Common Problems of Reverse Osmosis Systems
1. How often should a reverse osmosis system be cleaned?
Generally, the RO system should be cleaned when the standardized flux decreases by 10-15%, or the system desalination rate decreases by 10-15%, or the operating pressure and inter-stage pressure difference increase by 10-15%.
The cleaning frequency is directly related to the degree of pretreatment of the system. When SDI15 < 3, the cleaning frequency may be 4 times a year; when SDI15 is around 5, the cleaning frequency may need to be doubled, but the cleaning frequency depends on the actual situation of each project site.
Currently, the most effective technology for evaluating possible colloidal contamination in the feed water of RO/NF systems is to measure the Silt Density Index (SDI, also known as the fouling index) of the feed water. This is an important parameter that must be determined before RO design.
During the operation of RO/NF, regular measurements must be carried out (2-3 times a day for surface water). ASTM D4189-82 specifies the standard for this test.
The feed water specification for membrane systems is that the SDI15 value must be ≤5. Effective pretreatment technologies for reducing SDI include multimedia filters, ultrafiltration, and microfiltration. Adding polyelectrolytes before filtration can sometimes enhance the ability of the above physical filtration to reduce the SDI value.
3. Should reverse osmosis or ion exchange be used for general feed water?
Under many feed water conditions, both ion exchange resin and reverse osmosis are technically feasible. The choice of process should be determined by economic comparison. Generally, the higher the salt content, the more economical reverse osmosis is; the lower the salt content, the more economical ion exchange is.
Due to the widespread popularity of reverse osmosis technology, the combined process of reverse osmosis + ion exchange, multi-stage reverse osmosis, or reverse osmosis + other deep desalination technologies has become a recognized technically and economically more reasonable water treatment solution. Please consult a water treatment engineering company representative for more information.
4. 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 element, cleanability, feed water source, pretreatment, cleaning frequency, and operation and management level. 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-based liquid separation technology located between reverse osmosis and ultrafiltration. Reverse osmosis can remove the smallest solutes, with a molecular weight less than 0.0001 microns, while nanofiltration can remove solutes with a molecular weight of around 0.001 microns.
Nanofiltration is essentially a low-pressure reverse osmosis, used in situations where the purity of the treated water is not particularly strict. Nanofiltration is suitable for treating well water and surface water.
Nanofiltration is applicable to water treatment systems that do not require the high desalination rate of reverse osmosis, but it has a high ability to remove hardness components and is sometimes called a "softening membrane." Nanofiltration systems have low operating pressure and lower energy consumption than corresponding reverse osmosis systems.
6. What is the separation capacity of membrane technology?
Reverse osmosis is currently the most precise liquid filtration technology. Reverse osmosis membranes retain soluble salts and other inorganic molecules and organic substances with a molecular weight greater than 100. On the other hand, water molecules can freely pass through the reverse osmosis membrane. The typical removal rate of soluble salts is >95-99%. The operating pressure ranges from 7 bar (100 psi) for brackish water to 69 bar (1,000 psi) for seawater.
Nanofiltration can remove impurities with particles of 1 nm (10 Å) and organic substances with a molecular weight greater than 200-400. The removal rate of soluble 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, at 90-98%.
Ultrafiltration has a separation effect on macromolecules larger than 100-1,000 Å (0.01-0.1 microns). All soluble salts and small molecules can pass through the ultrafiltration membrane. Substances that can be removed include colloids, proteins, microorganisms, and macromolecular organic matter. The molecular weight cutoff of most ultrafiltration membranes is 1,000-100,000.
Microfiltration removes particles in the range of about 0.1-1 micron. Typically, suspended solids and large colloidal particles are 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 typical pressure difference across the membrane is 1-3 bar.
7. Who sells membrane cleaning agents or provides cleaning services?
Water treatment companies can provide specialized membrane cleaning agents and cleaning services. Users can purchase cleaning agents for membrane cleaning according to the recommendations of the membrane company or equipment supplier.
8. What is the maximum allowable concentration of silica in reverse osmosis membrane feed water?
The maximum allowable concentration of silica depends on temperature, pH, and antiscalants. Usually, without antiscalants, the maximum allowable concentration at the concentrate end is 100 ppm. Some antiscalants allow the silica concentration in the concentrate to be as high as 240 ppm. Please consult the antiscalant supplier.
