Source: Waste-to-energy water treatment
Part.1
Frequently Asked Questions about Reverse Osmosis Systems
1. How often should the reverse osmosis system be cleaned?
Under normal circumstances, when the normalized flux decreases by 10~15%, or the system desalination rate decreases by 10~15%, or the operating pressure and the 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 the SDI is 15<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?
The best technique currently available to evaluate the possibility of colloidal contamination in the influent water of an RO/NF system is to measure the siltation density index (SDI, also known as fouling index) of the influent water, which is an important parameter that must be determined prior to RO design.
During RO/NF operation, measurements must be taken at regular intervals (2~3 times per day for surface water), and ASTM D4189-82 specifies the standard for this test.
The water inlet specification for the membrane system is that the SDI15 value must be ≤5. Effective technologies to reduce SDI pretreatment include multi-media filters, ultrafiltration, microfiltration, etc. The addition of polydielectric prior to filtration can sometimes enhance the ability of the physical filtration described above to reduce the SDI value.
3. Should the reverse osmosis process or ion exchange process be used for general influent water?
In many influent conditions, the use of ion exchange resins 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 large 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 technical and economic more reasonable water treatment scheme, if you need to understand more, please consult the representative of the water treatment engineering company.
4. How many years can the reverse osmosis membrane element 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 inlet water source, the pretreatment, the frequency of cleaning, the level of operation management, etc. According to the 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 located between reverse osmosis contract ultrafiltration, reverse osmosis can remove the smallest solute, the molecular weight is less than 0.0001 microns, and nanofiltration can remove solutes with a molecular weight of about 0.001 microns.
Nanofiltration is essentially a low-pressure reverse osmosis, which is used when the purity of the produced water 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 require a high desalination rate like reverse osmosis, but has a high ability to remove hardness components, sometimes referred to as "softened membranes", and the operating pressure of the nanofiltration system is low, and the energy consumption is lower than that of the corresponding reverse osmosis system.
6. What kind of separation power does membrane technology have?
Reverse osmosis is the most sophisticated liquid filtration technology at present, reverse osmosis membrane on dissolved salts and other inorganic molecules and organic matter with a molecular weight greater than 100 to intercept the role, on the other hand, water molecules can freely pass through the reverse osmosis membrane, the removal rate of typical soluble salts is >95~99%. Operating pressures range from 7 bar (100 psi) when the feed water is brackish water to 69 bar (1,000 psi) when seawater.
Nanofiltration can remove impurities and organic matter with a molecular weight greater than 200~400 at 1 nm (10 angstroms), 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%, and the removal rate of salts containing divalent anions (such as MgSO4) is higher, which is 90~98%.
Ultrafiltration has a separation effect on macromolecules larger than 100~1,000 angstroms (0.01~0.1 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 1,000~100,000.
The range of microfiltration to remove particles is about 0.1~1 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, the microfiltration membrane is used to remove bacteria, microflocs or total suspended solids TSS, and the typical pressure on both sides of the membrane is 1~3bar.
7. Who sells membrane cleaners or provides cleaning services?
Water treatment companies can provide special membrane cleaning agents and cleaning services, and users can purchase their own cleaning agents for membrane cleaning according to the recommendations of membrane companies or equipment suppliers.
8. What is the maximum allowable silica concentration of reverse osmosis membrane influent?
The maximum allowable concentration of silica depends on temperature, pH and antiscalant, typically up to 100 ppm at the concentrate end without antiscalant, and up to 240 ppm for some antiscalants, please consult the antiscalant supplier.
9. What is the effect of chromium on RO film?
Some heavy metals such as chromium can catalyze the oxidation of chlorine, which in turn causes irreversible degradation of the diaphragm. This is because Cr6+ is less stable than Cr3+ in water. It seems that the oxidation of metal ions with high valences makes this destructive effect even stronger. Therefore, the concentration of chromium should be reduced or at least Cr6+ to Cr3+ should be reduced in the pretreatment section.
