Views: 3 Author: Site Editor Publish Time: 2022-06-23 Origin: Site
In general, when the standardized flux decreases by 10% to 15%, or the system desalination rate decreases by 10% to 15%, or the operating pressure and differential pressure between segments increases by 10% to 15%, the RO system should be cleaned.
Cleaning frequency is directly related to the degree of system pretreatment, when SDI15 <3, cleaning frequency may be 4 times a year; when SDI15 in about 5, cleaning frequency may have to double but cleaning frequency depends on the actual situation of each project site.
Currently the best technique to evaluate the possibility of colloidal contamination in the feed water of RO/NF system is to measure the sludge density index (SDI, also known as the fouling index) of the feed water, which is an important parameter that must be determined before the RO design.
This must be measured periodically during RO/NF operation (2 to 3 times daily for surface water), and ASTM D4189-82 provides the standard for this test.
The feed water regulations for membrane systems are that the SDI15 value must be ≤ 5. Effective technologies for SDI reduction pretreatment are multi-media filters, ultrafiltration, microfiltration, etc. Adding polyelectric media before filtration can sometimes enhance the above physical filtration and reduce the SDI value.
In many feed water conditions, the use of ion exchange resin or reverse osmosis are technically feasible, the choice of process should be determined by economic comparison, in general, the higher the salt content, the more economical reverse osmosis, the lower the salt content, the more economical ion exchange.
Due to the massive popularity of reverse osmosis technology, reverse osmosis + ion exchange process or multi-stage reverse osmosis or reverse osmosis + other depth desalination technology combination process has become a recognized technical and economic more reasonable water treatment program, for further information, please consult the water treatment engineering company representatives.
The service life of the membrane depends on the chemical stability of the membrane, physical stability of the element, cleanability, water source, pretreatment, cleaning frequency, operation management level, etc. According to economic analysis, it is usually more than 5 years.
Nanofiltration is a membrane liquid separation technology located between reverse osmosis and ultrafiltration. Reverse osmosis can remove the smallest solute with molecular weight less than 0.0001 microns, while nanofiltration can remove solutes with molecular weight around 0.001 microns.
Nanofiltration is essentially a low-pressure reverse osmosis for applications where the purity of the treated produced water is not particularly stringent. Nanofiltration is suitable for treating well water and surface water.
Nanofiltration is suitable for water treatment systems that do not require high desalination rates like reverse osmosis, but have high removal capacity for hardness components, sometimes called "softening membranes", nanofiltration systems operate at low pressure and consume less energy than the corresponding reverse osmosis systems.
Reverse osmosis is a relatively sophisticated liquid filtration technology, reverse osmosis membrane for dissolved salts and other inorganic molecules and molecular weight greater than 100 organic substances to play a retention role, on the other hand, water molecules can freely through the reverse osmosis membrane, the typical soluble salt removal rate of > 95% to 99%. The operating pressure ranges from 7 bar (100 psi) for brackish feed water to 69 bar (1,000 psi) for seawater.
Nanofiltration can remove impurities with particles of 1 nm (10Å) and organic matter with molecular weight greater than 200 to 400. The removal rate of dissolved solids is 20% to 98%, while the removal rate of salts containing monovalent anions (such as NaCl or CaCl2) is 20% to 80%, and the removal rate of salts containing divalent anions (such as MgSO4) is higher at 90% to 98%.
Ultrafiltration has a separation effect for macromolecules larger than 100 to 1,000Å (0.01 to 0.1 micron). All dissolved salts and small molecules can pass through the ultrafiltration membrane, and the substances that can be removed include colloids, proteins, microorganisms and macromolecular organics. Most ultrafiltration membranes have a retention molecular weight of 1 000 to 100 000.
Water treatment companies can provide special membrane cleaning agent and cleaning services, users can buy cleaning agent for membrane cleaning according to the recommendation of membrane companies or equipment suppliers.
The maximum permissible silica concentration depends on temperature, pH and scale inhibitor, usually the maximum permissible concentration is 100ppm at the concentrated water end without scale inhibitor, some scale inhibitors can allow the concentration of silica in concentrated water up to 240ppm, please consult the scale inhibitor supplier.
Some heavy metals such as chromium can play a catalytic role in the oxidation of chlorine, which in turn causes irreversible performance degradation of the membrane. This is because Cr6+ is less stable than Cr3+ in water. It seems that this destructive effect is stronger for metal ions with high oxidation valence. Therefore, the concentration of chromium should be reduced or at least Cr6+ should be reduced to Cr3+ in the pretreatment section.
The usual pretreatment system is composed as follows: coarse filtration (~80 microns) to remove large particles, adding sodium hypochlorite and other oxidants, then precision filtration by multi-media filter or clarifier, then adding sodium bisulfite to reduce residual chlorine and other oxidants, and finally installing a security filter before the high-pressure pump inlet.
The role of the security filter, as the name implies, is as the final insurance measure to prevent the occasional large particles on the high-pressure pump impeller and membrane elements of the destructive effect. Water sources containing more suspended particles usually require a higher degree of pretreatment to meet the specified influent requirements; water sources with high hardness content are recommended to be softened or acid and scale inhibitors are added, etc. For water sources with high microbial and organic content, activated carbon or anti-pollution membrane elements are also required.
Reverse osmosis (RO) is very dense and has a very high removal rate for viruses, phages and bacteria, at least 3log or more (removal rate >99.9%). However, it should be noted that in many cases, microbial re-growth on the produced water side of the membrane can still occur, depending on how it is assembled, monitored and maintained, meaning that the ability of a system to remove microorganisms depends critically on the design, operation and management of the system rather than the nature of the membrane elements themselves.
Read more: How to remove viruses from drinking water？
The higher the temperature, the higher the water production and vice versa. When operating at higher temperatures, the operating pressure should be lowered to keep the water production constant and vice versa. Please refer to the relevant section for the temperature correction factor TCF for water production variation.
Once particle and colloid fouling occurs in a reverse osmosis or nanofiltration system, it can seriously affect the water yield of the membrane and sometimes reduce the desalination rate.
Early symptoms of colloidal fouling are an increase in system differential pressure. The source of particles or colloids in the membrane feed water varies from place to place and often includes bacteria, sludge, colloidal silica, iron corrosion products, etc. Drugs used in the pretreatment section, such as polymeric aluminum and ferric chloride or cationic polyelectric media, may also cause fouling if they are not effectively removed in the clarifier or media filter.
In addition, cationic polyelectric media will also react with anionic scale inhibitors, and their precipitates will foul the membrane elements.