The pore size of RO reverse osmosis membrane is as small as nanometer (1 nanometer=10 * - 9 meters). Under certain pressure, water molecules can pass through the RO membrane, while inorganic salts, heavy metal ions, organics, colloids, bacteria, viruses and other impurities in the source water cannot pass through the RO membrane, so that the permeable pure water and the impermeable concentrated water can be strictly distinguished.

RO membrane is mostly used for water purifier in civil use. As one of the filtration devices for water purifier purification, it plays the most important role in the filtration from tap water to purified water. RO membrane can effectively remove calcium, magnesium, bacteria, organic substances, inorganic substances, metal ions and radioactive substances in water, and the crystal clear and sweet glycol purified by this device. The device is suitable for drinking water purification in families, hotels, hospitals and other enterprises and institutions.

The conductivity of pure water filtered by RO membrane of general tap water is 5 μ S/cm (RO membrane filtered outlet conductivity=inlet conductivity × Generally, the desalting rate of imported reverse osmosis membrane can reach more than 99%, and the operation within 5 years can guarantee more than 97%. For those with high requirements on water conductivity, the secondary reverse osmosis can be used. After simple treatment, the hydraulic conductivity is less than 1 μ S/cm), in line with the national laboratory level III water standard. After circulating filtration by atomic level ion exchange column, the outlet water resistivity can reach 18.2M. cm, which exceeds the national laboratory water standard (GB 6682-92).

Reasons for shortened service life of RO

1. Membrane performance damage caused by improper operation of reverse osmosis equipment

Residual gas in the reverse osmosis equipment operates under high pressure, and the formation of air hammer will damage the reverse osmosis membrane.

Two situations often occur:

A. After the equipment is emptied, when it is re-operated, the gas is not exhausted and the pressure is increased rapidly. The remaining air shall be exhausted under the pressure of 2~4 bar, and then the pressure shall be increased gradually for operation.

B. When the joint between the pre-treatment equipment and the high-pressure pump is poorly sealed or leaks (especially the water leakage of the microfilter and the pipeline behind it), when the pre-treatment water supply is not sufficient, if the microfilter is blocked, some air will be sucked in due to the vacuum in the poorly sealed place. The microfilter should be cleaned or replaced to ensure that the pipeline does not leak. In short, the flow meter should be operated gradually with no bubbles in it. If bubbles are found during the operation, the reasons should be checked gradually.

2. The method of reverse osmosis equipment shutdown is incorrect

A. The rapid depressurization was not thoroughly flushed during shutdown. Because the concentration of inorganic salt at the concentrated water side of the membrane is higher than that of raw water, it is easy to scale and pollute the membrane.

B. Rinse with pretreated water added with chemical reagent. Water containing chemical reagents may cause membrane pollution during equipment shutdown.

When the reverse osmosis equipment is ready to shut down, it should stop adding chemical reagents, gradually reduce the pressure to about 3 bar and wash with pretreated water for 10 minutes until the TDS of the concentrated water is close to the TDS of the raw water.

3. Microbial contamination caused by poor disinfection and maintenance of reverse osmosis equipment

This is a common problem in the use of composite polyamide film, because the polyamide film has poor residual chlorine resistance, does not properly add chlorine and other disinfectants in use, and users pay insufficient attention to the prevention of microorganisms, which is easy to lead to microbial pollution. At present, the pure water microorganism produced by many manufacturers exceeds the standard, which is caused by poor disinfection and maintenance.

The main performance is that the RO equipment is not maintained with disinfectant when leaving the factory; The entire pipeline and pretreatment equipment are not disinfected after the equipment is installed; Disinfection and maintenance measures are not taken for intermittent operation; The pretreatment equipment and reverse osmosis equipment are not disinfected regularly; The maintenance fluid fails or its concentration is insufficient.

4. Poor residual chlorine monitoring of reverse osmosis equipment

1. If the dosing pump fails or the solution fails, or the membrane is damaged due to residual chlorine when the activated carbon is saturated.