10. What kind of pretreatment is generally required for RO system?
The usual pretreatment system consists of the following components, coarse filtration (~80 microns) to remove large particles, addition of oxidants such as sodium hypochlorite, then precision filtration through a multi-media filter or clarifier, then addition of sodium bisulfite to reduce residual chlorine and other oxidants, and finally the installation of a security filter before the inlet of the high-pressure pump.
As the name suggests, the security filter serves as a final insurance measure against the damaging effect of accidental large particles on the impeller and membrane elements of the high-pressure pump. Water sources with a large amount of particulate suspended solids usually require a higher degree of pretreatment to meet the specified inlet water requirements; For water sources with high hardness content, it is recommended to use softening or adding acid and antiscalants, and for water sources with high content of microorganisms and organic matter, activated carbon or anti-fouling membrane elements are also required.
11. 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, phages and bacteria, at least above 3log (removal rate > 99.9%). However, it is also important to note that in many cases, microbial regrowth can still occur on the permeate side of the membrane, depending on how it is assembled, monitored, and maintained, meaning that the ability of a system to remove microorganisms depends critically on the proper design, operation, and management of the system rather than on the nature of the membrane element itself.
12. What is the effect of temperature on water production?
The higher the temperature, the higher the water yield and vice versa, and when operating at higher temperature conditions, the operating pressure should be lowered so that the water yield remains the same and vice versa. For the temperature correction factor TCF for changes in water production, please refer to the relevant section.
13. What is particle and colloidal contamination? How is it determined?
Fouling of particles and colloids in reverse osmosis or nanofiltration systems can seriously affect the water yield of the membrane and sometimes reduce the desalination rate.
The early symptoms of colloidal fouling are the increase in the differential pressure of the system, the source of particles or colloids in the membrane influent water source varies from place to place, often including bacteria, sludge, colloidal silicon, iron corrosion products, etc., and the drugs used in the pretreatment part such as polyaluminum and ferric chloride or cationic polydielectric may also cause fouling if they cannot be effectively removed in the clarifier or media filter.
In addition, cationic polydielectrics can also react with anionic scale inhibitors, and their precipitates can foul the membrane elements, and SDI15 is used to evaluate whether such fouling tendencies or pretreatment are qualified in water, please refer to the relevant sections for details.
14. How long is the maximum allowable shutdown without system flushing?
If the system uses antiscalant, when the water temperature is between 20~38°C, about 4 hours; below 20 °C, about 8 hours; If the system is not with antiscalants, about 1 day.
15. Can the reverse osmosis pure water system start and stop frequently?
The membrane system is designed for continuous operation, but in actual operation, there will always be 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 permeate or pre-treated water to replace the highly concentrated water containing antiscalants from the membrane elements.
Measures should also be taken to prevent water leakage in the system and the introduction of air, as irreversible loss of permeate flux can occur if the components run dry.
If the downtime is less than 24 hours, there is no need to take measures to prevent the growth of microorganisms. However, if the downtime exceeds the above provisions, the protective solution should be used as the system storage or the membrane system should be flushed regularly.
16. How to determine the direction of the brine sealing ring installed on the membrane element?
The brine sealing ring on the membrane element is required to be installed at the inlet end of the element, and the opening is facing the direction of the inlet water, and when the pressure vessel is fed with water, its opening (lip) will be further opened to completely seal the bypass flow of the inlet water from the membrane element and the inner wall of the pressure vessel.
17. How to remove silicon from water?
Silicon in water exists in two forms, active silicon (monomeric silicon) and colloidal silicon (multi-silicon): colloidal silicon does not have the characteristics of ions, but the scale is relatively large, colloidal silicon can be intercepted by fine physical filtration processes, such as reverse osmosis, and can also reduce the content in water through coagulation technology, such as coagulation clarifiers, but those separation technologies that rely on ionic charge characteristics, such as ion exchange resins and continuous electrodeionization processes (CDI), have very limited effect on the removal of colloidal silicon.