9. What is the effect of chromium on RO membranes?
Certain heavy metals, such as chromium, can catalyze the oxidation of chlorine, leading to irreversible performance degradation of the membrane. This is because Cr in water 6+ is less stable than Cr 3+ The higher the oxidation state of the metal ion, the stronger this destructive effect seems to be. Therefore, the concentration of chromium should be reduced in the pretreatment section, or at least Cr should be reduced to Cr 6+ reduced to Cr 3+。
10. What kind of pretreatment is generally required for RO systems?
A typical pretreatment system consists of the following: coarse filtration (~80 microns) to remove large particles; addition of oxidants such as sodium hypochlorite; then precision filtration through multimedia filters or clarifiers; addition of sodium bisulfite to reduce residual chlorine and other oxidants; finally, a security filter is installed before the high-pressure pump inlet.
As the name suggests, the role of a security filter is to act as a final safeguard to prevent accidental damage to the high-pressure pump impeller and membrane elements caused by large particles. Water sources with high suspended solids usually require a higher degree of pretreatment to meet the specified water intake requirements; for water sources with high hardness content, it is recommended to use softening or adding acid and scale inhibitors, etc. For water sources with high microbial and organic matter content, activated carbon or anti-pollution membrane elements are also needed.
11. Can reverse osmosis remove microorganisms such as viruses and bacteria?
Reverse osmosis (RO) is very dense and has a very high removal rate for viruses, bacteriophages, and bacteria, at least 3log or more (removal rate >99.9%). However, it should also be noted that in many cases, microorganisms may still regrow on the membrane permeate side. This mainly depends on the assembly, monitoring, and maintenance methods. In other words, the ability of a system to remove microorganisms depends on whether the system design, operation, and management are appropriate, not 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 production, and vice versa. When operating under higher temperature conditions, the operating pressure should be reduced to keep the water production constant, and vice versa. For the temperature correction factor TCF of water production changes, please refer to the relevant chapters.
13. What is particulate and colloidal fouling? How to determine it?
Once particulate and colloidal fouling occurs in reverse osmosis or nanofiltration systems, it will seriously affect the membrane's water production, and sometimes it will also reduce the desalination rate.
An early symptom of colloidal fouling is an increase in system pressure difference. The sources of particles or colloids in the membrane feed water vary from place to place and often include bacteria, sludge, colloidal silica, iron corrosion products, etc. The chemicals used in the pretreatment section, such as polyaluminum and ferric chloride or cationic polyelectrolytes, if not effectively removed in the clarifier or media filter, may also cause fouling.
In addition, cationic polyelectrolytes will also react with anionic scale inhibitors, and their precipitates will foul the membrane elements. The fouling tendency of this type of water or the qualification of pretreatment is evaluated using SDI15. Please refer to the detailed introduction in the relevant chapters.
14. How long can the system be shut down without flushing?
If the system uses scale inhibitors, it is about 4 hours when the water temperature is between 20-38℃, and about 8 hours when it is below 20℃. If the system does not use scale inhibitors, it is about 1 day.
15. Can the reverse osmosis pure water system be started and stopped 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 low pressure with its permeate or pre-treated qualified water to displace the high-concentration but scale-inhibitor-containing concentrate from the membrane elements.
Measures should also be taken to prevent water leakage from the system and the introduction of air, because if the elements are dehydrated, irreversible permeate flux loss may occur.
If the shutdown is less than 24 hours, no measures need to be taken to prevent microbial growth. However, if the shutdown time exceeds the above regulations, a protective solution should be used for system preservation or the membrane system should be rinsed regularly.
16. How to determine the direction of the brine seal on the membrane element?
The brine seal on the membrane element should be installed at the inlet end of the element, with the opening facing the inlet direction. When water is fed into the pressure vessel, the opening (lip) will further open, completely sealing the bypass flow of water from the membrane element and the inner wall of the pressure vessel.
17. How to remove silica from water?
Silica in water exists in two forms: reactive silica (monomeric silica) and colloidal silica (polymeric silica): Colloidal silica does not have ionic characteristics, but its size is relatively large. Colloidal silica can be retained by fine physical filtration processes, such as reverse osmosis, and its content in water can also be reduced by coagulation technology, such as coagulation clarifiers. However, those separation technologies that rely on ionic charge characteristics, such as ion exchange resins and continuous electrodeionization (CDI), are very limited in removing colloidal silica.
