Evaluation index and influencing factors of reverse osmosis membrane
1. Evaluation Index
Generally speaking, reverse osmosis membrane should have the following properties:
- The amount of water permeable per unit area is large, and the salt rejection rate is high;
- Good mechanical strength and small compaction effect of the porous support layer;
- Good chemical stability, resistant to acid, alkali corrosion and microbial corrosion;
- Uniform structure, long service life, slow performance degradation;
- The film is easy to make, the price is cheap, and the raw materials are sufficient.
Therefore, the evaluation index of reverse osmosis membrane can be analyzed from the following aspects:
1. Desalination rate and salt permeability
Salt rejection rate-the percentage of the concentration of soluble impurities removed from the system inlet water through the reverse osmosis membrane.
Salt permeability-the percentage of soluble impurities in the influent that penetrate the membrane.
Desalination rate = (1-salt content of product water / salt content of influent) × 100%
Salt penetration rate = 100%-desalination rate
The desalination rate of the reverse osmosis membrane element is determined when it is manufactured and formed. The desalination 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 desalination rate and the lower the water production. The removal rate of different substances by reverse osmosis is mainly determined by the structure and molecular weight of the material. The removal rate of high-valent ions and complex monovalent ions can be exceeded 99% for reverse osmosis membrane elements. For monovalent ions such as sodium ion and potassium ion , The removal rate of chloride ion 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 permeating through the membrane per unit time, usually expressed in tons/hour or gallons/day.
Permeation flow rate-Permeation flow rate is also an important indicator of the water production of reverse osmosis membrane elements. Refers to the permeate flow rate per unit membrane area, usually expressed in gallons per square foot per day (GFD). An excessively high permeate flow rate will cause the water velocity perpendicular to the membrane surface to increase, which will aggravate 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 at the time of design and is based on the preset water quality. The recovery rate is usually hoped to be maximized in order to improve economic efficiency, but it should be the limit value that the membrane system will not precipitate due to the supersaturation of impurities such as salts. Recovery rate = (product water flow / influent flow) × 100%
2. The influencing factors of reverse osmosis. The water flux and desalination rate of reverse osmosis membranes are the key operating parameters in the reverse osmosis process. These two parameters will be affected by pressure, temperature, recovery rate, feed water salt content, and feed water pH. influences.
1. Water inlet pressure
The inlet water pressure itself does not affect the salt permeation rate, but the increase in the inlet water pressure increases the net pressure that drives the reverse osmosis, which increases the water production while the salt permeation is almost unchanged. The increased water production dilutes the permeation The salt content of the membrane reduces the salt permeability and increases the salt rejection rate. When the inlet 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 permeation, which offsets the increased water production and makes the desalination rate no longer increase.
2. Inlet water temperature
Temperature has the most obvious influence on the operating pressure, desalination rate and pressure drop of reverse osmosis. The temperature rises, the permeability increases, and the net driving force required under 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, and the salt permeation rate increases, which is directly reflected by the increase in product water conductivity. Temperature also has a certain effect on the pressure drop of each stage of reverse osmosis. As the temperature rises, the viscosity of water decreases, and the pressure drop decreases. For devices where the turbulence degree of the reverse osmosis membrane channel is increased due to fouling, the effect of viscosity on the pressure drop More obvious. The water conductivity of reverse osmosis membranes is very sensitive to changes in the inlet water temperature. With the increase of water temperature, the water flux also linearly increases. Every time the inlet water temperature increases by 1℃, the water flux increases by 2.5% to 3.0%; The reason is that the viscosity of the water molecules passing through the membrane decreases and the diffusion performance increases. The increase in the temperature of the influent water will also lead to an increase in the salt permeability and a decrease in the desalination rate. This is mainly because the diffusion rate of salt through the membrane will be accelerated by the increase in temperature.
3. Influent pH
Various membrane modules have an allowable pH value range, and the pH value of the influent water has almost no effect on the water production; but even within the allowable range, the pH value has a greater impact on the desalination rate. On the one hand, the pH value has an effect on the conductivity of the product water. The rate also has a certain effect, because the reverse osmosis membrane itself mostly contains some active groups. The pH value can affect the electric field on the membrane surface and then affect the migration of ions. The pH value has a direct impact on the form of impurities in the influent, such as For dissociable organics, the rejection rate decreases with the decrease of pH value; on the other hand, because the dissolved CO2 in water is greatly affected by pH value, when the pH value is low, it exists in the form of gaseous CO 2 and easily penetrates the reverse osmosis membrane. Therefore, when the pH is low, the desalination rate is also lower. As the pH increases, gaseous CO 2 is converted into HCO 3-and CO 3 2-ions, and the desalination rate gradually rises. When the pH is between 7.5 and 8.5, the desalination rate reaches the highest.
4. Influent salt concentration
The osmotic pressure is a function of the salt or organic concentration in the water. The higher the salt content, the osmotic pressure increases. When the inlet pressure remains the same, the net pressure will decrease and the water production will decrease. The salt permeability is proportional to the salt concentration difference between the front and back sides of the reverse osmosis membrane. The higher the salt content of the influent, the greater the concentration difference, and the salt permeability increases, which leads to a decrease in the salt rejection rate. For the same system, the feed water salt content is different, the operating pressure and the product water conductivity are also different. For every increase in the feed water salt content by 100 ppm, the inlet water pressure needs to increase by about 0.007 MPa. At the same time, due to the increase in concentration, the product water conductivity The rate has also increased accordingly.
5. Suspended matter
Suspended matter in water refers to the substance remaining on the surface of the filter material while the water is being filtered, mainly composed of particles. The high content of suspended solids will cause the reverse osmosis and nanofiltration systems to quickly become severely blocked, affecting the system’s water production and water quality.
6. Recovery rate
The recovery rate has a great influence on the pressure drop of each stage. Under the condition that the total influent flow rate is maintained at a certain level, the recovery rate increases. As the flow of concentrated water flowing through the high pressure side of reverse osmosis decreases, the total pressure drop decreases. The recovery rate decreases and the total pressure drop increases. Actual operation shows that even if the recovery rate changes very small, such as 1%, the total pressure difference will change by about 0.02MPa. The effect of recovery rate on product water conductivity depends on salt permeation and product water volume. Generally speaking, an increase in system recovery rate will increase the salt content in the concentrated water and correspondingly increase the product water conductivity.