Application of Reverse Osmosis Membrane in Wastewater Treatment
Release time:
2024-01-20
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 pressure difference 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 about 5, the cleaning frequency may be doubled, but the cleaning frequency depends on the actual situation of each project site.
01 What is SDI?
At present, the best technology for evaluating the possibility of colloid pollution in the influent of RO/NF system is to measure the siltation density index (SDI, also known as fouling index) of the influent, which is an important parameter that must be determined before RO design. During the operation of RO/NF, it must be measured regularly (2-3 times a day for surface water). ASTM D4189-82 specifies the standard for this test. The water inlet regulation of the membrane system is that the SDI15 value must be ≤ 5. Effective techniques for reducing SDI pretreatment include multi-media filters, ultrafiltration, microfiltration, etc. The addition of poly dielectric prior to filtration can sometimes enhance the ability of the above physical filtration to lower the SDI value.
02 general water should choose reverse osmosis or ion exchange process?
In many influent conditions, the use of ion exchange resin or reverse osmosis is technically feasible, and the choice of process should be determined by economic comparison. In general, the higher the salt content, the more economical the reverse osmosis, and the lower the salt content, the more economical the ion exchange. Due to the popularity of reverse osmosis technology, the combination process of reverse osmosis ion exchange process or multi-stage reverse osmosis or other deep desalination technology has become a recognized technology and economic more reasonable water treatment scheme.
03 What is the difference between reverse osmosis and nanofiltration?
Nanofiltration is a membrane liquid separation technology between reverse osmosis F and ultrafiltration. Reverse osmosis can remove the smallest solute with a molecular weight of less than 0.0001 microns, and nanofiltration can remove solute with a molecular weight of about 0.001 microns. Nanofiltration is essentially a low-pressure reverse osmosis, used for the treatment of water purity is not particularly strict occasions, nanofiltration is suitable for the treatment of well water and surface water. Nanofiltration is suitable for water treatment systems that do not need high desalination rates like reverse osmosis, but it has a high removal capacity for hardness components, sometimes referred to as "softening membranes". The nanofiltration system has low operating pressure and lower energy consumption. The corresponding reverse osmosis system.
What is the separation capacity of membrane technology?
Reverse osmosis is currently the most sophisticated liquid filtration technology, reverse osmosis membrane on the solubility of salt and other inorganic molecules and molecular weight greater than 100 of the organic matter from the interception, on the other hand, water molecules can freely through the reverse osmosis membrane, the typical soluble salt removal rate of> 95 ~ 99%. The operating pressure ranges from 7bar(100psi) when the inlet water is brackish water to 69bar(1,000psi) when seawater. Nanofiltration can remove impurities with particles at 1nm(10 angstroms) and organic matter with molecular weight greater than 200~400. The removal rate of dissolved solids is 20 ~ 98%, the removal rate of salts containing monovalent anions (such as NaCl or CaCl2) is 20 ~ 80%, while the removal rate of salts containing divalent anions (such as MgSO4) is higher, 90 ~ 98%. Ultrafiltration separates large molecules larger than 100 to 1,000 angstroms (0.01 to 0.1 microns). 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 organic matter. Most ultrafiltration membranes have a molecular weight cut-off of 1,000 to 100,000. Microfiltration removes particles in the range of about 0.1 to 1 micron. Under normal circumstances, suspended solids and large particles of colloid can be retained while macromolecules and soluble salts can freely pass through the microfiltration membrane. Microfiltration membranes are used to remove bacteria, microflocs or total suspended solids TSS. The pressure on both sides of the membrane is typically 1 to 3bar.
05 reverse osmosis membrane water maximum allowable silica concentration?
The maximum allowable concentration of silica depends on temperature, pH value and scale inhibitor. Generally, the maximum allowable concentration at the concentrated water end is 100ppm when scale inhibitor is not added. Some scale inhibitors can allow the maximum concentration of silica in concentrated water to 240ppm. Please consult the scale inhibitor supplier.
06 What is the effect of chromium on RO membrane?
Certain heavy metals, such as chromium, catalyze the oxidation of chlorine and cause irreversible degradation of the membrane. This is because Cr6 is less stable than Cr3 in water. It seems that the high oxidation of metal ions, the destruction of the stronger. Therefore, the concentration of chromium should be reduced or at least Cr6 should be reduced to Cr3 in the pretreatment section.
07 reverse osmosis can 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 3log (removal rate> 99.9%). However, it should also be noted that in many cases, microorganisms may still appear on the water side of the membrane to breed again, which mainly depends on the way of assembly, monitoring and maintenance. That is to say, the ability of a certain system to remove microorganisms depends on Whether the system design, operation and management are appropriate rather than the nature of the membrane element itself.
08 What is particulate and colloidal contamination and how is it measured?
Reverse osmosis or nanofiltration system once the particles and colloids will seriously affect the water yield of the membrane, and sometimes reduce the desalination rate. The early symptom of colloidal fouling is the increase of system pressure difference. The sources of particles or colloids in the membrane influent water source vary from place to place, often including bacteria, sludge, colloidal silicon, iron corrosion products, etc. The drugs used in the pretreatment part, such as polyaluminum, ferric chloride or cationic polyelectric medium, may also cause fouling if they cannot be effectively removed in the clarifier or medium filter. In addition, cationic polydielectrics will also react with anionic scale inhibitors, and their precipitates will foul the membrane element. SDI15 is used to evaluate whether this kind of fouling tendency or pretreatment in water is qualified. Please refer to the detailed introduction in relevant chapters.
09 No system flushing, the longest system flushing is allowed, and the longest shutdown is allowed?
If the system uses scale inhibitor, when the water temperature is between 20~38 ℃, about 4 hours; below 20 ℃, about 8 hours; if the system does not use scale inhibitor, about 1 day. 15. How can the energy consumption of membrane systems be reduced?
It is sufficient to use low-energy membrane elements, but it should be noted that their salt removal rate is slightly lower than that of standard membrane elements.
010 How to remove silicon from water?
Silicon in water exists in two forms, active silicon (monomer silicon) and colloidal silicon (multi-component silicon): colloidal silicon has no ion characteristics, but the scale is relatively large, colloidal silicon can be trapped by fine physical filtration processes, such as Reverse osmosis can also reduce the content of water through coagulation technology, such as coagulation clarifier, but those separation technologies that need to rely on ion charge characteristics, such as ion exchange resin and continuous electrodeionization process (CDI), the removal of colloidal silicon effect is very limited.
The size of active silicon is much smaller than that of colloidal silicon, so that most physical filtration technologies such as coagulation clarification, filtration and air flotation cannot remove active silicon. The processes that can effectively remove active silicon are reverse osmosis, ion exchange and continuous electrodeionization processes.
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