Reactive silica is much smaller than colloidal silica, so most physical filtration technologies such as coagulation clarification, filtration, and flotation cannot remove reactive silica. Processes that can effectively remove reactive silica are reverse osmosis, ion exchange, and continuous electrodeionization.
18. What is the effect of pH on rejection rate, water production, and membrane life?
The pH range of reverse osmosis membranes is generally 2-11. The pH has little effect on the membrane performance itself. This is one of the significant characteristics different from other membrane products. However, the properties of many ions in water are greatly affected by pH. For example, when weak acids such as citric acid are at low pH, they are mainly in a non-ionic state, while at high pH values, they dissociate and become ionic. Since the higher the charge of the same ion, the higher the rejection rate of the membrane, and the lower the charge or no charge, the lower the rejection rate of the membrane, therefore, the pH has a significant impact on the rejection rate of certain impurities.
19. What is the relationship between feed water TDS and conductivity?
When obtaining the feed water conductivity value, it must be converted into a TDS value for input during software design. For most water sources, the conductivity/TDS ratio is between 1.2 and 1.7. For ROSA design, seawater uses a ratio of 1.4, while brackish water uses a ratio of 1.3 for conversion, which usually gives a good approximate conversion rate.
20. How do you know if the membrane is fouled?
The following are common symptoms of fouling:
Water production decreases at standard pressure
Operating pressure must be increased to achieve standard water production
Increased pressure drop between feed water and concentrate
Increased weight of membrane element
Significant change (increase or decrease) in membrane rejection rate
When the element is removed from the pressure vessel, water is poured onto 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 path is completely blocked)
21. How to prevent microbial growth in the original packaging of membrane elements?
When the protective solution becomes cloudy, it is likely due to microbial growth. Membrane elements protected with sodium bisulfite should be checked every three months.
When the protective solution becomes cloudy, the element should be removed from the storage bag and re-immersed in fresh protective solution. The concentration of the protective solution is 1% (weight) food-grade sodium bisulfite (not cobalt-activated). Soak for about 1 hour and reseal. The element should be drained before repackaging.
22. What are the feed water requirements for RO membrane elements and IX ion exchange resins?
Theoretically, the following impurities should not be present in the water entering the RO and IX systems:
Oxidants, such as residual chlorine
Oil or grease (must be below the detection limit of the instrument)
Organic matter and iron-organic complexes
Metal oxides such as iron, copper, and aluminum corrosion products
Inlet water quality has a significant impact on the life and performance of RO elements and IX resins.
23. What impurities can RO membranes remove?
RO membranes can effectively remove ions and organic matter. Reverse osmosis membranes have a higher removal rate than nanofiltration membranes. Reverse osmosis typically removes 99% of salts in the feed water, and the removal rate of organic matter in the feed water is ≥99%.
24. How do you know which cleaning method to use for your membrane system?
To achieve the best cleaning results, it is crucial to select the appropriate cleaning agent and cleaning steps. Incorrect cleaning can actually worsen system performance. Generally, for inorganic scaling contaminants, acidic cleaning solutions are recommended; for microbial or organic contaminants, alkaline cleaning solutions are recommended.
25. Why is the pH of RO product water lower than the pH of feed water?
In a closed system, the relative content of CO2, HCO3 - and CO3 2- changes with pH. At low pH, CO2 is the main component; in the medium pH range, it is mainly HCO3 - , and in the high pH range, it is mainly CO3 2- 。
Since RO membranes can remove soluble ions but not soluble gases, the CO2 content in RO product water is basically the same as the CO2 content in RO feed water, but HCO3 - and CO3 2- can often be reduced by 1-2 orders of magnitude. This will disrupt the equilibrium between CO2 and HCO3 in the feed water. - and CO3 2- In a series of reactions, CO2 will combine with H2O to cause the following reaction equilibrium transfer until a new equilibrium is established.
If the feed water contains CO2, the pH of the product water of the RO membrane element will always decrease. For most RO systems, the pH of the reverse osmosis product water will decrease by 1-2 pH units. When the feed water alkalinity and HCO3 - are high, the pH of the product water will decrease even more. In very few feed waters, containing less CO2, HCO3 - or CO3 2- , the change in product water pH is less noticeable.
The pH of reverse osmosis effluent is low. A metering pump is added to add NaOH to adjust the pH to alkaline, because when the pH is between 7.5 and 8, the desalination effect of reverse osmosis can reach its optimum.
