I. Evaluation Indicators
Generally speaking, reverse osmosis membranes should have the following properties:
① High water permeability per unit area and high desalination rate;
② Good mechanical strength, small compaction effect of the porous support layer;
③ Good chemical stability, resistant to acid, alkali corrosion and microbial erosion;
④ Uniform structure, long service life, slow performance degradation;
⑤ Easy to make, cheap, and sufficient raw materials.
Therefore, the evaluation indicators of reverse osmosis membranes can be analyzed from the following aspects:
1. Salt rejection and salt passage
Salt rejection - The percentage of soluble impurities removed from the system feed water through the reverse osmosis membrane.
　 Salt passage - The percentage of soluble impurities in the feed water that pass through the membrane.
　 Salt rejection = (1 - product water salinity / feed water salinity) × 100%
　 Salt passage = 100% - Salt rejection
The salt rejection rate of the reverse osmosis membrane element is determined when it is manufactured. The high and low salt rejection rate depends on the density of the ultra-thin desalination layer on the surface of the reverse osmosis membrane element. The denser the desalination layer, the higher the salt rejection rate, and the lower the water yield. The removal rate of different substances by reverse osmosis is mainly determined by the structure and molecular weight of the substance. The high-valent ions and complex monovalent ions removal rate of the reverse osmosis membrane element can exceed 99%, and the removal rate of monovalent ions such as sodium ions, potassium ions, and chloride ions is slightly lower, but it also exceeds 98%; the removal rate of organic matter with a molecular weight greater than 100 can also reach 98%.
2. Water production (water flux)
Water production (water flux) - Refers to the production capacity of the reverse osmosis system, that is, the amount of water passing through the membrane per unit time, usually expressed in tons/hour or gallons/day.
Permeate flux - Permeate flux is also an important indicator of the water production of reverse osmosis membrane elements. It refers to the flow rate of permeate per unit membrane area, usually expressed in gallons per square foot per day (GFD). Too high a permeate flux will lead to an increase in the water flow velocity perpendicular to the membrane surface, exacerbating membrane fouling.
3. Recovery rate
Recovery rate - Refers to the percentage of feed water converted into product water or permeate in the membrane system. The recovery rate of the membrane system is determined during design and is based on the preset feed water quality. The recovery rate is usually expected to be maximized to improve economic efficiency, but it should be limited by the fact that precipitation will not occur due to supersaturation of salts and other impurities in the membrane system.
Recovery rate = (Product water flow rate / Feed water flow rate) × 100%

II. Influencing Factors of Reverse Osmosis

The water flux and salt rejection rate of the reverse osmosis membrane are key operating parameters in the reverse osmosis process. These two parameters will be affected by factors such as pressure, temperature, recovery rate, feed water salinity, and feed water pH.
1. Feed water pressure
　 The feed water pressure itself does not affect the salt passage, but the increase in feed water pressure increases the net pressure driving reverse osmosis, increasing the water production, while the salt passage remains almost unchanged. The increased water production dilutes the salt through the membrane, reducing the salt passage and increasing the salt rejection. When the feed water pressure exceeds a certain value, due to the excessively high recovery rate, the concentration polarization is increased, which will lead to an increase in salt passage, offsetting the increased water production, so that the salt rejection will no longer increase.
2. Feed water temperature

Temperature has the most significant impact on the operating pressure, salt rejection, and pressure drop of reverse osmosis. As the temperature rises, the osmotic performance increases, and the net driving force required at a certain water flux decreases, so the actual operating pressure decreases. At the same time, the solute permeation rate also increases with the increase in temperature, the salt passage increases, and it is directly manifested as an increase in the conductivity of the product water.
Temperature also has a certain impact on the pressure drop in each section of reverse osmosis. As the temperature increases, the viscosity of water decreases, and the pressure drop decreases. For devices where the turbulence degree is enhanced due to blockage of the reverse osmosis membrane channel, the impact of viscosity on pressure drop is more significant.