The size of activated silicon is much smaller than that of colloidal silicon, so most physical filtration technologies such as coagulation clarification, filtration and air flotation cannot remove active silicon, and the processes that can effectively remove active silicon are reverse osmosis, ion exchange and continuous electrodeionization.
18. What is the effect of pH on removal rate, water yield and membrane life?
Reverse osmosis membrane products correspond to the pH range, generally 2~11, pH has little impact on the membrane performance itself, which is one of the significant characteristics different from other membrane products, but the characteristics of many ions in water are greatly affected by pH, for example, when citric acid and other weak acids are mainly non-ionic under low pH conditions, and dissociate and are ionic at high pH. Due to the high degree of charge of the same ion, the removal rate of the membrane is high, and the removal rate of the membrane is low if the degree of charge is low or uncharged, so the pH has a great influence on the removal rate of some impurities.
19. What is the relationship between influent TDS and conductivity?
When an influent conductivity value is obtained, it must be converted into a TDS value so that it can be entered during software design. For most water sources, the ratio of conductivity/TDS is between 1.2~1.7, and for ROSA design, the ratio of 1.4 is selected for seawater and 1.3 ratio is used for brackish water, which can usually obtain a good approximate conversion rate.
20. How do I know if the membrane has been contaminated?
The following are common symptoms of contamination:
At standard pressures, the water yield decreases
In order to achieve the standard water production, the operating pressure v must be increased
The pressure drop between the inlet water and the concentrated water increases v
The weight of the membrane element increases v
Significant change (increase or decrease) in membrane rejection rate
When the element is removed from the pressure vessel, water is poured on the inlet side of the upright membrane element, and the water cannot flow through the membrane element, only overflowing from the end face (indicating that the inlet flow channel is completely blocked)
21. How to prevent the growth of microorganisms in the original packaging of the membrane element?
When the protective solution is cloudy, it is likely due to microbial growth. Membrane elements protected with sodium bisulfite should be inspected every three months.
When the protective solution is turbid, the element should be removed from the preservation sealed bag, re-soaked in fresh protective solution, the concentration of the protective solution is 1% (weight) food-grade sodium bisulfite (not cobalt activated), soaked for about 1 hour, and resealed and sealed, the element should be drained before repackaging.
22. What are the water inlet requirements for RO membrane elements and IX ion exchange resins?
Theoretically, the entry into the RO and IX systems should be free of the following impurities:
Suspended solids
colloid
calcium sulphate
algae
bacteria
Oxidizing agents, such as residual chlorine, etc
Oils or lipids (must be below the lower detection limit of the instrument)
Organic matter and iron - complex of organic matter
Metal oxides such as iron, copper, aluminum corrosion products
The quality of the feed water will have a huge impact on the life and performance of the RO element and IX resin.
23. What impurities can be removed by RO membrane?
RO membranes can remove ions and organic matter well, reverse osmosis membranes have a higher removal rate than nanofiltration membranes, reverse osmosis can usually remove 99% of the salt in the feed water, and the removal rate of organic matter in the influent water is ≥99%.
24. How do you know what cleaning method to use for your membrane system?
In order to get the best cleaning results, it is important to choose the right cleaning agent and cleaning procedure, the wrong cleaning can actually deteriorate the performance of the system, in general, inorganic scale contaminants, acidic cleaning fluids are recommended, microbial or organic contaminants, alkaline cleaning fluids are recommended.
25. Why is the pH value of RO permeate water lower than that of influent water?
In a closed system, the relative contents of CO2, HCO3- and CO32- vary with pH, with CO2 accounting for the majority at low pH, HCO3- in the medium pH range, and CO32- in the high pH range.
