How often should a reverse osmosis system be cleaned?
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 the pressure difference between segments increase by 10 to 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 on each project site.
What is SDI?
At present, the best technique for evaluating the possibility of colloidal contamination in the inlet water of the RO/NF system is to measure the silting density index (SDI, also known as the fouling index) of the inlet water, which is an important parameter that must be determined before the RO is designed, and must be measured regularly during the operation of the RO/NF (2 to 3 times a day for surface water), and ASTM D4189-82 specifies the standard for this test.
The water intake regulation for membrane systems is that the SDI15 value must be ≤5. Effective techniques to reduce SDI pretreatment are multi-media filters, ultrafiltration, microfiltration, etc. Adding polyelectrics prior to filtration can sometimes enhance the physical filtration described above and reduce the ability to reduce SDI values.
Should the general inlet water be a reverse osmosis process or an ion exchange process?
In many water inlet conditions, the use of ion exchange resin or reverse osmosis is technically feasible, the choice of process should be determined by economic comparison, in general, the higher the salt content, the more economical the reverse osmosis, the lower the salt content, the more economical the ion exchange.
Due to the widespread popularity of reverse osmosis technology, the use of reverse osmosis + ion exchange process or multi-stage reverse osmosis or reverse osmosis + other deep desalination technology combination process has become a recognized technology and economic more reasonable water treatment program, for more in-depth understanding, please consult the representative of the water treatment engineering company.
How many years can reverse osmosis membrane elements generally be used? The service life of the membrane depends on the chemical stability of the membrane, the physical stability of the components, the cleanability, the water source, the pretreatment, the cleaning frequency, the level of operation management, etc. According to economic analysis is usually more than 5 years.
How many years can reverse osmosis membrane elements generally be used?
The service life of the membrane depends on the chemical stability of the membrane, the physical stability of the components, the cleanability, the water source, the pretreatment, the cleaning frequency, the level of operation management, etc. According to economic analysis is usually more than 5 years.
What is the difference between reverse osmosis and nanofiltration?
Nanofiltration is a membrane liquid separation technology located between reverse osmosis contract ultrafiltration, reverse osmosis can remove the smallest solute, molecular weight less than 0.0001 microns, nanofiltration can remove solutes with molecular weight of about 0.001 microns.
Nanofiltration is essentially a low-pressure reverse osmosis for applications where the purity of the water production after treatment is not particularly strict, and nanofiltration is suitable for treating well water and surface water. Nanofiltration is suitable for water treatment systems that do not have to have a high desalination rate like reverse osmosis, but have a high removal capacity for hardness components, sometimes referred to as "softening membranes", and nanofiltration systems operate at low pressure and consume less energy than the corresponding reverse osmosis systems.
What is the separation capability of membrane technology?
Reverse osmosis is currently the most sophisticated liquid filtration technology, reverse osmosis membrane on dissolved salts and other inorganic molecules and molecular weight greater than 100 organic matter play a role in interception, on the other hand, water molecules can freely pass through the reverse osmosis membrane, the typical soluble salt removal rate of > 95 ~ 99%.
Operating pressures range from 7 bar (100 psi) in brackish inlet water to 69 bar (1,000 psi) at seawater. Nanofiltration can remove impurities and organic matter with molecular weight greater than 200 to 400 particles at 1 nm (10 angstroms), the removal rate of dissolved solids is 20 to 98%, the removal rate of monovalent anions-containing salts (such as NaCl or CaCl2) is 20 to 80%, and the removal rate of divalent anions -- such as MgSO4 is higher, which is 90 to 98%.
Ultrafiltration has a separation effect on macromolecules greater than 100 to 1,000 angstroms (0.01 to 0.1 microns). 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 residual molecular weight of 1,000 to 100,000. Microfiltration removes particles in the range of about 0.1 to 1 microns, under normal circumstances, suspended solids and large particle colloids can be intercepted while macromolecules and dissolved salts can freely pass through the microfiltration membrane, microfiltration membrane is used to remove bacteria, micro flocculates or total suspended solids TSS, typical of the pressure on both sides of the membrane is 1 to 3 bar.
Who sells membrane cleaners or provides cleaning services?
Water treatment companies can provide special membrane cleaners and cleaning services, users can purchase their own cleaning agents for membrane cleaning according to the recommendations of membrane companies or equipment suppliers.
What is the maximum allowable silica concentration in the reverse osmosis membrane inlet water?
The maximum permissible silica concentration depends on temperature, pH and scale inhibitor, usually the maximum allowable concentration at the water end is 100ppm when no scale inhibitor is added, and some scale inhibitors can allow the maximum concentration of silica in concentrated water to be 240ppm, please consult the scale inhibitor supplier.
What is the effect of chromium on RO membranes?
