The rapid industrialization of China in the past decades has caused the continuous deterioration of water resources. In some areas, heavy metal pollution, organic pollution (even antibiotic pollution) is serious. According to the Ministry of Environmental Protection, about 280 million residents use unsafe drinking water. Therefore, the water purification products in the Chinese market are the primary solution to the problem of drinking water safety. Among the various technologies of water purifiers, it is a reverse osmosis membrane technology that can solve the problems of heavy metal pollution, pesticides and antibiotics pollution with low commercialization cost and stable performance.
Zhongyikang predicts that following the market scale of reverse osmosis pure water machine in 2014, the market scale will reach 6.8 billion yuan, and the growth rate will reach 130%. In 2015, the market size of pure water machine will reach 16.1 billion, with a year-on-year growth rate of 75%, becoming market share. The largest, year-on-year growth category. The domestic/commercial reverse osmosis membrane industry has made great progress, and many membrane companies and film reeling plants have been launched. However, as the core component of the water purifier, the quality of the reverse osmosis membrane component directly affects the overall quality of the water purifier, and it is also related to the healthy development of the entire industry.
Household reverse osmosis membrane performance evaluation index
The use of household reverse osmosis membranes has its own particularity. The pretreatment and operating pressure of household water purifiers are different from those of large-scale industrial systems. In combination with actual use, the reverse osmosis membrane can be evaluated from the following points.
1. Desalination rate Desalination rate = (1 - salt content of water production / salt content of influent water) × 100%. To put it simply, the reverse osmosis membrane removes salt from the influent water.
For example, the household membrane desalination rate is 97%, which means that the raw water TDS/conductivity is retained by the membrane by 97%, and 3% passes through the membrane into the produced water. The raw water TDS is 500 mg/l, and the produced water TDS is 500 x 3% = 15 mg/l.
The salt rejection of the membrane element has been determined at the time of its manufacture. The degree of salt rejection depends on the density of the ultra-thin desalination layer on the surface of the membrane element, and the denser the desalination layer, the higher the salt rejection rate (but at the same time the lower the water production). Household reverse osmosis membranes have a low salting rate, so the actual salt rejection rate is generally 96-98%.
2, water production
Water production (water flux): The capacity of the reverse osmosis membrane, that is, the amount of membrane water per unit time, usually expressed in tons per hour or gallons per day. For example, JRW-1812-75 type membrane, water production ≥ 75GPD (gallon / day).
1 gallon = 3.785 liters, converted to national standard units, 75 gallon membrane flux: 75 gallons / day × 3.785 liters = 283.9 liters / tian, that is, 197 ml / min. It is very simple to judge whether a membrane flux is up to standard. The measuring cylinder (empty pure water bottle can also be used) actually measures the amount of water produced per minute. After a simple calculation, it can be determined whether the membrane flux is up to standard.
3, the service life of the membrane relative to the desalination rate, flux two factors, the long service life of the membrane, is a more important indicator to evaluate the performance of a household membrane. Under the same conditions of use, the reverse osmosis membrane has a long service life and a large cumulative water production, which is far more important than the initial salt rejection rate. It is like a marathon, and the last thing to win is to stick to the end, although at first he may not be the fastest player to run. Therefore, the desalination rate of the membrane and the decay rate of the flux are the true indicators for determining the merits of a membrane. This needs to be tested from long-term operation.
Influencing factors of reverse osmosis membrane performance <br>The desalination rate and water flux of reverse osmosis membrane are the key operating parameters in reverse osmosis process, but these two parameters will be subject to pressure, temperature, recovery rate, salt content of feed water, water supply. The impact of the PH factor.
1. Inlet pressure The inlet pressure itself does not affect the salt permeate, but the increase in the influent pressure causes the net pressure to drive reverse osmosis to increase, resulting in an increase in water production, while the salt permeation is almost constant, increasing production. The amount of water dilutes the salt that passes through the membrane, lowers the salt permeability and increases the salt rejection. When the inlet pressure exceeds a certain value, the concentration polarization is increased due to the excessive recovery rate, which leads to an increase in salt permeation, offsetting the increased water production, and the salt rejection rate is no longer increased.
For example, the same household film, due to the different brands of feed pumps used, the test data will be biased. Because the actual lift and flow rate of some brands of household machine pumps are not up to the nominal, the film shows a low salt rejection rate.
2. The inlet water temperature has the most obvious influence on the reverse osmosis operating pressure, salt rejection rate and pressure drop. As the temperature rises, the permeation performance increases, and the net driving force required under a certain water flux decreases, so the actual operating pressure is lowered. As the water temperature increases, the water flux increases linearly. When the influent water temperature rises by 1 °C, the water production 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 also leads to an increase in the salt permeability and a decrease in the salt rejection rate, mainly because the diffusion rate of the salt permeating through the membrane is accelerated by the increase in temperature. At the same time, the solute permeation rate also increases with the increase of temperature, and the salt permeation increases, which directly indicates that the product water conductivity increases.