Because the RO membrane can remove dissolved ions but not dissolved gases, the CO2 content in the RO permeate is basically the same as the CO2 content in the RO influent water, but HCO3- and CO32- can often be reduced by 1~2 orders of magnitude, which will break the balance between CO2, HCO3- and CO32- in the influent water, in the series of reactions, CO2 will combine with H2O to undergo the transfer of the following reaction equilibrium until a new equilibrium is established.
CO2 + H2O ---HCO3- + H+
If the influent contains CO2, the pH value of the permeate of the RO membrane element will always decrease, and the pH value of the permeate of most RO systems will drop by 1~2 pH, and the pH value of the permeate will drop even more when the alkalinity and HCO3- of the influent water are high. The very little influent contains less CO2, HCO3- or CO32 – so that less change in the pH of the permeate is seen.
The pH value of reverse osmosis effluent is low, and NaOH is added to the metering pump to adjust the pH to alkalinity, because when the pH value is between 7.5~8, the desalination effect of reverse osmosis can be the best.
26. How can the energy consumption of the membrane system be reduced?
Low-energy membrane elements are sufficient, but it should be noted that they have a slightly lower desalination rate than standard membrane elements.
Part.2
Summary of common problems with reverse osmosis equipment
1. Why does the sealing ring in reverse osmosis equipment swell?
In the water treatment equipment, the reverse osmosis equipment needs three types of sealing rubber rings in the membrane shell to realize the sealing and isolation of each section in the membrane shell. In order to reduce the installation resistance, the system should be coated with water or glycerin on the surface of each sealing ring during installation.
It should be noted that petroleum jelly or other petroleum-based grease lubricants should be used with caution as the lubricant will cause cracking of fresh water pipes, especially the expansion of the sealing ring. The expansion of the rubber ring generally does not directly affect the operation effect of the system, but it will affect the reloading of the system after unloading, that is, it is difficult for the expanded rubber ring to enter the slot during loading.
2. Is the water yield of each element in the process of reverse osmosis equipment consistent?
In the reverse osmosis equipment, because there is a pressure difference between the feed end and the concentrated end of the membrane element, that is, the membrane pressure drop, and because the salt content of the concentrated water of each element is higher than the salt content of the feed water, the osmotic pressure of the feed water of each element continues to rise along the system process.
If the freshwater back pressure and osmotic pressure are ignored, the water yield of each membrane element along the system flow will be proportional to the difference between the respective operating pressure and osmotic pressure, that is, the water yield of each membrane element will gradually decrease.
3. Will pH have an impact on the removal rate and life of reverse osmosis membranes?
Reverse osmosis equipment as the main filtration process in water treatment equipment, then in the raw water into the reverse osmosis membrane, the pH value of the raw water will not bring harm to the reverse osmosis membrane, generally speaking, the material of the reverse osmosis membrane is mostly composite membrane material, this membrane material in the use of the product is according to the range of pH value, generally 2-11, then the pH value to the membrane itself is less harmful and impactful.
As for the effect of pH on the desalination rate of reverse osmosis membrane, it is affected by the pH value of the characteristics of many ions in the water, which is determined by the acidity and alkalinity of the ions themselves, decomposition, and the degree of charge, which are all factors that will lead to the reduction of the desalination rate of the membrane.
It can be seen that pH still has a great influence on the desalination rate of some impurities. In the same way, if the removal rate of CO2 from the reverse osmosis membrane is zero, increase the pH value of the raw water to convert CO2 to CO32-, so that the reverse osmosis membrane can effectively desalination. However, special attention should be paid to the fouling of the reverse osmosis membrane.
4. How should the reverse osmosis equipment be operated when it is running for the first time?
The method of low pressure and low flow is used to drive away the air in the pipeline, and only the air in the pipeline does not exist in the equipment can operate normally. First of all, the pressure should be kept between 0.2~0.4MPa. When using low-pressure flushing and exhausting, the resulting brine and permeate are discharged into the sewer.
If the pressure rises rapidly during operation, this is due to the presence of air in the membrane element, which will generate a water flow and radial impact force. In this case, the outer skin of the membrane may be ruptured, resulting in a situation where the reverse osmosis membrane cannot be repaired.
