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 system pretreatment. 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. However, the cleaning frequency depends on the actual situation of each project site.

2. What is SDI?

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 combination 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 scheme. For further information, please consult a water treatment engineering company representative. 4.

4. How many years can a reverse osmosis membrane element generally 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 feed water source, the pretreatment, the cleaning frequency, and the level of operation and management. 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 suitable for 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 feed 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 separates 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 the temperature, pH value, and antiscalant. Usually, when no antiscalant is added, 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 Cr6+ is less stable than Cr3+ in water. It seems that the higher the oxidation state of the metal ion, the stronger this destructive effect is. Therefore, the concentration of chromium should be reduced in the pretreatment section, or at least Cr6+ should be reduced to Cr3+.

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 an oxidant such as sodium hypochlorite; then precision filtration through a multimedia filter or clarifier; addition of sodium bisulfite to reduce residual chlorine and other oxidants; finally, a security filter is installed before the high-pressure pump inlet. The role of the security filter, as its name suggests, is as a final safety measure to prevent accidental large particles from damaging the high-pressure pump impeller and membrane elements. Water sources with high particulate matter usually require a higher degree of pretreatment to meet the specified feed water requirements. For water sources with high hardness content, softening or the addition of acid and antiscalants is recommended. For water sources with high microbial and organic matter content, activated carbon or anti-fouling 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 3 log 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, rather than the nature of the membrane element itself.

12. How does temperature affect permeate production?

The higher the temperature, the higher the permeate production, and vice versa. When operating under higher temperature conditions, the operating pressure should be reduced to keep the permeate production unchanged, and vice versa. Please refer to the relevant chapters for the temperature correction factor TCF for permeate production changes.

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 permeate production and sometimes reduce the desalination rate. An early symptom of colloidal fouling is an increase in system pressure drop. The sources of particles or colloids in the membrane feed water vary from place to place and often include bacteria, silt, colloidal silica, iron corrosion products, etc. The chemicals used in the pretreatment section, such as polyaluminum and ferric chloride or cationic polyelectrolytes, may also cause fouling if they cannot be effectively removed in the clarifier or media filter. In addition, cationic polyelectrolytes will also react with anionic antiscalants, and their precipitates will foul the membrane elements. The fouling tendency of this kind 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 antiscalants, approximately 4 hours when the water temperature is between 20 and 38℃; approximately 8 hours below 20℃; approximately 1 day if the system does not use antiscalants. 15. 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 desalination rate is slightly lower than that of standard membrane elements.

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 antiscalant-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 flushed 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 enters the pressure vessel, the opening (lip) will further open, completely sealing the bypass of water from the membrane element to 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, separation technologies that rely on ionic charge characteristics, such as ion exchange resins and continuous electrodialysis (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 electrodialysis.

18. What is the effect of pH on removal rate, permeate production, and membrane life?

The pH range of reverse osmosis membrane products 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 removal rate of the membrane, and the lower or uncharged, the lower the removal rate of the membrane, the pH has a significant impact on the removal 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 better approximate conversion rate.

20. How do you know if the membrane is fouled?

The following are common symptoms of fouling:

Permeate production decreases under standard pressure. To achieve standard permeate production, operating pressure must be increased. The pressure drop between feed water and concentrate increases. The weight of the membrane element increases. The membrane removal rate changes significantly (increases or decreases).

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 RO and IX systems:

Suspended solids, colloids, calcium sulfate, algae, bacteria, oxidants such as residual chlorine, oil or grease (must be below the detection limit of the instrument), organic matter and iron-organic complexes, iron, copper, aluminum corrosion products and other metal oxides, influent water quality will have a huge 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 from the feed water, and the removal rate of organic matter in the feed water is ≥99%.

24. How do you know which cleaning method should be used for your membrane system?

To achieve the best cleaning results, it is very important to choose the right cleaning agents 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?

Understanding the equilibrium between CO2, HCO3-, and CO3= provides the best answer to this question. In a closed system, the relative amounts of CO2, HCO3-, and CO3= change with pH. At low pH, CO2 predominates; in the medium pH range, HCO3- predominates; and at high pH, CO3= predominates. Since RO membranes can remove dissolved ions but not dissolved gases, the CO2 content in RO product water is basically the same as that in RO feed water, but HCO3- and CO3= are often reduced by 1-2 orders of magnitude. This will disrupt the equilibrium between CO2, HCO3-, and CO3= in the feed water. In a series of reactions, CO2 will combine with H2O to cause the following reaction equilibrium shift until a new equilibrium is established.

If the feed water contains CO2, the pH of the RO product water 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.

Very few feed waters contain less CO2, HCO3-, or CO3=, so the change in product water pH is less noticeable. In some countries and regions, there are regulations for the pH of drinking water, generally 6.5-9.0. In our understanding, this is to prevent corrosion of water pipes, while drinking water with low pH itself will not cause any health problems. As is well known, many commercially available carbonated beverages have a pH between 2 and 4.