26. How can the energy consumption of the membrane system be reduced?
Low-energy membrane elements can be used, but it should be noted that their salt rejection rate is slightly lower than that of standard membrane elements.
Summary of Common Problems in Reverse Osmosis Equipment
1. Why do the O-rings in reverse osmosis equipment swell?
In water treatment equipment, reverse osmosis equipment uses three types of sealing O-rings to achieve sealing and isolation between different sections in the membrane shell. To reduce installation resistance, clean water or glycerin should be applied to the surface of each O-ring during system installation.
It should be noted that lubricants such as Vaseline or other petroleum-based oils should be used cautiously, otherwise it will cause cracking of the freshwater pipe, especially swelling of the sealing O-rings. The swelling of the O-rings generally does not directly affect the operation of the system, but it will affect the reloading after the system is unloaded, that is, the swollen O-rings are difficult to enter the slot during loading.
2. Is the water production of each component in the reverse osmosis process consistent?
In reverse osmosis equipment, due to the pressure difference between the feed water end and the concentrate end of the membrane element, that is, the transmembrane pressure drop, and because the salt content of each element's concentrate is higher than the salt content of the feed water, the osmotic pressure of each element's feed water continuously increases along the system process.
If the freshwater backpressure and osmotic pressure are ignored, the water production of each membrane element along the system process will be proportional to the difference between its working pressure and osmotic pressure, that is, the water production of each membrane element gradually decreases.
3. Does pH affect the removal rate and lifespan of reverse osmosis membranes?
As the main filtration process in water treatment equipment, reverse osmosis equipment, when raw water enters the reverse osmosis membrane, will the pH value of the raw water cause damage to the reverse osmosis membrane? Generally speaking, the material of reverse osmosis membranes is mostly composite membrane material. When using this membrane material, if it is within the pH range specified by the product, generally 2-11, then the damage and impact of pH on the membrane itself is relatively small.
Regarding the impact of pH on the desalination rate of reverse osmosis membranes, it is due to the influence of pH on the properties of various ions in water. This is determined by the acidity and basicity, decomposability, and degree of charge of the ions themselves, all of which can lead to a decrease in the desalination rate of the membrane.
Therefore, pH has a significant impact on the desalination rate of certain impurities. Similarly, if the CO2 removal rate of the reverse osmosis membrane is zero, increasing the pH of the raw water to convert CO2 to CO3 2- allows the reverse osmosis membrane to effectively desalinate. However, special attention must be paid to the scaling problem of the reverse osmosis membrane at this time.
4. How should the reverse osmosis equipment be operated for the first time?
Use low pressure and low flow rate to remove air from the pipeline. Only when there is no air in the pipeline can the equipment operate normally. First, the pressure should be maintained between 0.2~0.4MPa. When using low-pressure flushing and venting, the concentrated water and produced water should be discharged into the sewer.
If the pressure rises rapidly during operation, this indicates that there is air in the membrane element, which will generate a radial impact force of the water flow. In this case, the outer casing of the membrane may rupture, resulting in irreparable damage to the reverse osmosis membrane.
During the first use, the operating pressure of the membrane must be adjusted to 0.2~0.4MPa for flushing, and it must be ensured that the reverse osmosis system automatically flushes the reverse osmosis membrane at low pressure each time it starts.
5. How to replace the security filter core in the reverse osmosis equipment?
After a period of use, the security filter may become clogged due to its age and changes in the water quality being processed. The filter core should be replaced based on the pressure difference before and after the filter. When the pressure difference is greater than 0.03MPa,
Replacement method: Turn off the reverse osmosis system. Release the pressure by pressing the pressure relief valve button on the equipment until the pressure gauge reads zero. Use a professional wrench to rotate and open the filter bottle. Remove the old filter core and install the new one. Use a professional wrench to tighten the filter bottle by rotating it.
6. How to clean and disinfect the reverse osmosis system?
Generally, cleaning of the reverse osmosis system requires professional technicians. Customers should not clean it themselves. If the reverse osmosis system needs cleaning, contact professional manufacturers. The following conditions are prerequisites for chemical cleaning.
Prerequisites for chemical cleaning: When the system's water production is reduced by 5-10% compared to the initial or last cleaning operation. When the system's desalination rate is reduced by 2.5-5% compared to the initial or last cleaning operation. When the pressure difference in each section of the system is 1-2 times higher than the initial or last cleaning operation. When the system needs to be shut down for a long time, it should be protected with a protective solution.
