How to extend the service life of reverse osmosis membrane

Reverse osmosis membranes are an important component of water treatment equipment and are widely used in electronics, power, automotive, food, medical and other fields. All mechanical equipment has a service life, and the service life of water treatment equipment, in addition to design and material factors, is also closely related to the life of the reverse osmosis membrane. Here, Water Treatment New Vision shares with you how to maintain the reverse osmosis membrane and extend the service life of reverse osmosis water treatment equipment methods and techniques.

After the reverse osmosis equipment is tested, two methods are used to protect the membrane . After the equipment is tested for two days (15-24h), it is maintained with a 2% formaldehyde solution; or after running for 2-6h, it is maintained with a 1% NaHSO3 aqueous solution (the air in the equipment pipeline should be drained, ensuring that the equipment does not leak, and all inlet and outlet valves are closed). Both methods can achieve satisfactory results.

Residual gas in the reverse osmosis equipment running under high pressure can form water hammer, which will damage the membrane. There are two common situations: when the equipment is drained and restarted, the gas is not completely drained before the rapid pressure increase operation. The remaining air should be drained at a pressure of 2-4 bar before gradually increasing the pressure. When the joint between the pretreatment equipment and the high-pressure pump is not well sealed or leaks (especially the microfiltration device and the subsequent pipeline leakage), when the reverse osmosis pretreatment water supply is insufficient, the microfiltration is blocked, and due to the vacuum, some air will be sucked in at the place where the seal is not good. The microfiltration device should be cleaned or replaced to ensure that the pipeline does not leak. 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.

Because the concentration of inorganic salts on the membrane concentrate side is higher than that of the raw water, it is easy to scale and pollute the membrane. Rinse with pretreated water by adding chemical reagents. Because water containing chemical reagents may cause membrane pollution during equipment shutdown. When the reverse osmosis equipment is ready to shut down, the addition of chemical reagents should be stopped, the pressure should be gradually reduced to about 3 bar, and the pre-treated water should be rinsed for 10 min until the TDS of the concentrate is very close to the TDS of the raw water.

A common problem in the use of composite polyamide membranes is that because polyamide membranes have poor resistance to residual chlorine, the correct addition of disinfectants such as chlorine is not used, and users do not pay enough attention to the prevention of microorganisms, which easily leads to microbial contamination. At present, many pure water microbial exceedances produced by water treatment equipment manufacturers are caused by poor disinfection and maintenance. The reverse osmosis equipment does not use disinfectant maintenance; the entire pipeline and pretreatment equipment are not disinfected after the equipment is installed; intermittent operation does not adopt disinfection and maintenance measures; the pretreatment equipment and reverse osmosis equipment are not disinfected regularly; the maintenance solution fails or the concentration is insufficient.

In the process of using reverse osmosis equipment, in addition to the normal attenuation of performance, the attenuation of equipment performance caused by pollution is more serious. Common pollution mainly includes chemical scaling, organic matter and colloid pollution, and microbial pollution. Different pollutions show different symptoms.

① Colloid pollution: When colloid pollution occurs, it is usually accompanied by the following two characteristics: the microfiltration device in the pretreatment is blocked very quickly, especially the pressure difference increases very quickly, and the SDI value is usually above 2.5.

② Microbial pollution: When microbial pollution occurs, the total number of bacteria in the permeate and concentrate of the reverse osmosis equipment is relatively high, and maintenance and disinfection have not been carried out according to the requirements.

③ Calcium scaling: It can be judged according to the raw water quality and design parameters.

Reverse osmosis equipment pretreatment debugging

The quality of pretreatment is the key to the stable operation of reverse osmosis equipment. When the reverse osmosis equipment uses groundwater as its water source, the pretreatment of quartz sand and activated carbon is not a problem, but it is different when using surface water.

1. Reverse osmosis equipment pretreatment chemicals

The chemicals used in the treatment process, including coagulants, flocculants, oxidants, reducing agents, scale inhibitors, etc., especially coagulants and scale inhibitors, the selection of these chemicals, the amount of chemicals added, and even the chemical preparation methods will have a great impact on the operation of reverse osmosis.