When using it for the first time, it is necessary to adjust the pressure of the membrane operation to 0.2~0.4MPa for flushing, and ensure that the reverse osmosis system will automatically flush the reverse osmosis membrane function at low pressure every time it is started.
5. How to replace the security filter element in reverse osmosis equipment?
After a period of use, the quality of the treated water will vary due to the age of the security filter. At this time, the filter element will be fouled, and the replacement of the filter element should be based on the pressure difference before and after the filter, and when the pressure difference is greater than 0.03MPa, it should be considered to be replaced.
Replacement method: Turn off the reverse osmosis system. To remove the pressure, press the button of the pressure relief valve on the device until the indication on the pressure gauge is zero. Use a professional wrench to rotate the filter bottle to open it. Remove the old filter and replace it with the new one. Use a professional wrench to rotate and tighten the filter bottle of the filter.
6. How to clean and disinfect the reverse osmosis system?
Generally, the cleaning of the reverse osmosis system is required to be carried out by professional technicians, whether the customer can clean it by himself, if the reverse osmosis system needs to be cleaned, it should contact the professional manufacturer personnel to complete. The following conditions are the prerequisite for chemical cleaning, which can be cleaned if they occur.
Prerequisites for chemical cleaning: When the water yield of the system is reduced by 5-10% compared to the initial cleaning or the last cleaning run. When the desalination rate of the system is reduced by 2.5-5% compared to the initial or last cleaning run. When the pressure difference between the sections of the system is 1-2 times higher than the initial or after the last cleaning. The system is protected with a protective solution in the event of a long-term outage.
Note: Water yield and desalination rate are affected by water temperature, so the results of the test should be obtained at the same water temperature.
7. Is the fluoride removal effect of reverse osmosis equipment good?
If people use superfluorinated water, it will cause harm to the human body, in order to avoid people from the harm of fluoride in their lives, reverse osmosis equipment can be used to remove fluoride. Most of the fluoride ions in groundwater come from the erosion and dissolution of surrounding rock, and there are also a large number of soluble ions in the water, so the influence of fluoride removal on other molecules must be considered when defluoridation. In groundwater with excessive salt content, the fluoride removal rate of reverse osmosis equipment is not too high. However, compared with other methods, the reverse osmosis method is easy to operate and has a good treatment effect.
8. What are the basic requirements for the effluent quality of purified water equipment?
The purified water produced by purified water equipment is used in medicine, biomedical and other industries. But what are the basic criteria for purified water quality? Both the Chinese Pharmacopoeia and the European Pharmacopoeia clearly state that the raw water used in pharmaceutical applications must at least meet the standards for drinking water.
If it doesn't meet the target, it will evolve until it reaches the target. Except for E. coli, no more than 100 bacteria/ml are allowed together. In the process of making water in purified water equipment, there will also be internal pollution, and various water treatment devices in purified water equipment may become pollution sources.
Therefore, the purified water equipment must be cleaned and disinfected frequently, and in addition, the end of the purified water equipment should also be equipped with a sterilization and disinfection device.
9. What are the characteristics of the effluent quality of purified water equipment?
The effluent quality prepared by the purified water equipment meets the health standards stipulated by the state and the actual production standards of the enterprise. Effluent of purified water equipment - Purified water has two characteristics. One of the characteristics is that the number of disinfection and sterilization equipment installed in purified water equipment is gradually increasing.
Another feature is that the piping distribution system of the purified water plant replaces the traditional water supply line with a circulating pipeline, both of which are designed to control microbial contamination and the increase of bacterial endotoxins.
At the same time, it is also necessary to pay attention to the influence of the flow rate in the pipe on the reproduction of microorganisms, that is, the flow rate in the pipeline is too low or blocked, which may cause the reproduction of microorganisms to increase and affect the water quality.