Remarks: Water production and desalination rate are affected by water temperature, so the test results should be obtained under the same water temperature.
7. How effective is the defluorination effect of reverse osmosis equipment?
Excessive fluoride in water can harm human health. To avoid the harm of fluoride in daily life, reverse osmosis equipment can be used for defluorination. Fluoride ions in groundwater mostly come from the erosion and dissolution of surrounding rocks, and there are also many soluble ions in the water. Defluorination must consider the impact on other molecules. The defluorination rate is not very high in groundwater with high salinity using reverse osmosis equipment. However, compared with other methods, the reverse osmosis method is simple to operate and has good treatment effects.
8. What are the basic requirements for the water quality of purified water equipment?
Purified water produced by purified water equipment is used in the pharmaceutical and biomedical industries. But what are the basic standards for purified water quality? The Chinese Pharmacopoeia and the European Pharmacopoeia clearly state that the raw water for pharmaceutical water must at least meet the drinking water standards.
If it does not meet the standards, it should be pre-treated until it meets the standards. In addition to the clear regulations for E. coli, other bacteria are not allowed to exceed 100/ml. During the water production process of purified water equipment, there may be internal contamination, and various water treatment devices in the purified water equipment may become pollution sources.
Therefore, the purified water equipment must be cleaned and disinfected frequently. In addition, a sterilization and disinfection device should be installed at the end of the purified water equipment.
9. What are the characteristics of the water quality of purified water equipment?
The water quality of the purified water produced by the purified water equipment meets the national hygiene standards and the actual production standards of the enterprise. The effluent of the purified water equipment - purified water has two characteristics. One characteristic is that the number of disinfection and sterilization equipment installed in the purified water equipment is gradually increasing.
Another characteristic is that the pipeline distribution system of the purified water equipment is replaced by a circulating pipeline instead of the traditional water supply pipeline. These two characteristics are to control microbial contamination and the increase of bacterial endotoxins.
At the same time, attention should be paid to the influence of the flow rate in the pipe on the reproduction of microorganisms, that is, if the flow rate in the pipe is too low or blocked, it may cause the reproduction of microorganisms to increase, affecting the water quality.
10. What are the key points for selecting the installation location of the water softener?
The water softener has good effluent water quality and stable system operation, and is used by many enterprises. When installing the water softener, the following points should be noted:
1) The water softener should be as close to the drain as possible.
2) If other water treatment equipment is needed, the installation location should be reserved.
3) Because salt is frequently added to the salt tank, a salt placement location should be set.
4) Do not install the water softener within 3 meters of the boiler, otherwise hot water will flow back into the water softener, causing damage.
5) The water softener should be placed in an environment where the room temperature is below 1°C and above 49°C.
11. What are the precautions for using a water softener?
A water softener is a water treatment device that removes Ca, Mg, and other ions from water, reducing water hardness and softening water. After prolonged use of the water softener, certain precautions should be followed.
1) The water inlet of the water softener should be kept open during use, unless it is necessary to close it for equipment maintenance. If water is not needed, the water outlet valve can be closed.
2) If the water output fails to meet standards due to incomplete regeneration of the water softener, manual regeneration can be performed.
3) When the water softener is not in use for a long time, the brine should be sucked into the tank for protection.
4) When putting it back into use, just prepare the same amount of brine as for normal operation, open the water inlet valve first, then connect the power supply, let the equipment reset itself, then manually regenerate it once before opening the water outlet valve.
5) If the industrial salt used is too dirty, the salt tank 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 ultrapure water equipment produces high-quality water, suitable for the production needs of various enterprises, and its application range is gradually expanding. This article mainly introduces the standard instructions for the installation of ultrapure water equipment.
1) The equipment installation site should be chosen in a flat, clean environment, close to the power and water sources.
2) Do not place it near fire sources or any heating elements to avoid affecting its operating efficiency due to heat.
3) In northern regions, the equipment must not be installed outdoors to prevent the equipment from freezing and damaging the instruments and filter elements.
4) The equipment installation location should allow for convenient drainage, keeping the equipment's drain pipe unobstructed.
5) The working pressure of the water pump used in the system should be ensured to be 1.0-1.2Mpa, ensuring that the water pump operates within its rated head range.
