Reverse osmosis membrane lifespan and identification of pollution types

Reverse osmosis membranes are the core components of reverse osmosis, a type of artificial semi-permeable membrane that simulates biological semi-permeable membranes. They are generally made of polymeric materials such as cellulose acetate membranes, aromatic polyhydrazide membranes, and aromatic polyamide membranes. The diameter of the surface micropores is generally between 0.5 and 10 nm, and the permeability is related to the chemical structure of the membrane itself. Some polymeric materials have good salt rejection but poor water permeability. Some polymeric materials have a chemical structure with more hydrophilic groups, resulting in relatively faster water permeability. Therefore, a satisfactory reverse osmosis membrane should have an appropriate permeation rate or desalination rate.

I. Characteristics of Reverse Osmosis Membranes:

1. High desalination rate at high flow rates;

2. High mechanical strength and service life;

3. Function at lower operating pressures;

4. Resistance to chemical or biochemical effects;

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

6. Easily available membrane raw materials, simple processing, and low cost.

Reverse osmosis membranes are of two types: asymmetric membranes and homogeneous membranes. Currently used membrane materials are mainly cellulose acetate and aromatic polyamide. The components include hollow fiber type, spiral wound type, plate and frame type, and tubular type. They can be used in separation, concentration, and purification unit operations in the chemical industry, mainly used in pure water preparation and water treatment industries.

II. Reverse Osmosis Principle

Reverse osmosis, also known as reverse permeation, is a membrane separation operation that separates the solvent from the solution using pressure difference as the driving force. When pressure is applied to one side of the membrane, exceeding its osmotic pressure, the solvent will undergo reverse osmosis against the direction of natural osmosis. This results in the permeated solvent, i.e., permeate, on the low-pressure side of the membrane and the concentrated solution, i.e., concentrate, on the high-pressure side. If reverse osmosis is used to treat seawater, freshwater is obtained on the low-pressure side of the membrane, and brine on the high-pressure side.

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

N=Kh（Δp－Δπ）

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

Osmotic pressure of dilute solution π is:

π=iCRT

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

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

Reverse osmosis membranes can retain various inorganic ions, colloidal substances, and macromolecular solutes in water, thereby 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 (see brine), 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 juices, as well as the separation and concentration of biochemical and biological preparations.

III. Reverse Osmosis Membrane Lifespan

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 have found 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 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 the performance. If the pollution time is one year (a certain pure water machine), the salt flux increases from the initial 2 mg/L to 37 mg/L (raw water is 140 mg/L～160 mg/L), and the water production decreases from 230 L/h to 50 L/h. After cleaning with citric acid, the salt flux decreases to 7 mg/L, and the water production increases to 210 L/h. Furthermore, pollution is often not single, and its symptoms are also different, making pollution identification more difficult.

IV. Identification of Reverse Osmosis Membrane Pollution Types

The identification of pollution types should be judged comprehensively based on the 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 in the equipment pipeline should be drained, 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 and use. If the type of contamination cannot be determined, cleaning + disinfection + 0.1% HCl (pH 3) steps are usually used.