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25 Typical Problems & Solutions of Reverse Osmosis

Views:1895     Author:Kevin Chen     Publish Time: 2020-06-09      Origin:Site

1. How often should the Reverse Osmosis System be cleaned?

Theoretically speaking, when the standardized flux drops by 10 to 15%, or the system desalination rate drop by 10 to 15%, or the operating pressure and the pressure difference between sections increases by 10 to 15%, the RO system should be cleaned.

The cleaning frequency has a direct relationship with the quality of the pretreatment. When SDI15 <3, the cleaning frequency maybe 4 times a year; when SDI15 is around 5, the cleaning frequency may be doubled. And of course, it depends on the actual situation of the system.

2. What is SDI?

The best technique currently available to evaluate the possible colloidal pollution of the RO / NF system influent is to measure the silt density index (SDI, also known as the fouling index) of the influent, which is an important parameter that must be determined before the RO design During RO / NF operation, measurements must be taken periodically (2 to 3 measurements per day for surface water). ASTM standard test method D 4189-82 specifies the standard for this test.

The water inlet regulation of the membrane system is that the SDI15 must be ≤5. The effective technologies for reducing SDI pretreatment are multi-media filters, ultrafiltration, microfiltration, etc. Adding a polyelectrolyte before filtering can sometimes enhance the physical filtering and reduce the SDI.

3. The Reverse Osmosis or Ion Exchange, which is better?

Both of them can work well under most conditions. The choice of the process should be determined by economic comparison. General speaking, if the salinity is high, the best choice is ro. Because it is more economical. If the salinity is low, the ion exchange is the best choice.

With the widespread popularity of ro technology, the combined process using ro + ion exchange process or multi-stage ro or ro + other deep desalination technologies has become a recognized solution.

4. What is the difference between Reverse Osmosis and Nanofiltration?

Nanofiltration is a membrane liquid separation technology between ro & ultrafiltration. Ro can remove the smallest solutes with a molecular weight of fewer than 0.0001 microns, and nanofiltration can remove solutes with a molecular weight of about 0.001 microns.

Nanofiltration is essentially a kind of low-pressure ro, which is used on occasions where the purity of the produced water is not particularly strict. Nanofiltration is suitable for treating well water and surface water. Nanofiltration is suitable for water treatment systems that do not need high desalination rates like ro, but has a high ability to remove hardness components. Sometimes it is called a “softened membrane”. Compare with the ro system, the nanofiltration system has low operating pressure and lower energy consumption.

5. What separation capabilities does membrane have?

Reverse osmosis is currently the most precise liquid filtration technology. Ro membranes retain the inorganic molecules such as soluble salts and organic matter with a molecular weight greater than 100. On the other hand, water molecules can pass through the ro membrane freely. Typical solubility the removal rate of salt is >95~99%.

Operating pressures range from 7 bar (100 psi) with brackish water to 69 bar (1,000 psi) with seawater. Nanofiltration can remove impurities at 1nm (10 angstroms) and organic matter 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% to 80%, and the removal rate of salts containing divalent anions (such as MgSO4) is relatively high, 90% to 98%.

Ultrafiltration can separate macromolecules larger than 100-1,000 angstroms (0.01-0.1 microns). All soluble salts and small molecules can pass through ultrafiltration membranes. Removable substances include colloids, proteins, microorganisms and macromolecular organics. The cut-off molecular weight of most ultrafiltration membranes is 1,000 to 100,000. Microfiltration removes particles in the range of about 0.1 to 1 micron. Generally, suspended matter and large particle colloids can be trapped and large molecules and soluble salts can pass freely through the microfiltration membrane. The microfiltration membrane is used to remove bacteria and microparticles. Flocs or total suspended solids TSS, typical pressure on both sides of the membrane is 1 to 3 bar.

RO system3

6. Who provides cleaning services?

Water treatment companies can provide special membrane cleaning services, and users can purchase cleaning agents for membrane cleaning themselves according to the recommendations of the membrane company or equipment supplier.

7. What is the maximum allowed silica concentration for reverse osmosis membrane water ingress?

The maximum allowable silica concentration depends on the temperature, pH value and scale inhibitor. Usually, the maximum allowable concentration of concentrated water end is 100ppm without scale inhibitor. Some scale inhibitors can allow the highest concentration of silica in concentrated water at 240ppm. For more info, you can consult the scale inhibitor supplier.