Usually, we pay attention to detecting the iron content of the influent. In fact, high aluminum content in the influent will also cause pollution of the reverse osmosis membrane. The pollution of aluminum to the membrane is due to the precipitation of aluminum hydroxide, and aluminum hydroxide precipitates usually exist in the form of colloids, which is an amphoteric hydroxide with very low solubility in the pH range of 6.5-6.7. If the aluminum coagulation process is carried out at too high or too low pH values, aluminum ions will enter the reverse osmosis device and cause pollution of the reverse osmosis membrane. Therefore, for pretreatment systems using aluminum salts as coagulants, the pH value is controlled at 6.5-6.7 to minimize the solubility of aluminum. Pay attention to adjusting the amount of chemicals added according to the water quality, and if possible, regularly check the aluminum content in the pre-treated water and control it below 0.05mg/L.

In order to prevent scaling on the concentrate side, we usually add scale inhibitors. Current scale inhibitors are usually a mixture of some organic acids and organic phosphates to achieve the purpose of scale inhibition and dispersion. If the selection or control is improper, these organic substances will block the reverse osmosis membrane elements, and it will also become a breeding ground for bacteria and microorganisms, bringing greater harm to the operation of reverse osmosis.

2. Temperature

It is possible that everyone knows that temperature has a great influence on the flux of reverse osmosis membrane elements, so the temperature should be checked when calculating the water production to make a comparison. Therefore, in places with low water temperature in winter, heating equipment will be designed in the pretreatment system of reverse osmosis, which can effectively ensure that the reverse osmosis equipment can also achieve the design output in winter.

In fact, the precipitation of SiO₂ in the membrane element is also closely related to the inlet water temperature of the reverse osmosis device. The concentration of silicon dioxide in the concentrate cannot exceed 100mg/l at 25℃ and 25mg/l at 5℃. Therefore, when there is no heating equipment in the pretreatment system, in winter, we should pay great attention to the pollution of silicon dioxide precipitates to the membrane elements, and strictly control the content of silicon dioxide in the concentrate, which should not exceed the solubility at that temperature.

Operation and Management of Reverse Osmosis Equipment

1. Regular Inspection of Reverse Osmosis Equipment

Regularly inspect and replace the security filter cartridges in a timely manner to prevent leakage caused by installation or quality issues from causing particulate contamination of the reverse osmosis membrane. When the inlet pressure difference of the security filter is greater than 0.15 MPa, the filter cartridge should be replaced. Generally, it should be checked once a month, and the service life of the filter cartridge should not exceed 6 months. During operation, the security filter should also be frequently checked for gas, and air should not be allowed to enter.

2. Inspection of Reverse Osmosis Membrane Elements

(1) Generally, every six months (the time can be shortened if necessary), the first and second stage membrane elements of each reverse osmosis set should be inspected.
(2) Open the pressure vessel end cover (use special tools and operate by skilled technicians).
(3) Check the inlet section for mechanical impurities, metal oxide deposits, bacterial and microbial growth, changes in membrane element color, and scaling on the membrane.
(4) If necessary, the reverse osmosis membrane element can be removed for detailed inspection. When removing the inlet end membrane element, it must not be directly pulled out. It must be pushed out of the pressure vessel in the direction of water flow, and the same applies during installation.
(5) Detailed records should be made after each inspection for comparison.
3. Regular calibration of various meters to ensure accurate and reliable instruments.

4. Regular analysis and statistics of reverse osmosis equipment operation data.

Operating pressure, recovery rate (or concentrate discharge), inlet SDI (pollution index), pH, residual chlorine, and temperature are the main operating control parameters of the reverse osmosis device; desalination rate, water production, and pressure difference are the three main monitoring performance parameters. These must be strictly adhered to during operation and management, and operating conditions must not be changed arbitrarily. In particular, it is necessary to prevent increasing the recovery rate to increase water production, leading to scaling on the reverse osmosis membrane surface; to prevent continued operation when the SDI value exceeds the standard, leading to blockage of the reverse osmosis membrane; and to prevent continued operation above the allowable pressure difference, causing destructive damage to the membrane elements.