Some heavy metals such as chromium can catalyze the oxidation of chlorine, which in turn causes the irreversible performance of the diaphragm to decay. This is because Cr6+ is less stable than Cr3+ in water. It seems that oxidation of metal ions with high prices makes this destructive effect stronger. Therefore, the concentration of chromium should be reduced in the pretreatment section or at least Cr6+ should be reduced to Cr3+.
What kind of pretreatment is generally required for RO systems?
The usual pretreatment system is composed of the following, coarse filter (~80 microns) to remove large particles, add sodium hypochlorite and other oxidants, and then through the multi-media filter or clarification tank for precision filtration, and then add sodium bisulfite to reduce residual chlorine and other oxidants, and finally install a security filter before the high-pressure pump inlet.
The role of the security filter, as the name suggests, is as the ultimate insurance measure to prevent the accidental damaging effect of large particles on the impeller and membrane elements of the high-pressure pump. Water sources containing more particulate suspended solids usually require a higher degree of pretreatment to meet the specified water intake requirements; Water sources with high hardness content, it is recommended to use softening or adding acid and anti-scaling agents, etc., for water sources with high content of microorganisms and organic matter, activated carbon or anti-pollution membrane elements are also needed.
Can reverse osmosis remove microorganisms such as viruses and bacteria?
Reverse osmosis (RO) is very dense, with a very high removal rate for viruses, bacteriophages and bacteria, at least above 3logs (removal rate >99.9%). However, it should also be noted that in many cases, microorganisms may still re-breed on the water-producing side of the membrane, depending mainly on the way of assembly, monitoring and maintenance, that is, the ability of a system to remove microorganisms depends critically on the appropriate design, operation and management of the system rather than the nature of the membrane elements themselves.
How does temperature affect water production?
The higher the temperature, the higher the water production, and vice versa, when operating at higher temperature conditions, the operating pressure should be lowered so that the water yield remains unchanged, and vice versa. Please refer to the relevant section on the temperature correction coefficient TCF for changes in water production.
What is particle and colloid contamination? How is it determined?
Once the reverse osmosis or nanofiltration system is fouled by particles and colloids, it will seriously affect the water production of the membrane and sometimes reduce the desalination rate.
The early symptoms of colloidal fouling are an increase in the pressure difference of the system, the source of particles or colloids in the membrane inlet water source varies from place to place, often including bacteria, silt, colloidal silicon, iron corrosion products, etc., and the drugs used in the pretreatment part such as polymeric aluminum and ferric chloride or cationic polyelectric media, if they cannot be effectively removed in the clarification tank or media filter, may also cause contamination.
In addition, the cationic polyelectric will also react with the anionic scale inhibitor, and its sediment will foul the membrane element, and whether the water is suitable for such fouling tendency or pretreatment is evaluated by SDI15, please refer to the detailed description of the relevant chapter.
How long is the maximum allowed downtime without system flushing?
If the system uses a retardant, when the water temperature is between 20 and 38 ° C, about 4 hours; At 20 ° C or less, about 8 hours; If the system is not using a scale inhibitor, about 1 day.
How can the energy consumption of the membrane system be reduced?
Low-energy membrane elements are sufficient, but it should be noted that their desalination rate is slightly lower than that of standard membrane elements.
Can the reverse osmosis pure water system start and stop frequently?
Membrane systems are designed to operate continuously, but in practice, there is always a certain frequency of start-up and shutdown.
When the membrane system is shut down, it must be flushed at a low pressure with its produced or pretreated water to replace the membrane element with a high concentration of concentrated water containing a scale inhibitor.
Measures should also be taken to prevent the introduction of air due to water leakage in the system, as the loss of water in the components may result in an irreversible loss of water flux. If the downtime is less than 24 hours, there is no need to take measures to prevent microbial growth. However, if the downtime exceeds the above provisions, the protective liquid should be used as a system preservation or regular flushing of the membrane system.
How to determine the direction of the brine seal installed on the membrane element?
It is required that the brine seal on the membrane element is installed at the inlet end of the element, and the opening is facing the direction of inlet water, and when the pressure vessel is fed water, its opening (lip) will be further opened to completely seal the side flow of the inlet water from the membrane element and the inner wall of the pressure vessel.
How to remove silicon from water?
Silicon in water exists in two forms, active silicon (monomeric silicon) and colloidal silicon (polysilicon): colloidal silicon has no ion characteristics, but the scale is relatively large, colloidal silicon can be trapped by a fine physical filtration process, such as reverse osmosis, can also reduce the content in water through coagulation technology, such as coagulation clarification tank, but those that need to rely on ion charge characteristics of separation techniques, such as ion exchange resin and continuous electrodeionization process (CDI), the effect of removing colloidal silicon is very limited.