2. Damage of membrane performance caused by untimely cleaning and incorrect cleaning method

In addition to the normal attenuation of equipment performance, the degradation of equipment performance caused by pollution is more serious during the use of equipment. The common pollution mainly includes chemical scale, organic matter and colloid pollution, microbial pollution, etc. Different pollution shows different symptoms. The symptoms of membrane pollution proposed by different membrane companies also have certain differences.

In the project, we found that the duration of pollution is different, and the symptoms are also different. For example, when the membrane is polluted by calcium carbonate scale and the pollution time is one week, the main performance is the rapid decrease of desalting rate, the slow increase of pressure difference, and the insignificant change of water yield. The performance can be completely restored after cleaning with citric acid. The pollution time is one year (a water purifier), the salt flux increases from the initial 2mg/L to 37mg/L (the raw water is 140mg/L~160mg/L), and the water yield decreases from 230L/h to 50L/h. After cleaning with citric acid, the salt flux decreases to 7mg/L, and the water yield increases to 210L/h.

Moreover, pollution is often not single, and its symptoms are also different, making it more difficult to identify pollution.

The identification of pollution type shall be based on the raw water quality, design parameters, pollution index, operation records, equipment performance changes and microbial indicators:

(1) Colloid pollution: Colloid pollution is usually accompanied by the following two characteristics:

A. The micro-filter is blocked quickly in the pretreatment, especially the pressure difference increases rapidly.

B. The SDI value is usually above 2.5.

(2) Microbial pollution: when microbial pollution occurs, the total number of bacteria in the permeated water and concentrated water of RO equipment is relatively high, and it must not be maintained and disinfected as required.

(3) Calcium scale: it can be judged according to the raw water quality and design parameters. For carbonate water, if the recovery rate is 75%, scale inhibitor is added in the design, and the LSI of the concentrate should be less than 1; When the scale inhibitor is not added, the LSI of the concentrated solution shall be less than zero, and generally no calcium scale will be generated.

(4) The performance change of different parts of the test assembly can be judged by inserting 1/4 inch PVC plastic pipe into the assembly.

(5) Determine the type of pollution according to the change of equipment performance.

(6) Acid cleaning (such as citric acid and dilute HNO3) can be used to determine calcium scale according to the cleaning effect and cleaning solution, which is further confirmed by the analysis of cleaning solution composition.

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

After determining the type of pollution, it can be cleaned according to the method in Table 1, and then disinfected for use. When the type of pollution cannot be determined, the steps of cleaning (3)+disinfection+0.1% HCl (pH 3) are usually used for cleaning.

Prevention of membrane performance damage and maintenance

1. Prevent damage to membrane performance

New reverse osmosis membrane elements are usually soaked in 1% NaHSO3 and 18% glycerol aqueous solution and stored in sealed plastic bags. If the plastic bag is unbroken and stored for about one year, its life and performance will not be affected. When the plastic bag is opened, it should be used as soon as possible to avoid negative effects on the components due to the oxidation of NaHSO3 in the air. Therefore, the membrane should be unsealed before use.

After the test run of reverse osmosis equipment, we used two methods to protect the membrane. Test run the equipment for two days (15~24h), and then use 2% formaldehyde solution for maintenance; Or use 1% NaHSO3 aqueous solution for maintenance after 2~6h operation (the air in the equipment pipeline shall be drained to ensure that the equipment does not leak, and all inlet and outlet valves shall be closed). Both methods can achieve satisfactory results. The first method is more expensive and can be used when the idle time is long, and the second method can be used when the idle time is short.

2. Maintenance

After a period of normal operation, the membrane elements will be contaminated by suspended or insoluble substances that may exist in the feedwater. When the system performance drops by 10% under standard conditions, or scaling or fouling obviously occurs, it should be cleaned in time. Regular water flushing and chemical dosing cleaning can restore the performance of membrane elements and extend the service life of membrane elements.

During the short-term shutdown of the system, the system shall be flushed every 5 days, and the valve shall be closed after flushing to prevent scaling and blocking.

The system is shut down for a long time (more than 30 days), and 1% sodium bisulfite solution is added to prevent bacterial proliferation.