10. What are the key points in the selection of the installation location of water softener equipment?
The effluent quality of the softened water equipment is good, the system is stable, and it is used by many enterprises. When installing water softener equipment, the following points should be noted:
1) The water softener should be as close to the drainage as possible.
2) If other water treatment equipment is needed, the installation space should be reserved.
3) Since salt is often added to the salt box, the position of salt should be set.
4) Do not install the demineralized water equipment within 3 meters of the boiler, otherwise the hot water will flow back into the demineralized water equipment, causing damage.
5) Keep the water demineralized water equipment in an environment where the room temperature is lower than 1°C and above 49°C.
11. What are the precautions for the use of softened water equipment?
Water softening equipment is a water treatment equipment that removes Ca and Mg plasma in water, reduces the hardness of water, and softens water. After a long period of use, softened water equipment should follow certain precautions.
1) The water inlet of the demineralized water equipment should be opened for a long time when in use, unless it needs to be closed when the equipment is overhauled. If no water is required, the outlet valve can be closed.
2) If the effluent is unqualified due to incomplete regeneration of the softened water equipment, it can be regenerated manually.
3) When the softened water equipment is out of service for a long time, the brine needs to be sucked into the tank for protection.
4) When it is put into use again, you only need to prepare the same salt water as it is working normally, open the water inlet valve first, then connect the power supply, let the equipment reset itself and then manually regenerate it again to open the outlet valve.
5) If the industrial salt used is too dirty, the salt bucket should be cleaned once a year to ensure the smooth progress of the salt absorption process.
12. What are the installation standards for ultrapure water equipment?
The effluent quality of the ultrapure water equipment is good, which is suitable for the production needs of various enterprises, and the scope of use is gradually expanded. This article mainly introduces the standard instructions for the installation of ultrapure water equipment.
1) The equipment installation site should be selected in a flat, clean environment, close to the power supply, water source environment.
2) Do not be close to the fire source and any heating element to avoid the heat affecting the efficiency of its operation.
3) The equipment should not be installed outdoors in the northern area to prevent the equipment from freezing inside and damaging the instrument and filter elements.
4) The installation position of the equipment should be convenient for drainage, so that the drainage pipe of the equipment can be kept smooth.
5) The working pressure of the pump used in the system is ensured to be 1.0-1.2Mpa, and the pump works within its rated head.
13. Booster pump and high-pressure pump do not absorb water
Solution: If it is 380V voltage, check whether the booster pump and high-pressure pump are reversed, if it is found that the reversal should be reversed, the three power terminals of the pump should be replaced by two arbitrarily, if it is not reversed, the exhaust valve of the pump needs to be opened or filled with water.
The booster pump and high-pressure pump with 220V voltage will not be reversed, as long as the exhaust valve of the pump is opened to vent or the pump body can be filled with water.
14. The high-pressure pump does not start
Solution: Check whether the relay docked with the high-pressure pump is engaged, and whether the wire end of the wiring bolt is loose, falling off or the underwater indicator light is on; If the water deficit indicator light is on, it means that the raw water source is not enough for the booster pump, in order to prevent the high-pressure pump from idling and damaging the high-pressure pump, the water cut-off protector is used to cut off the power supply of the high-pressure pump, so as to protect the high-pressure pump. If sufficient water source is provided to make the pressure of the water cut-off protector meet the working pressure requirements of the high-pressure pump, and the indicator light of the water-off protector is off, the high-pressure pump can be started.
15. The high-pressure pump makes an abnormal sound
Solution: Check whether the high-pressure pump is idling, sometimes the high-pressure pump will make some abnormal noise when the water does not fully enter, usually disappear automatically in 1~3 minutes, if it does not disappear after 3 minutes, the exhaust valve of the high-pressure pump should be opened for deflation or water injection.
16. The pipeline bursts
Solution: Due to the poor water quality and more impurities in some areas, or due to the failure to replace the fleece spray filter element and clean the RO membrane for a long time, the RO membrane will be blocked, which will lead to an increase in the pressure in the pipeline and a burst of the pipeline.
