressure should be gradually increased during operation, ensuring there are no air bubbles in the flow meter. If air bubbles are found during operation, the pressure should be gradually decreased to check the cause.
2.2 Incorrect Shutdown Procedures
A. Rapid pressure reduction without thorough flushing during shutdown. Because the concentration of inorganic salts on the concentrate side of the membrane is higher than that of the raw water, scaling and membrane fouling are easily formed. B. Flushing with pretreated water containing chemical reagents. Water containing chemical reagents may cause membrane fouling during equipment shutdown.
When preparing to shut down, stop adding chemical reagents, gradually reduce the pressure to approximately 3 bar, and flush with pretreated water for 10 minutes until the TDS of the concentrate is very close to that of the raw water.
2.3 Inadequate Disinfection and Maintenance 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, improper addition of disinfectants such as chlorine during use, coupled with insufficient attention to microbial prevention by users, easily leads to microbial contamination. Currently, many manufacturers produce purified water with excessive microbial levels, which is caused by inadequate disinfection and maintenance.
The main manifestations are: RO equipment was not maintained with disinfectant at the factory; the entire pipeline and pretreatment equipment were not disinfected after installation; disinfection and maintenance measures were not taken during intermittent operation; pretreatment equipment and reverse osmosis equipment were not disinfected regularly; and the maintenance solution was ineffective or insufficiently concentrated.
2.4 Inadequate Residual Chlorine Monitoring
For example, a malfunctioning pump or ineffective chemical solution for adding NaHSO3, or saturated activated carbon, can damage the membrane due to residual chlorine.
3. Damage to Membrane Performance Due to Untimely Cleaning and Incorrect Cleaning Methods
During equipment use, in addition to normal performance degradation, performance degradation caused by fouling is more severe. Common fouling types include chemical scale, organic and colloidal fouling, and microbial fouling. Different types of fouling present different symptoms. Different membrane companies also describe different symptoms of membrane fouling, as shown in Table 1.
In engineering practice, we have found that the symptoms vary depending on the duration of fouling. For example, when a membrane becomes fouled with calcium carbonate scale within one week, the main symptoms are a rapid decrease in desalination rate and a slow increase in pressure differential, while the permeate flow rate shows little change. Cleaning with citric acid can completely restore its performance. In a water purifier with a fouling period of one year (140-160 mg/L raw water), the salt flux increased from an initial 2 mg/L to 37 mg/L, and the permeate flow rate decreased from 230 L/h to 50 L/h. After cleaning with citric acid, the salt flux decreased to 7 mg/L, and the permeate flow rate increased to 210 L/h.
Furthermore, fouling is often not singular, and its symptoms can vary, making fouling identification more difficult.
Identifying the type of contamination requires a comprehensive assessment of raw water quality, design parameters, contamination index, operating records, equipment performance changes, and microbiological indicators:
(1) Colloidal contamination: Colloidal contamination is typically accompanied by the following two characteristics: A. Rapid clogging of the microfilter during pretreatment, especially a rapid increase in differential pressure; B. SDI values are usually above 2.5.
(2) Microbial contamination: Microbial contamination results in high total bacterial counts in both the permeate and concentrate of the RO equipment, indicating a lack of proper maintenance and disinfection.
(3) Scale buildup: This can be determined based on raw water quality and design parameters. For carbonate-type water, if the recovery rate is 75% and scale inhibitors were added during design, the LSI of the concentrate should be less than 1; without scale inhibitors, the LSI of the concentrate should be less than zero, and scale buildup is generally not expected.
(4) A 1/4-inch PVC plastic tube can be inserted into the assembly to test performance changes in different parts of the assembly for assessment.
(5) The type of contamination can be determined based on changes in equipment performance. (6) Acid washing (such as citric acid or dilute HNO3) can be used. The scale can be determined based on the cleaning effect and the cleaning solution, and further confirmed by component analysis of the cleaning solution.
(7) Chemical analysis of the cleaning solution: Take three samples—raw water, original cleaning solution, and cleaning solution—for analysis.
After determining the type of contamination, cleaning can be performed according to the methods in Table 1, followed by disinfection before use. When the type of contamination cannot be determined, the cleaning process is usually performed using cleaning (3) + disinfection + 0.1% HCl (pH 3). The authors also found the cleaning solution (1) + cleaning solution (3) + disinfection to be very effective.