13. Booster pump/high-pressure pump not drawing water
Solution: For 380v voltage, check if the booster pump/high-pressure pump is reversing. If it is reversing, change any two of the three power connection terminals of the pump. If it is not reversing, open the pump's vent valve to vent or fill the pump with water.
220v voltage booster pumps/high-pressure pumps will not reverse. Just open the pump's vent valve to vent or fill the pump body with water.
14. High-pressure pump not starting
Solution: Check if the relay connected to the high-pressure pump is engaged, and if the wiring terminals are loose, detached, or if the low water indicator light is on. If the low water indicator light is on, it means that the raw water source is insufficient for the booster pump. To prevent the high-pressure pump from idling and damaging it, a water shortage protector cuts off the power supply to the high-pressure pump, thus protecting it. If a sufficient water source is provided, and the pressure of the water shortage protector reaches the working pressure requirement of the high-pressure pump, the water shortage protector indicator light will go out, and the high-pressure pump can be started.
15. High-pressure pump making unusual noise
Solution: Check if the high-pressure pump is idling. Sometimes, the high-pressure pump may make some unusual noise when the water has not fully entered. This usually disappears automatically in 1-3 minutes. If it does not disappear after 3 minutes, open the vent valve of the high-pressure pump to vent or add water.
Solution: Due to the poor water quality and high impurity content in some areas, or due to the long time without replacing the felt spray filter element and cleaning the RO membrane, the RO membrane may become blocked, resulting in increased pressure in the pipeline and causing the pipeline to burst.
In this case, first check if the felt spray filter element needs cleaning or replacement, and then clean the RO membrane. Poor quality raw water can cause frequent blockage of the RO membrane. In this case, we should add an ion exchange device or add scale inhibitors to the raw water to remove impurities and improve the water quality and extend the service life of the RO membrane.
17. Water output decreasing
Solution: Some equipment users may find that the equipment's water output is decreasing (this phenomenon basically does not occur in equipment using tap water as raw water). This is because some groundwater has poor water quality and high impurity content, causing partial blockage of the RO membrane, thus reducing the equipment's water output.
At this time, we should regularly backwash the pretreatment, replace the felt spray filter element, clean the RO membrane, or replace the raw water from poor quality groundwater with tap water (in the absence of tap water, it is better to configure a set of ion exchange and scale inhibition system, which will basically eliminate this phenomenon).
18. Fine white or black suspended particles appear in the purified water
Solution: This is caused by pipeline contamination leading to bacterial growth. In this case, dissolve caustic soda and put it into the precision filter. Operate the regulating valve to close the wastewater and open the pure water to a high point. Start the high-pressure pump and introduce the water flowing out of the pure water outlet into the precision filter for water circulation for about 30 minutes. Equipment with a pipeline sterilizer should promptly turn on the pipeline sterilizer to sterilize the pure water pipeline.
19. Residual gas in the reverse osmosis equipment operating under high pressure, forming water hammer that can damage the membrane
There are usually two situations:
1) After the equipment is drained, when it is restarted, the gas is not completely discharged before rapid pressurization operation. The remaining air should be discharged at a pressure of 2-4 bar before gradually increasing the pressure.
2) When the joint between the pretreatment equipment and the high-pressure pump is not well sealed or leaks (especially leakage in the microfiltration unit and subsequent pipelines), when the pretreatment water supply is insufficient, such as microfiltration blockage, some air may be sucked in due to vacuum at the poorly sealed location. The microfiltration unit 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 flow meter. If bubbles are found during operation, the pressure should be gradually reduced to check the cause.
20. Incorrect shutdown method of reverse osmosis equipment damages the membrane
1) Rapid depressurization during shutdown without thorough rinsing. Because the concentration of inorganic salts on the concentrate side of the membrane is higher than that of the raw water, it is easy to scale and pollute the membrane.
2) Rinsing with pretreated water containing added chemical reagents. Water containing chemical reagents may cause membrane pollution during equipment shutdown.
When preparing to shut down the reverse osmosis water treatment equipment, stop adding chemical reagents, gradually reduce the pressure to around 3 bar, and rinse with pre-treated water for 10 minutes until the TDS of the concentrate is very close to the TDS of the raw water.
21. Microbial contamination due to inadequate disinfection and maintenance of reverse osmosis equipment
This is a common problem in the use of composite polyamide membranes, because polyamide membranes have poor chlorine resistance. If chlorine and other disinfectants are not added correctly during use, and users do not pay enough attention to the prevention of microorganisms, microbial contamination is easily caused. Many manufacturers' pure water microbial levels exceed the standard, which is caused by poor disinfection and maintenance.