In this case, the first thing to do is to check whether the spray filter already needs to be cleaned or replaced, and then the RO membrane can be cleaned. Poor quality raw water will lead to frequent blockage of RO membranes, at this time, we should add ion exchange equipment or add scale inhibitors to the raw water to eliminate impurities in the water, so as to improve the quality of permeate and increase the service life of RO membranes.
17. The water output is getting smaller and smaller
Solution: Some equipment users will find that the water output of the equipment is getting smaller and smaller (the equipment that uses tap water as raw water basically does not have this phenomenon), this is because some groundwater quality is poor and there are more impurities, resulting in partial blockage of the RO membrane, so that the water output of the equipment is reduced.
At this time, we should regularly backwash the pretreatment, replace the velvet spray filter, clean the RO membrane, or replace the raw water from poor quality groundwater to tap water (in the absence of tap water, it is good to configure a set of ion exchange scale inhibition system, which will basically eliminate the occurrence of such phenomena).
18. Fine white or black suspended particles appear in pure water
Solution: This is due to contamination of the pipes, which leads to the growth of bacteria. In this case, the caustic soda should be dissolved and put into the precision filter, the control valve should be operated to close the wastewater, the pure water should be boiled to a high point, the high-pressure pump should be started, and the water flowing out of the pure water outlet should be introduced into the precision filter for about 30 minutes. Equipment equipped with a pipeline disinfection machine should open the pipeline disinfection machine in time to disinfect the purified water pipeline.
19. There is residual gas in the reverse osmosis equipment running under high pressure, and the formation of air hammer will damage the membrane
There are two common scenarios:
1) After the equipment is emptied, when it is re-run, the gas is quickly boosted and operated without exhaustion. The remaining air should be exhausted under the pressure of 2~4bar, and then gradually increase the pressure.
2) When the joint between the pretreatment equipment and the high-pressure pump is not well sealed or leaks (especially the leakage of the microfilter and the pipeline behind it), when the pretreatment water supply is not sufficient, if the microfiltration is blocked, part of the air will be sucked in due to the vacuum in the place where the sealing is not good. The microfilter should be cleaned or replaced to ensure that the pipeline does not leak.
In short, the pressure should be gradually increased when there are no bubbles in the flowmeter, and the bubbles found in the operation should be gradually depressurized to check the cause.
20. The method of shutting down the reverse osmosis equipment is incorrect and damages the membrane
1) Rapid pressure reduction during shutdown without thorough flushing. Because the concentration of inorganic salts on the concentrated side of the membrane is higher than that of the raw water, it is easy to scale and pollute the membrane.
2) Rinse with pretreated water with chemical reagents. Membrane contamination can occur during equipment outages due to chemically laden water.
When the reverse osmosis water treatment equipment is ready to shut down, the chemical reagent should be stopped, and the pressure should be gradually reduced to about 3bar and rinsed with pretreated water for 10min until the TDS of the concentrated water is very close to the TDS of the raw water.
21. Poor disinfection and maintenance of reverse osmosis equipment leads to microbial contamination
This is a common problem in the use of composite polyamide membranes, because the polyamide membrane has poor residual chlorine resistance, chlorine and other disinfectants are not correctly added in use, and users do not pay enough attention to the prevention of microorganisms, which can easily lead to microbial pollution. The pure water produced by many manufacturers exceeds the microbial standard, which is caused by poor disinfection and maintenance.
The main manifestations are: when leaving the factory, the RO equipment is not maintained with disinfectant; The entire pipeline and pretreatment equipment were not disinfected after the equipment was installed; Intermittent operation without disinfection and maintenance measures; Failure to regularly disinfect pretreatment equipment and reverse osmosis equipment; The maintenance fluid is not effective or the concentration is insufficient.