The size of active silicon is much smaller than that of colloidal silicon, so that most physical filtration technologies such as condensation clarification, filtration and air flotation cannot remove active silicon, and the process that can effectively remove active silicon is reverse osmosis, ion exchange and continuous electrodeionization process.
What is the effect of pH on removal rate, water yield and membrane life?
Reverse osmosis membrane products corresponding to the pH range, generally 2 ~ 11, pH has little impact on the membrane performance itself, which is one of the significant features different from other membrane products, but the characteristics of many ions in the water are greatly affected by pH, such as when citric acid and other weak acids are mainly non-ionic under low pH conditions, and dissociated at high pH and are ionic.
Because the same ion, the degree of charge is high, the removal rate of the membrane is high, the degree of charge is low or not charged, the removal rate of the membrane is low, so the pH has a great influence on the removal rate of some impurities.
What is the relationship between inlet water TDS and conductivity?
When obtaining the inlet conductivity value, it must be converted to a TDS value so that it can be entered at the time of software design. For most water sources, the conductivity/TDS ratio is between 1.2 and 1.7, and for rosa design, the 1.4 ratio is used for seawater and 1.3 ratio for brackish water, which usually results in a better approximate conversion rate.
How do I know if the membrane has been contaminated?
The following are common symptoms of contamination:
Under standard pressure, the water yield decreases In order to achieve the standard water yield, it is necessary to increase the operating pressure v The pressure drop between the inlet water and the concentrated water increases v, and the weight of the membrane element increases the v film removal rate changes significantly (increase or decrease)
When the element is removed from the pressure vessel, the water is poured on the inlet side of the erected membrane element, and the water cannot flow through the membrane element, only overflowing from the end face (indicating that the inlet runner is completely blocked).
How to prevent the growth of microorganisms in the original packaging of the membrane element?
When the protective liquid is cloudy, it is likely to be due to the breeding of microorganisms. Membrane elements protected with sodium bisulfite should be viewed every three months.
When the protective liquid appears turbid, the element should be removed from the preservation seal bag, re-soaked in a fresh protective liquid, the protective liquid concentration is 1% (by weight) food grade sodium bisulfite (not activated by cobalt), soaked for about 1 hour, and resealed and sealed, the component should be drained before repacking.
What are the water intake requirements for RO membrane elements and IX ion exchange resins?
Theoretically, entering the RO and IX systems should not contain the following impurities: suspended solids, colloids, calcium sulfate, algae, bacteria, oxidants, such as residual chlorine, etc., oils or lipids (must be below the lower detection limit of the instrument), organic and iron - organic complexes, iron, copper, aluminum corrosion products and other metal oxides, inlet water quality on the RO elements and IX resin life and performance will have a huge impact.
What impurities can RO membranes remove?
RO membrane can remove ions and organic matter well, reverse osmosis membrane has a higher removal rate than nanofiltration membrane, reverse osmosis can usually remove 99% of the salt in the feed water, and the removal rate of organic matter in the inlet water ≥ 99%.
How do I know which cleaning method to use for your membrane system?
In order to obtain the best cleaning effect, it is very important to choose the cleaning agent and cleaning steps that can be symptomatic, the wrong cleaning will actually worsen the performance of the system, in general, inorganic scaling contaminants, it is recommended to use acidic cleaning liquid, microorganisms or organic contaminants, it is recommended to use alkaline cleaning liquid.
Why is the pH of RO produced water lower than the pH of inlet water?
When the balance between CO2, HCO3- and CO3= is understood, the best answer to this question can be found, in a closed system, the relative content of CO2, HCO3- and CO3= changes with the change of pH value, under low pH conditions, CO2 accounts for the main part, in the medium pH range, mainly HCO3-, high pH range, mainly CO3 =.
Since the RO membrane can remove dissolved ions and can not remove the solubility gas, the CO2 content in the RO produced water is basically the same as the content of CO2 in the RO inlet water, but HCO3- and CO3= can often be reduced by 1 to 2 orders of magnitude, which will break the balance between CO2, HCO3- and CO3= in the inlet water, in a series of reactions, CO2 will be combined with H2O to undergo the following reaction equilibrium transfer, until a new equilibrium is established.
HCO3- + H+ H2OàCO2 +
If the inlet water contains CO2, the pH of the RO will always decrease, for most RO systems reverse osmosis water production pH value will have a decrease of 1 to 2 pH values, when the inlet water alkalinity and HCO3- high, the pH value of the water production will be greater.
A very small number of inlet water, containing less CO2, HCO3 - or CO3 = so that the change in the pH value of the water produced is less, some countries and regions, for the drinking water pH value has regulations, generally 6.5 ~ 9.0, according to our understanding, this is to prevent the corrosion of water pipelines, and drinking low pH water, itself will not cause any health problems, as we all know, many commercially available carbonated beverages have a pH value between 2 and 4.
Source: Sewage Treatment Assistant