The same household has the same water purifier. When the water temperature is low in winter, the water production is much lower than when the summer water temperature is high. For example, it can be found from the temperature correction system table that when the water temperature is 10 ° C, the temperature correction coefficient is 1.743 at a pressure of 0.4 MP, that is, 10 ° C (actual water production amount × 1.743) = standard water production amount (25 ° C).
3. Influent pH Various membrane modules have an allowable pH range. The influent pH has little effect on the water yield; however, even within the allowable range, the desalination rate has a large effect. On the one hand, the pH value is The conductivity of the product water also has a certain effect. This is because the reverse osmosis membrane itself mostly has some reactive groups. The pH value can affect the electric field on the membrane surface and affect the migration of ions. The pH value has the form of impurities in the influent water. Direct influence, such as the rejection of the dissociable organic matter, decreases with the decrease of pH value; on the other hand, the dissolved CO2 in water is greatly affected by the pH value, and the pH is less than 3.5 in the form of carbon dioxide gas, which is easy to pass through. The membrane is permeable, so the salt rejection rate is low when the pH is low. As the pH increases, the carbon dioxide gas is converted into bicarbonate (HCO-3) and carbonate (CO2-3) ions, and the salt rejection rate is gradually increased at pH 7.5. ~ 8.5, the salt rejection rate reached zui high.
4. Influent salt concentration The osmotic pressure is a function of the concentration of salt or organic matter in the water. The higher the salt content, the higher the osmotic pressure, and the lower the inlet pressure, the net pressure will decrease and the water production will decrease. The salt permeability is proportional to the difference in salt concentration between the positive and negative sides of the membrane. The higher the salt content of the influent water, the greater the concentration difference and the higher the salt permeability, resulting in a decrease in the salt rejection rate. For the same system, the salt content of the feed water is different, and the operating pressure and the product water conductivity are also different. For every 100 ppm increase of the salt content of the feed water, the inlet pressure needs to increase by about 0.007 MPa, and the water conductivity of the product increases due to the increase of the concentration. It also increases accordingly.
For example, the difference in salt concentration between the water quality in the South and the water quality in the North is large. Taking the groundwater in the Liaocheng area of ​​Shandong as an example, the TDS refers to even more than 2000mg/l. Compared with some places in the south, the TDS value is less than 100mg/l. The same machine with the same membrane, the actual water production and desalination rate. The difference is large. In order to achieve the same amount of water produced, the influent pressure must be increased.
5. Recovery rate recovery has a great influence on the desalination rate of the membrane. Under the condition that the total influent flow rate is kept constant, the recovery rate increases, which means that the initial water production increases and the concentrated water volume decreases, but it will cause residual film. The salt content on the concentrated water side becomes high, and the concentration polarization phenomenon is serious, causing fouling of the membrane and increasing the natural osmotic pressure, which will offset the pushing effect of the inlet water pressure, slow down or stop the reverse osmosis process, and reduce the permeate flux. Or even stop. At the same time, the salt rejection rate of the membrane is lowered.
The household reverse osmosis membrane usually recovers within 30% (waste water ratio 1:3). If the recovery rate is increased by directly closing the concentrate valve, the water production will be increased in a short period of time, but the membrane will be quickly fouled. The amount of water drops until the final scale is severely blocked.
Factors affecting the life of reverse osmosis membrane
It can be seen from the above five points that the influent pressure, temperature, recovery rate and other indicators have a great influence on the desalination rate and flux of the membrane, so which factors have a great influence on the service life of the membrane?
1. The water quality of the influent water is different, and the service life of the membrane is very large. It is very intuitive that the water quality is different in the south and the north. In some areas, the membrane is completely water-free for one month. It may be caused mainly by two reasons. One is high turbidity in water, high in colloidal content, and the pretreatment is not perfect enough to cause fouling of the membrane; secondly, the hardness in water is high and the structure tends to be serious, causing fouling of the membrane.
2. The recovery rate of reverse osmosis membrane can affect the desalination rate and flux of the membrane, and it will affect the service life of the membrane. If the recovery rate is too high, the more concentrated the concentration polarization of the concentrated side of the membrane, the more easily the membrane is quickly fouled and the service life is directly reduced. The recovery rate of household water purifiers is generally less than 30%, and some manufacturers have improved the recovery rate by more than 60% through process improvement. However, no matter how high recovery rate is achieved, it is not possible to improve the recovery by directly closing the concentrated water valve. rate.
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