The product water conductivity of the reverse osmosis membrane is very sensitive to changes in the feed water temperature. As the water temperature increases, the water flux also increases linearly. For every 1℃ increase in the feed water temperature, the product water flux increases by 2.5%～3.0%; this is because the viscosity of the water molecules passing through the membrane decreases and the diffusion performance is enhanced. The increase in feed water temperature will also lead to an increase in salt passage and a decrease in salt rejection, mainly because the diffusion speed of salt through the membrane will increase with the increase in temperature.
3. Feed water pH

All membrane components have an allowable pH range. The feed water pH has almost no effect on the water production; however, even within the allowable range, the pH has a significant effect on the salt rejection. On the one hand, the pH also has a certain effect on the conductivity of the product water, because most reverse osmosis membranes themselves carry some active groups, and the pH can affect the electric field on the membrane surface and thus affect the migration of ions. The pH directly affects the form of impurities in the feed water, such as for dissociable organic matter, its retention rate decreases with the decrease in pH; on the other hand, the dissolved CO2 in the water is greatly affected by the pH. When the pH is low, it exists in the form of gaseous CO2 and is easy to pass through the reverse osmosis membrane, so the salt rejection is also lower when the pH is low. As the pH increases, gaseous CO2 is converted into HCO3- and CO32- ions, and the salt rejection gradually increases. When the pH is between 7.5 and 8.5, the salt rejection reaches the highest.

4. Feed water salt concentration

Osmotic pressure is a function of the concentration of salts or organic matter in the water. The higher the salt content, the higher the osmotic pressure. With constant influent pressure, the net pressure will decrease, and the water yield will decrease. The salt passage rate is proportional to the difference in salt concentration on both sides of the reverse osmosis membrane. The higher the salt content in the influent water, the greater the concentration difference, and the higher the salt passage rate, thus leading to a decrease in desalination rate. For the same system, with different feed water salinity, the operating pressure and product water conductivity will also differ. For every 100 ppm increase in feed water salinity, the influent pressure needs to be increased by approximately 0.007 MPa. At the same time, due to the increase in concentration, the product water conductivity will also increase accordingly.
5. Suspended Solids

Suspended solids in water refer to substances that remain on the surface of the filter material during filtration, mainly composed of particles. High suspended solids content can quickly cause severe blockage in reverse osmosis and nanofiltration systems, affecting the system's water yield and water quality.

6. Recovery Rate

The recovery rate has a significant impact on the pressure drop in each section. Under the condition that the total influent flow rate remains constant, an increase in the recovery rate will reduce the total pressure drop due to the reduced concentrate flow rate through the high-pressure side of the reverse osmosis. Conversely, a decrease in the recovery rate will increase the total pressure drop. Actual operation shows that even a small change in the recovery rate, such as 1%, can cause a change of about 0.02 MPa in the total pressure difference. The impact of the recovery rate on product water conductivity depends on the salt passage and product water volume. Generally speaking, an increase in the system recovery rate will increase the salt content in the concentrate and correspondingly increase the conductivity of the product water.

  Installation and Removal of Membrane Elements

Installation gaps must be eliminated before installation is complete. Even qualified membrane housings and membrane elements may have dimensional deviations. When the system is running, the membrane elements will slide back and forth in the membrane housing due to the installation gap, impacting the membrane housing end plates, thus causing malfunctions.

Membrane elements must be installed using the correct method. Otherwise, after the system starts running, a series of malfunctions may occur, including rapid system contamination, high conductivity, membrane element shell breakage, membrane element end plate breakage, low system water yield, high system operating pressure, and membrane element central tube breakage.

The following should be considered during installation:

Use the correct installation direction: Push from the inlet end of the membrane housing towards the concentrate end. Installing the membrane element in reverse will damage the concentrate sealing ring. The concentrate end of the membrane element without a black sealing ring should enter the membrane housing first, followed by the inlet end of the membrane element with a black sealing ring. If reversed, it may lead to insufficient tangential flow velocity during system operation, increased concentration polarization, and increased pollution rate.

Use the correct lubricant; glycerol (glycerin) is recommended. Strictly prohibit the use of detergents, petroleum jelly, and other oily lubricants. Detergents are cationic surfactants that will reduce the water yield of electronegative membrane elements, while other oily lubricants will cause the membrane element central tube to become brittle and damaged.

