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Problem Solving Collection about Reverse Osmosis Equipment


1. How often should the reverse osmosis water purifier system be cleaned?


Generally, when the standardized flux decreases by 10-15%, or the desalination rate decreases by 10-15%, or the operating pressure and inter-section pressure difference increase by 10-15%, the RO system should be cleaned. The frequency of cleaning is directly related to the degree of system pretreatment. When SDI15 < 3, the frequency of cleaning may be 4 times a year. When SDI15 is around 5, the frequency of cleaning may be doubled, but the frequency of cleaning depends on the actual situation of each project site.


2. What's SDI?

At present, the most effective method for evaluating colloidal contamination in RO/NF system is to measure the siltation density index (SDI), which is an important parameter that must be determined before RO design. During the operation of RO/NF, regular measurements must be made (2-3 times daily for surface water). ASTMD4189-82 stipulates the standard for this test. The water intake requirement of membrane system is that SDI15 value must be less than or equal to 5. The effective technologies to reduce SDI pretreatment include multi-media filter, ultrafiltration, microfiltration and so on. Adding polydielectrics before filtration can sometimes enhance the ability of physical filtration and reduce SDI value.


3. Should reverse osmosis process or ion exchange process be used for general influent?


Under many influent conditions, it is technically feasible to use ion exchange resin or reverse osmosis. The choice of process should be determined by economic comparison. Generally, the higher the salt content, the more economical the reverse osmosis is. The lower the salt content, the more economical the ion exchange is. Due to the extensive popularization of reverse osmosis technology, the combination of reverse osmosis + ion exchange process or multi-stage reverse osmosis or reverse osmosis + other deep desalination technology has become recognized as a more technically and economically rational water treatment scheme. For further understanding, please consult the representative of water treatment engineering company. 4. Reverse osmosis membranes can be used for several years? The service life of membranes depends on the chemical stability of membranes, physical stability of components, cleanability, water source, pretreatment, cleaning frequency, operation and management level. According to economic analysis, it is usually more than five years.


4. How many years can reverse osmosis membranes be used?


The service life of the membrane depends on the chemical stability of the membrane, the physical stability of the components, cleanability, water source, pretreatment, cleaning frequency, operation and management level. According to economic analysis, it is usually more than five years.


5. What is the difference between reverse osmosis and nanofiltration?


Nanofiltration is a membrane liquid separation technology between reverse osmosis contract ultrafiltration. Reverse osmosis can remove the smallest solute with molecular weight less than 0.0001 micron and nanofiltration can remove solute with molecular weight about 0.001 micron. Nanofiltration is essentially a low-pressure reverse osmosis, which is suitable for treating well water and surface water when the purity of treated water is not particularly strict. Nanofiltration is suitable for water treatment systems with high desalination rate, such as reverse osmosis, but it has a high removal capacity for hardness components, sometimes called "softening membrane". The operating pressure of nanofiltration system is low and the energy consumption is lower than that of the corresponding reverse osmosis system.


6. What is the separation capability of membrane technology?


Reverse osmosis is the most precise liquid filtration technology at present. Reverse osmosis membranes can intercept inorganic molecules such as soluble salts and organic substances with molecular weight greater than 100. On the other hand, water molecules can freely penetrate the reverse osmosis membranes. The typical removal rate of soluble salts is more than 95-99%. The operating pressure ranges from 7 bar (100 psi) when the intake water is brackish water to 69 bar (1,000 psi) when the seawater is seawater. Nanofiltration can remove impurities and organic matter with molecular weight greater than 200-400 at 1nm (10A). The removal rate of soluble solids is 20-98%. The removal rate of salts containing monovalent anions (such as NaCl or CaCl2) is 20-80%. The removal rate of salts containing bivalent anions (such as MgSO4) is 90-98%. Ultrafiltration can separate macromolecules larger than 100-1,000 E (0.01-0.1 micron). All soluble salts and small molecules can pass through ultrafiltration membranes. Removable substances include colloids, proteins, microorganisms and macromolecules. The molecular weight of most ultrafiltration membranes ranged from 1,000 to 100,000. The removal range of particles by microfiltration is about 0.1-1 micron. Usually, suspended matter and colloids of large particles can be intercepted while macromolecules and soluble salts can freely pass through the microfiltration membrane. The microfiltration membrane is used to remove bacteria, microflocculants or total suspended solids TSS. Typical pressure on both sides of the membrane is 1-3 bar.


8. What is the maximum permissible concentration of silica in reverse osmosis membrane?


The maximum allowable concentration of silicon dioxide depends on temperature, pH value and scale inhibitor. The maximum allowable concentration of silicon dioxide in concentrated water is 100 ppm without scale inhibitor. Some scale inhibitors can allow the maximum concentration of silicon dioxide in concentrated water to be 240 ppm. Please consult the scale inhibitor supplier.


9. How does chromium affect RO film?


Some heavy metals, such as chromium, can catalyze the oxidation of chlorine, which leads to the irreversible performance degradation of the diaphragm. This is because Cr6 + is less stable than Cr3 + in water. It seems that the destructive effect of metal ions with high oxidation valence is stronger. Therefore, the concentration of chromium should be reduced in the pretreatment part or at least Cr6+ should be reduced to Cr3+.


10. What kind of pretreatment does RO system need?


Usually, the pretreatment system consists of coarse filtration (~80 microns) to remove large particles, adding oxidants such as sodium hypochlorite, then precise filtration through multi-media filter or clarifier, adding oxidants such as sodium bisulfite to reduce residual chlorine, and finally installing a security filter before the entrance of high-pressure pump. As the name implies, the function of security filter is the ultimate insurance measure to prevent accidental large particles from destroying the impeller and membrane components of high-pressure pump. Water sources with more suspended particulate matter usually need higher pretreatment to meet the required intake requirements; water sources with high hardness content should be softened or acidified and scale inhibitors. Activated carbon or anti-fouling membrane elements are also needed for water sources with high microbial and organic matter content.


11. Can RO remove microorganisms such as viruses and bacteria?


Reverse osmosis (RO) is very dense and has a very high removal rate of viruses, bacteriophages and bacteria, at least more than 3 logs (removal rate > 99.9%). However, it should also be noted that in many cases, microbial regeneration may occur on the water side of the membrane production, which mainly depends on the way of assembly, monitoring and maintenance. That is to say, the ability of a system to remove microorganisms depends on the design, operation and management of the system, rather than the nature of the membrane element itself.


12. How does temperature affect water production?


The higher the temperature, the higher the water yield, and vice versa. When operating at higher temperature, the operating pressure should be lowered to keep the water yield unchanged, and vice versa. For the temperature correction factor TCF of water production change, please refer to the relevant chapters.


13. What is particle and colloid pollution? How to determine?


Once the fouling of particles and colloids occurs in reverse osmosis or nanofiltration systems, the water yield of membranes will be seriously affected, and sometimes the desalination rate will be reduced. The early symptoms of colloidal fouling are the increase of pressure difference in the system. The sources of particles or colloids in the membrane water source vary from place to place, often including bacteria, sludge, colloidal silicon, iron corrosion products, etc. Drugs used in the pretreatment part, such as polyaluminium and ferric trichloride or cationic polydielectric, may also cause fouling if they cannot be effectively removed in clarifying pools or media filters. In addition, cationic polydielectrics also react with anionic scale inhibitors. The sediments of cationic polydielectrics will foul the membrane plugging elements. SDI15 is used to evaluate the fouling tendency or pretreatment qualification in water. Please refer to the detailed introduction in relevant chapters.



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