Reverse osmosis membrane is the core component of reverse osmosis, a kind of artificial semi-permeable membrane with special characteristics, which simulates biological semi-permeable membrane. It is generally made of polymer materials, such as cellulose acetate membrane, aromatic polyhydrazide membrane, and aromatic polyamide membrane. The diameter of surface micropores is generally between 0.5 and 10 nm, and the permeability is related to the chemical structure of the membrane itself. Some polymer materials have good salt rejection, but the water permeability is not good. Some polymer materials have more hydrophilic groups in their chemical structure, so the water permeability is relatively fast. Therefore, a satisfactory reverse osmosis membrane should have appropriate permeation or desalination rate.

I. Reverse osmosis membrane should have the following characteristics:

1. High desalination rate at high flow rate;

2. High mechanical strength and service life;

3. Function at lower operating pressure;

4. Resistant to chemical or biochemical effects;

5. Less affected by factors such as pH and temperature;

6. Easy to obtain membrane raw materials, simple processing, and low cost.

The structure of reverse osmosis membrane includes asymmetric membrane and homogeneous membrane. The currently used membrane materials are mainly cellulose acetate and aromatic polyamide. Its components include hollow fiber type, spiral wound type, plate and frame type, and tubular type. It can be used for separation, concentration, and purification in chemical unit operations, mainly used in pure water preparation and water treatment industry.

II. Reverse osmosis principle

Reverse osmosis, also known as reverse osmosis, is a membrane separation operation that separates solvent from solution by pressure difference. When pressure is applied to one side of the membrane, if the pressure exceeds its osmotic pressure, the solvent will undergo reverse osmosis against the direction of natural osmosis. Thus, the permeated solvent, i.e., permeate, is obtained on the low-pressure side of the membrane, and the concentrated solution, i.e., concentrate, is obtained on the high-pressure side. If seawater is treated by reverse osmosis, fresh water is obtained on the low-pressure side of the membrane, and brine is obtained on the high-pressure side.

The permeation rate of solvent during reverse osmosis, i.e., liquid flow energy N, is:

N=Kh（Δp－Δπ）

Where Kh is the hydraulic permeability coefficient, which increases slightly with increasing temperature; Δp is the static pressure difference between the two sides of the membrane; Δπ is the osmotic pressure difference between the two sides of the membrane.

The osmotic pressure π of a dilute solution is:

π=iCRT

Where i is the number of ions generated by the ionization of solute molecules; C is the molar concentration of solute; R is the molar gas constant; T is the standard temperature.

Asymmetric membranes and composite membranes are usually used in reverse osmosis. The equipment used in reverse osmosis is mainly hollow fiber or spiral wound membrane separation equipment.

Reverse osmosis membrane can intercept various inorganic ions, colloidal substances and macromolecular solutes in water, thus obtaining purified water. It can also be used for pre-concentration of macromolecular organic solutions. Due to its simple process and low energy consumption, reverse osmosis has developed rapidly in the past 20 years. It has been widely used in seawater and brackish water desalination, boiler water softening and wastewater treatment, and combined with ion exchange to produce high-purity water. Its application range is expanding, and it has begun to be used in the concentration of dairy products and fruit juice, as well as the separation and concentration of biochemical and biological preparations.

III. Reverse osmosis membrane life

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 and colloidal pollution, and microbial pollution. Different pollutions show different symptoms. Different membrane companies have different symptoms of membrane pollution. In engineering, we find that the duration of pollution is different, and the symptoms are also different.

For example, when calcium carbonate scaling occurs on the membrane, if the pollution time is one week, it mainly shows a rapid decrease in desalination rate, a slow increase in pressure difference, and no significant change in water production. The performance can be fully restored by cleaning with citric acid. If the pollution time is one year (a certain pure water machine), the salt flux 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 flux decreases to 7mg/L, and the water production increases to 210L/h. Furthermore, pollution is often not single, and its symptoms are also different, which makes the identification of pollution more difficult.

IV. Identification of reverse osmosis membrane pollution type

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

1. Colloidal pollution

When colloidal pollution occurs, the following two characteristics are usually accompanied: A. The microfiltration filter in the pretreatment is blocked very quickly, especially the pressure difference increases very quickly. B. The SDI value is usually above 2.5.

2. Microbial pollution

When microbial contamination occurs, the bacterial count in both the permeate and concentrate water of the RO equipment is relatively high, and regular maintenance and disinfection are not performed as required. This prevents damage to the ultrafiltration RO membrane performance. New reverse osmosis membrane elements are usually immersed in a 1% NaHSO3 and 18% glycerol aqueous solution and stored in a sealed plastic bag. If the plastic bag remains unbroken, 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 prevent the NaHSO3 from oxidizing in the air and adversely affecting the element. Therefore, the membrane should be opened just before use. After the equipment commissioning test, we have used two methods to protect the membrane. The equipment is tested for two days (15-24h), and then maintained with a 2% formaldehyde solution; or after running for 2-6h, it is maintained with a 1% NaHSO3 aqueous solution (air should be drained from the equipment pipeline, ensuring that the equipment is leak-proof, and all inlet and outlet valves are closed). Both methods can achieve satisfactory results. The former method is more costly and is used when the equipment is idle for a long time, while the latter method is used when the equipment is idle for a shorter time.

3. Calcium scaling

This can be determined based on the raw water quality and design parameters. For carbonate-type water, if the recovery rate is 75%, and a scale inhibitor is added during design, the LSI of the concentrate should be less than 1; if no scale inhibitor is added, the LSI of the concentrate should be less than zero, and calcium scaling will generally not occur.

4. A 1/4-inch PVC plastic tube can be inserted into the component to test the performance changes at different parts of the component.

5. Determine the type of contamination based on changes in equipment performance.

6. Acid washing (such as citric acid, dilute HCl) can be used. The calcium scaling can be determined based on the cleaning effect and cleaning solution, and further confirmed through analysis of the cleaning solution components.

7. Chemical analysis of the cleaning solution

Samples of raw water, original cleaning solution, and cleaning solution are taken for analysis. After determining the type of contamination, cleaning can be performed according to the method in 1, followed by disinfection. If the type of contamination cannot be determined, cleaning + disinfection + 0.1% HCl (pH 3) steps are usually used.