8. How does chromium affect RO membranes?

Certain heavy metals such as chromium can catalyze the oxidation of chlorine, which in turn causes the irreversible performance of the membrane sheet to deteriorate. This is because Cr6 + is less stable than Cr3 + in water. It seems that oxidizing metal ions with higher valences have a stronger destructive effect. Therefore, the chromium concentration should be reduced or at least the Cr6 + should be reduced to Cr3 + in the pretreatment part.

9. What kind of pretreatment does an RO system require?

The general pretreatment system consists of the following: coarse filtration (around 80 microns) to remove large particles, adding oxidants such as sodium hypochlorite, and then precision filtering through a multi-media filter or clarification tank, and then adding sodium bisulfite to reduce residual chlorine and other oxidants. Finally, install a security filter before the high-pressure pump inlet.

The security filter is used as the ultimate safety measure to prevent the accidental large particles from damaging the impeller of the high-pressure pump and membrane elements. Water sources with more particulate suspended matter usually require a higher degree of pretreatment to meet the specified water intake requirements; water sources with high hardness content are recommended to use softening or acid and scale inhibitors. For the source water with high microbial and organic content, activated carbon or anti-pollution membrane components are also required

10. 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, phages and bacteria, at least above 3log (removal rate> 99.9%). However, it should also be noted that in many cases, microorganisms may still breed again on the water-producing side of the membrane, which mainly depends on the way of assembly, monitoring and maintenance, that is, the ability to remove microorganisms depends on the proper design, operation and management of the system rather than the membrane element itself.

11. How does temperature affect water production?

The higher the temperature, the higher the water yield, and vice versa. When operating at higher temperatures, the operating pressure should be reduced to keep the water yield unchanged, and vice versa. Please refer to the relevant info for the temperature correction factor TCF for changes in water production.

12. What is particle and colloid contamination?

Once the ro or nanofiltration system fouling by particles and colloids, it will seriously affect the flux, and sometimes reduce the desalination rate.

The early symptoms of colloidal fouling are an increase in the differential pressure of the system. The source of particles or colloids in the membrane feedwater source varied, often including bacteria, sludge, colloidal silicon, and iron corrosion products. The drugs used in the pretreatment part such as polymerization Aluminum and ferric chloride or cationic polydielectrics can also cause fouling if they cannot be effectively removed in a clarifier or media filter.

In addition, the cationic polyelectrolyte will also react with the anionic scale inhibitor. The sediment will foul the membrane element. The tendency of fouling or pretreatment in water is evaluated by SDI15.

RO system2

13. Without system flushing, how long is the maximum downtime allowed?

If the system uses a post-blocking agent, when the water temperature is between 20 and 38 ℃, it will be about 4 hours; below 20 ℃, it will be about 8 hours; if the system does not use a scale inhibitor, it will be about 1 day.

14. How can the energy consumption of the membrane be reduced?

Low-energy membrane elements are sufficient, but it should be noted that their salt rejection rates are slightly lower than standard membrane elements.

15. Can the reverse osmosis system be started and stopped frequently?

The system is designed based on continuous operation, but there will always be a certain frequency of startup and shutdown during operation.

When the system is shut down, it must be flushed under a low-pressure condition with its produced water or pre-treated qualified water to flush the high-concentration concentrated water containing scale inhibitors from the membrane elements.

Measures should also be taken to prevent water from leaking & introducing air, because if the components lose water and dry out, irreversible water production flux loss may occur. If the downtime is less than 24 hours, no precautions against microbial growth are required. However, if the downtime exceeds the above requirements, a protective liquid should be used for system preservation or flush the membrane regularly.

16. How to determine the direction of the seal ring(O-Ring)?

It is required that the seal ring on the membrane element should be installed on the water inlet end of the element. When the pressure vessel is filled with water, it will seal the water from the membrane element By-pass between pressure vessel inner walls.

17. How to remove silicon from water?

Silicon in water exists in two forms, active silicon (monomeric silicon) and colloidal silicon (polysilicon):

colloidal silicon does not have ions characteristics, but its size is large, and colloidal silicon can be retained by physical filtration processes, like ro filtration. It can also be reduced by coagulation techniques, such as coagulation clarifiers. But those separation technologies that rely on ionic charge characteristics, such as ion exchange resins and continuous electric deionization (CDI), are very limited in removing colloidal silicon.