22. The residual chlorine monitoring of reverse osmosis equipment is not effective
If the pump with NaHSO3 fails or the chemical solution fails, or the membrane is damaged due to residual chlorine when the activated carbon is saturated.
23. Damage to the membrane performance caused by untimely cleaning and incorrect cleaning methods
In the process of using the equipment, in addition to the normal attenuation of performance, the attenuation of equipment performance caused by pollution is more serious.
The usual pollution of EDI high-purity water equipment mainly includes chemical scale, organic matter and colloidal pollution, microbial pollution, etc. Different pollution exhibits different symptoms. The symptoms of membrane fouling proposed by different membrane companies are also different.
In the project, we found that the length of the pollution time is different, and the symptoms are also different.
For example, when the membrane is polluted with calcium carbonate scale, and the pollution time is one week, it is mainly manifested as a rapid decrease in the desalination rate, a slow increase in the pressure difference, and no obvious change in water production, and the performance can be fully restored by cleaning with citric acid. The pollution time is one year (a water purifier), the salt flux increases from the initial 2mg/L to 37mg/L (raw water is 140mg/L~160mg/L), the water yield decreases from 230L/h to 50L/h, and after cleaning with citric acid, the salt flux decreases to 7mg/L, and the water yield rises to 210L/h.
In addition, pollution is often not singular, and its symptoms are also different, making it more difficult to identify pollution.
To identify the type of pollution, it is necessary to judge the raw water quality, design parameters, pollution index, operation records, equipment performance changes and microbial indicators.
1) Colloidal pollution: When colloidal pollution occurs, it is usually accompanied by the following two characteristics: A. The microfilter is blocked very quickly in the pretreatment, especially the pressure difference increases quickly, B. The SDI value is usually above 2.5.
2) Microbial contamination: When microbial contamination occurs, the total number of bacteria in the permeable water and concentrated water of the RO equipment is relatively high, and it must not be maintained and disinfected as required.
3) Calcium scale: It can be judged according to the quality of raw water and design parameters. For carbonate water, if the recovery rate is 75%, the LSI of the concentrate should be less than 1 if the antiscalant is added to the design; The LSI of the concentrate should be less than zero when the antiscalant is not added, and calcium scale will generally not be generated.
4) You can use 1/4 inch PVC plastic pipe inserted into the module to test the performance changes of different parts of the module to judge.
5) Determine the type of contamination according to the change in equipment performance.
6) Pickling (such as citric acid, dilute HNO3) can be used to judge the calcium scale according to the cleaning effect and cleaning solution, which is further confirmed by the composition analysis of the cleaning solution.
7) Chemical analysis of the cleaning solution: take the raw water, the cleaning stock solution, and the cleaning liquid, and analyze three samples.
Once the type of contamination has been determined, it can be cleaned as described in Table 1 and then disinfected. When the type of contamination cannot be determined, cleaning is usually done in the step of cleaning (3) and sterilizing 0.1% HCl (pH 3).
24. Improper storage and maintenance methods lead to the decline of membrane performance
New reverse osmosis membrane elements are typically soaked with 1% NaHSO3 and 18% glycerol in aqueous solution and stored in a sealed plastic bag. In the case that the plastic bag is not broken, it will be stored for about 1 year, and its life and performance will not be affected. When the plastic bag is opened, it should be used as soon as possible to avoid the adverse effects on the components due to the oxidation of NaHSO3 in the air. Therefore, the film should be opened as much as possible before use.
After the trial operation of the reverse osmosis equipment, we have used two methods to protect the membrane. The equipment was put into trial operation for two days (15~24h), and then maintained with 2% formaldehyde solution; Or after running for 2~6h, use 1% NaHSO3 aqueous solution for maintenance (the air in the equipment pipeline should be drained to ensure that the equipment does not leak, and all inlet and outlet valves should be closed).
Both methods can achieve satisfactory results. The first method is more expensive and is used when there is a long idle time, and the second method is used when the idle time is shorter.