Installation gaps must be eliminated before installation is complete. Even qualified membrane housings and membrane elements may have dimensional deviations. When the system is running, the membrane elements will slide back and forth in the membrane housing due to the installation gap, impacting the membrane housing end plates, thus causing malfunctions. Before the inlet side membrane housing end cap is locked, a gasket must be installed on the adapter connecting the membrane housing and the membrane element to eliminate the installation gap.

Methods to Extend the Service Life of Reverse Osmosis Membranes After the reverse osmosis equipment is tested, we have used two methods to protect the membrane. After the equipment is tested for two days (15-24 hours), it is maintained using a 2% formaldehyde solution; or after running for 2-6 hours, it is maintained using a 1% NaHSO3 aqueous solution (air should be purged from the equipment piping, ensuring the equipment is leak-free, and all inlet and outlet valves are closed).

1. Preventing Membrane Damage

　　New reverse osmosis membrane elements are usually immersed in a 1% NaHSO3 and 18% glycerin 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 use as possible.

After the reverse osmosis equipment is tested, we have used two methods to protect the membrane. After the equipment is tested for two days (15-24 hours), it is maintained using a 2% formaldehyde solution; or after running for 2-6 hours, it is maintained using a 1% NaHSO3 aqueous solution (air should be purged from the equipment piping, ensuring the equipment is leak-free, and all inlet and outlet valves are closed). Both methods achieve satisfactory results. The first method is more expensive and is used for longer idle times, while the second method is used for shorter idle times.

2. Improper Operation of Reverse Osmosis Equipment Causing Membrane Damage

2.1 Residual gases in the reverse osmosis equipment running under high pressure can form water hammer, damaging the membrane.

　　Two situations often occur: A. When the equipment is drained and restarted, the gas is not completely purged before rapid pressurization. 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 leaking (especially leakage in the microfiltration unit and subsequent piping), if the pretreatment water supply is insufficient, such as microfiltration blockage, air may be sucked in at the poorly sealed location due to vacuum. The microfiltration unit should be cleaned or replaced, and the piping 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.

2.2 Incorrect Shutdown Method of 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 contaminate the membrane. B. Rinsing with pretreated water containing added chemical reagents. Water containing chemical reagents may cause membrane contamination during equipment downtime.

　　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 pretreated water for 10 minutes until the TDS of the concentrate is very close to the TDS of the raw water.

2.3 Microbial contamination due to poor disinfection and maintenance of reverse osmosis equipment

　　This is a common problem in the use of composite polyamide membranes, because polyamide membranes have poor resistance to residual chlorine. Improper addition of chlorine or other disinfectants during use, coupled with insufficient attention to microbial prevention by users, easily leads to microbial contamination. Currently, many manufacturers' pure water microbial levels exceed standards, which is caused by poor disinfection and maintenance.

　　This is mainly manifested as: at the time of leaving the factory, the RO equipment was not maintained with disinfectant; after the equipment was installed, the entire pipeline and pretreatment equipment were not disinfected; intermittent operation did not adopt disinfection and maintenance measures; the pretreatment equipment and reverse osmosis equipment were not disinfected regularly; the maintenance solution failed or the concentration was insufficient.

2.4 Inadequate monitoring of residual chlorine in reverse osmosis equipment

　　For example, if the NaHSO3 pump fails, the solution fails, or the activated carbon is saturated, the membrane is damaged by residual chlorine.

3 Membrane damage due to untimely cleaning and incorrect cleaning methods

　　During the use of the 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. Different membrane companies have some differences in the symptoms of membrane pollution they propose.

In our engineering projects, we have found that the duration of pollution varies, and its symptoms also vary. For example: when the membrane is contaminated with calcium carbonate scale, if the pollution time is one week, it mainly manifests as 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. If the pollution time is one year (a certain pure water machine), the salt passage increases from the initial 2mg/L to 37mg/L (raw water is 140mg/L～160mg/L), and the water production decreases from 230L/h to 50L/h. After cleaning with citric acid, the salt passage decreases to 7mg/L, and the water production increases to 210L/h.