The main manifestations are: Upon leaving the factory, the RO equipment was not maintained with disinfectant; after the equipment was installed, the entire pipeline and pretreatment equipment were not disinfected; intermittent operation did not adopt disinfection and maintenance measures; the pretreatment equipment and reverse osmosis equipment were not disinfected regularly; the maintenance solution failed or the concentration was insufficient.
22. Inadequate monitoring of residual chlorine in reverse osmosis equipment
For example, if the NaHSO3 pump fails or the solution fails, or the activated carbon is saturated, the membrane is damaged due to residual chlorine.
23. Membrane performance damage caused by untimely cleaning and incorrect cleaning methods
During the use of the equipment, in addition to the normal attenuation of performance, the attenuation of equipment performance caused by pollution is more serious.
The common pollution of EDI ultrapure water equipment mainly includes chemical scaling, organic matter and colloid pollution, and microbial pollution. Different pollutions show different symptoms. Different membrane companies have some differences in the symptoms of membrane pollution.
In the project, we found that the duration of pollution is different, and the symptoms are also different.
For example: When the membrane is contaminated with calcium carbonate scale, and the pollution time is one week, the main manifestation is a rapid decrease in desalination rate, a slow increase in pressure difference, and no significant change in water production. Cleaning with citric acid can fully restore performance. The pollution time is one year (a certain pure water machine), the salt flux increased from the initial 2mg/L to 37mg/L (raw water is 140mg/L～160mg/L), the water production decreased from 230L/h to 50L/h, and after cleaning with citric acid, the salt flux decreased to 7mg/L, and the water production increased to 210L/h.
Furthermore, pollution is often not single, and its symptoms are also different, making pollution identification more difficult.
To identify the type of pollution, the raw water quality, design parameters, pollution index, operation records, equipment performance changes, and microbial indicators should be comprehensively judged:
1) Colloid pollution: When colloid pollution occurs, it is usually accompanied by the following two characteristics: A, the microfiltration filter in the pretreatment is blocked very quickly, especially the pressure difference increases quickly, B, the SDI value is usually above 2.5.
2) Microbial pollution: When microbial pollution occurs, the total number of bacteria in the permeate and concentrate of the RO equipment is relatively high, and maintenance and disinfection have not been carried out as required.
3) Calcium scaling: It can be judged based on the raw water quality and design parameters. For carbonate-type water, if the recovery rate is 75%, and scale inhibitors are added during design, the LSI of the concentrate should be less than 1; if no scale inhibitors are added, the LSI of the concentrate should be less than zero, and calcium scaling will generally not occur.
4) A 1/4-inch PVC plastic tube can be inserted into the component to test the performance changes of different parts of the component.
5) Judge the type of pollution based on the changes in equipment performance.
6) Acid washing (such as citric acid, dilute HNO3) can be used, and the effect of cleaning and cleaning solution can be used to judge calcium scaling, and further confirmed by analyzing the components of the cleaning solution.
7) Chemical analysis of the cleaning solution: Take raw water, original cleaning solution, and cleaning solution samples for analysis.
After determining the type of pollution, the equipment can be cleaned according to the methods in Table 1, and then disinfected and used. When the type of pollution cannot be determined, the cleaning (3) disinfection 0.1% HCl (pH=3) steps are usually used for cleaning.
24. Improper storage and maintenance methods lead to a decline in membrane performance
New reverse osmosis membrane elements are usually soaked in a 1% NaHSO3 and 18% glycerol aqueous solution and stored in a sealed plastic bag. If the plastic bag is not broken, storage for about one year will not affect its life and performance. When the plastic bag is opened, it should be used as soon as possible to avoid adverse effects on the components due to the oxidation of NaHSO3 in the air. Therefore, the membrane should be opened before use as much as possible.
After the reverse osmosis equipment is tested, we have used two methods to protect the membrane. The equipment is tested and run for two days (15～24h), and then maintained with a 2% formaldehyde solution; or after running for 2～6h, it is maintained with a 1% NaHSO3 aqueous solution (the air in the equipment pipeline should be drained, ensuring that the equipment does not leak, and closing all inlet and outlet valves).
Both methods can achieve satisfactory results. The first method is more expensive and is used when the idle time is long, while the second method is used when the idle time is shorter.