The size of active silicon is much smaller than that of colloidal silicon. In this way, most physical filtration technologies such as coagulation clarification, filtration, and air flotation cannot remove active silicon. The processes that can effectively remove active silicon are ro, ion exchange, and continuous Electrodeionization process.

18. How does pH affect the removal rate, water production, and membrane life span?

The reverse osmosis membrane product has a corresponding pH range, generally 2 to 11, and the pH has little effect on the membrane performance itself. This is one of the distinctive features different from other membrane products, but the 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, they dissociate and become ionic under high pH.

For the ion, the degree of charge is high, and the removal rate of the membrane is high. If the degree of charge is low or uncharged, the removal rate of the membrane is low, so the pH has a great effect on the removal rate of some impurities.

19. What is the relationship between influent TDS and conductivity?

When the water 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 conductivity / TDS ratio is between 1.2 and 1.7. For ROSA design, 1.4 ratios are used for seawater and 1.3 ratios for brackish water. Generally, a good approximate conversion rate can be obtained.

RO system

20. How do I know if the membrane is polluted?

Under standard pressure, the water production drops.

When the element is taken out of the pressure vessel, pour water on the water inlet side of the erected membrane element. Water cannot flow through the membrane element and only overflow from the end face (indicating that the water inlet flow path is completely blocked).

21. How to prevent the growth of microorganisms in the original packaging of membrane elements?

When the protective solution becomes cloudy, it is most likely due to the growth of microorganisms. Membrane elements protected with sodium bisulfite should be checked every three months.

When the protective solution is turbid, remove the membrane from the storage sealed bag and re-immerse it in the fresh protective solution. The protective solution is 1% concentration of food-grade sodium bisulfite. Soak for about 1 hour, then reseal. The membrane should be drained before repacking.

22. What are the water inlet requirements for RO membrane elements and IX ion exchange resins?

In theory, entering the RO and IX systems should not contain the following impurities: suspended matter, colloids, calcium sulfate, algae, bacteria, oxidants, such as residual chlorine, oil or lipid substances (must be lower than the detection limit of the instrument), organic matter, iron, complexes with iron-organic compounds, metal oxides such as iron, copper, and aluminum corrosion products, and water quality will have a huge impact on the life and performance of RO elements and IX resins.

23. What impurities can RO membrane remove?

RO membrane can remove ions and organic matter very well. RO membrane has a higher removal rate than the nanofiltration membrane. Ro usually can remove 99% of salt in feed water and can remove at least 99% organic matter in water.

24. Which cleaning method should I use?

In order to obtain the best cleaning effect, it is very important to choose the correct cleaning agent and cleaning steps. Incorrect cleaning will actually worsen the system performance. Generally speaking, for inorganic scale pollutants, it is recommended to use acidic cleaning fluids. For microbial or organic pollutants, alkaline cleaning solution is recommended.

25. Why is the pH value of RO produced water lower than the pH of incoming water?

When we understand the balance between CO2, HCO3-, and CO3=, we can find the best answer to this question. In a closed system, the relative content of CO2, HCO3-, and CO3= changes with changes in pH. Under the condition of low pH value, CO2 is the main part, in the middle pH range, it is mainly HCO3-, and in the high pH range, it is mainly CO3=.

Due to RO membrane can remove soluble ions but not soluble gases, the CO2 content in the RO produced water is basically the same as the CO2 content in the RO inlet water, but HCO3- and CO3= can often be reduced. This will break the balance between CO2, HCO3- and CO3= in the incoming water. In a series of reactions, CO2 will combine with H2O to undergo the reaction equilibrium shifts until a new equilibrium is established.

If the feedwater contains CO2, the pH value of the RO product water will always decrease. For most RO systems, the pH value of the ro product water will drop by 1 or 2 pH. When the feedwater alkalinity and HCO3- are high, the pH of the produced water drops even more.

A very small amount of incoming water, containing less CO2, HCO3- or CO3= so that there is less change in the pH value of the produced water. In some countries and regions, there are regulations for the pH value of drinking water, generally 6.5 to 9.0. This is to prevent corrosion of the water pipeline, and drinking low pH water will not cause any health problems by itself. It is well known that many commercially available carbonated drinks have a pH between 2 and 4.


Shanghai CM Environmental Technology Co., Ltd. is a high-tech enterprise specializing in reverse osmosis membrane, ultrafiltration membrane, nanofiltration and MBR membrane assembly, membrane production and sales service, and also a system design and proprietary service provider. 

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