1. How often should the reverse osmosis system be cleaned?
In general, when the standardized flux decreases by 10-15%, or the desalination rate of the system decreases by 10-15%, or the operating pressure and differential pressure between sections increase by 10-15%, the RO system should be cleaned. The cleaning frequency is directly related to the degree of system pretreatment. When SDI15<3, the cleaning frequency may be 4 times a year; When SDI15 is around 5, the cleaning frequency may be doubled, but the cleaning frequency depends on the actual situation of each project site.
2. What is SDI?
At present, the best possible technology for effective evaluation of colloid pollution in the inflow of RO/NF system is to measure the sedimentation density index (SDI, also known as the pollution blockage index) of the inflow, which is an important parameter that must be determined before RO design. During the operation of RO/NF, it must be measured regularly (for surface water, it is measured 2-3 times a day). ASTM D4189-82 specifies the standard for this test. The inlet water of membrane system is specified as SDI15 value must be ≤ 5. Effective technologies to reduce SDI pretreatment include multi-media filter, ultrafiltration, microfiltration, etc. Adding polydielectric before filtering can sometimes enhance the above physical filtering and reduce the SDI value.
3. Generally, reverse osmosis process or ion exchange process should be used for inlet water?
In many influent conditions, the use of ion exchange resin or reverse osmosis is technically feasible, and the selection of process should be determined by economic comparison. Generally, the higher the salt content, the more economical the reverse osmosis is, and the lower the salt content, the more economical the ion exchange is. Due to the popularity of reverse osmosis technology, the combination process of reverse osmosis+ion exchange process or multi-stage reverse osmosis or reverse osmosis+other deep desalination technologies has become a recognized technical and economic more reasonable water treatment scheme. For further understanding, please consult the representative of the Water Treatment Engineering Company.
4. How many years can reverse osmosis membrane elements be used?
The service life of the membrane depends on the chemical stability of the membrane, the physical stability of the element, the cleanability, the water source of the inlet, the pretreatment, the cleaning frequency, the operation management level, etc. According to economic analysis, it is usually more than 5 years.
5. What is the difference between reverse osmosis and nanofiltration?
Nanofiltration is a membrane liquid separation technology between reverse osmosis and ultrafiltration. Reverse osmosis can remove the smallest solute with a molecular weight of less than 0.0001 μ m. Nanofiltration can remove solutes with a molecular weight of about 0.001 μ m. Nanofiltration is essentially a kind of low pressure reverse osmosis, which is used in situations where the purity of produced water after treatment is not particularly strict. Nanofiltration is suitable for treating well water and surface water. Nanofiltration is applicable to water treatment systems with high desalination rate that are unnecessary like reverse osmosis. However, it has a high ability to remove hardness components, sometimes called "softened 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. The reverse osmosis membrane 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 pass through the reverse osmosis membrane, and the removal rate of typical soluble salts is>95-99%. The operating pressure ranges from 7bar (100psi) when the inlet water is brackish water to 69bar (1000psi) when the inlet water is seawater. Nanofiltration can remove impurities of particles at 1nm (10A) and organic matters with molecular weight greater than 200~400. The removal rate of soluble solids is 20~98%, that of salts containing univalent anions (such as NaCl or CaCl2) is 20~80%, and that of salts containing bivalent anions (such as MgSO4) is 90~98%. Ultrafiltration can separate macromolecules larger than 100~1000 angstroms (0.01~0.1 μ m). All soluble salts and small molecules can pass through the ultrafiltration membrane, and the substances that can be removed include colloids, proteins, microorganisms and macromolecular organics. The molecular weight of most ultrafiltration membranes is 1000~100000. The range of particles removed by microfiltration is about 0.1~1 μ m. Generally, suspended solids and large particle colloids can be intercepted while macromolecules and soluble salts can freely pass through the microfiltration membrane. The microfiltration membrane is used to remove bacteria, micro flocs or TSS. The pressure on both sides of the membrane is typically 1~3 bar.
7. What is the maximum allowable silicon dioxide concentration of reverse osmosis membrane inlet water?
The maximum allowable concentration of silicon dioxide depends on temperature, pH value and scale inhibitor. Generally, the maximum allowable concentration of concentrated water is 100ppm without scale inhibitor. Some scale inhibitors can allow the maximum concentration of silicon dioxide in concentrated water to be 240ppm.
8. What is the effect of chromium on RO film?
Some heavy metals, such as chromium, will catalyze the oxidation of chlorine, thus causing irreversible degradation of the membrane. This is because Cr6+is less stable than Cr3+in water. It seems that the destructive effect of metal ions with high oxidation price is stronger. Therefore, the concentration of chromium should be reduced in the pretreatment section or at least Cr6+should be reduced to Cr3+.
9. What kind of pretreatment is generally required for RO system?
The usual pre-treatment system consists of coarse filtration (~80 μ m) to remove large particles, adding oxidants such as sodium hypochlorite, then fine 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 inlet of high-pressure pump. As the name implies, the safety filter is the final insurance measure to prevent accidental large particles from damaging the high-pressure pump impeller and membrane element. Water sources with more suspended particles usually require higher degree of pretreatment to meet the specified requirements for water inflow; For water sources with high hardness content, it is recommended to use softening or adding acid and scale inhibitor. For water sources with high microbial and organic content, activated carbon or anti pollution membrane elements should also be used.
10. Can reverse osmosis 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 log (removal rate>99.9%). However, it should also be noted that in many cases, microorganisms may still breed again on the water producing side of the membrane, which mainly depends on the way of assembly, monitoring and maintenance. In other words, the ability of a system to remove microorganisms depends on whether the system design, operation and management are appropriate rather than the nature of the membrane element itself.
11. What is the impact of temperature on water yield?
The higher the temperature is, the higher the water yield is, and vice versa. When operating at a higher temperature, the operating pressure should be lowered to keep the water yield unchanged, and vice versa.
12. What is particle and colloid pollution? How to measure?
Once the fouling of particles and colloids occurs in the reverse osmosis or nanofiltration system, the water yield of the membrane will be seriously affected, and sometimes the desalination rate will be reduced. The early symptom of colloid fouling is the increase of system differential pressure. The source of particles or colloids in the membrane inlet water source varies from place to place, often including bacteria, sludge, colloidal silicon, iron corrosion products, etc. Drugs used in the pretreatment part, such as polyaluminum chloride, ferric chloride or cationic polyelectrolyte, may also cause fouling if they cannot be effectively removed in the clarifier or media filter.
13. How to determine the direction of installing brine seal ring on membrane element?
The brine seal ring on the membrane element is required to be installed at the water inlet end of the element, and the opening faces the water inlet direction. When the pressure vessel is fed with water, its opening (lip edge) will be further opened to completely seal the side flow of water from the membrane element to the inner wall of the pressure vessel.
Post time: Nov-14-2022