Summary of common problems of reverse osmosis in sewage treatment

1. How often should the reverse osmosis system be cleaned?

Under normal circumstances, when the standardized flux decreases by 10 to 15%, or the system salt rejection rate decreases by 10 to 15%, or the operating pressure and the differential pressure between the sections increase by 10 to 15%, the RO system should be cleaned. The cleaning frequency is directly related to the degree of system pretreatment. When SDI15<3, the cleaning frequency may be 4 times per year; when SDI15 is around 5, the cleaning frequency may be doubled but the cleaning frequency depends on each item. The actual situation at the scene.

2. What is SDI?

The current effective evaluation of the RO/NF system in the in-water colloidal pollution is a good technique for measuring the sediment density index (SDI, also known as the fouling plug index), which is an important parameter that must be determined before the RO design. During the RO/NF operation, measurements must be taken periodically (two to three times per day for surface water), and 4189-82 specifies the standard for this test. The inlet water regulation of the membrane system is that the SDI15 value must be ≤5. Effective techniques for reducing SDI pretreatment include multi-media filters, ultrafiltration, microfiltration, and the like. The addition of a poly dielectric prior to filtration sometimes enhances the above-described physical filtration and the ability to reduce SDI values.

3 Should the general influent water be reverse osmosis or ion exchange?

Under many influent conditions, the use of ion exchange resin or reverse osmosis is technically feasible. The choice of process should be determined by economic comparison. Under normal circumstances, the higher the salt content, the more economical the reverse osmosis, the salt The lower the amount, the more economical the ion exchange. Due to the large-scale popularization of reverse osmosis technology, the combination of reverse osmosis + ion exchange process or multi-stage reverse osmosis or reverse osmosis + other deep desalination technology has become a recognized technology and economical water treatment plan.

4. How many years can a 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 component, the cleanability, the influent water source, the pretreatment, the cleaning frequency, the operational management level, and the like. 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 technique between reverse osmosis contract ultrafiltration. Reverse osmosis can remove small solute of zui. The molecular weight is less than 0.0001 micron. Nanofiltration can remove solute with molecular weight of about 0.001 micron. Nanofiltration is essentially a low-pressure reverse osmosis for the treatment of post-production water purity is not particularly strict, nanofiltration is suitable for the treatment of well water and surface water. Nanofiltration is suitable for water treatment systems that do not require high desalination rates like reverse osmosis, but has a high removal capacity for hardness components, sometimes referred to as "softening membranes". Nanofiltration systems have low operating pressures and lower energy consumption. Corresponding reverse osmosis system.

6. What is the separation capability of membrane technology?

Reverse osmosis is currently Zui's sophisticated liquid filtration technology. The reverse osmosis membrane retains inorganic molecules such as dissolved salts and organic substances with a molecular weight greater than 100. On the other hand, water molecules can pass through the reverse osmosis membrane freely, typical solubility. The salt removal rate is >95 to 99%. Operating pressures range from 7 bar (100 psi) for brackish water to 69 bar (1,000 psi) for seawater. Nanofiltration can remove impurities of 1 nm (10 angstroms) and organic matter with molecular weight greater than 200-400. The removal rate of soluble solids is 20-98%, and the removal rate of salts containing monovalent anions (such as NaCl or CaCl2). 20 to 80%, and the salt containing dianion (such as MgSO4) has a high removal rate of 90 to 98%. Ultrafiltration has a separation effect on macromolecules larger than 100 to 1,000 angstroms (0.01 to 0.1 micrometers). All soluble salts and small molecules can pass through the ultrafiltration membrane. The removable materials include colloids, proteins, microorganisms and macromolecular organics. Most ultrafiltration membranes have a molecular weight cut-off of 1,000 to 100,000. Microfiltration removes particles in the range of about 0.1 to 1 micron. In general, suspended solids and large particle colloids can be trapped while macromolecules and soluble salts are free to pass through the microfiltration membrane. Microfiltration membranes are used to remove bacteria and micro. Flocs or total suspended solids TSS, the pressure on both sides of a typical membrane is 1-3 bar.

7. Who sells film cleaners or provides cleaning services?

Water treatment companies can provide special membrane cleaners 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 concentration of silica in the reverse osmosis membrane?

The zui is 100ppm, and some scale inhibitors can allow the concentration of silica in the concentrated water to be 240ppm. Please consult the scale inhibitor supplier.

9. What effect does chromium have on RO membranes?

Certain heavy metals, such as chromium, catalyze the oxidation of chlorine, which in turn causes irreversible degradation of the membrane. This is because the stability of Cr6+ in water is lower than that of Cr3+. It seems that the metal ions with high oxidation sites have a stronger destructive effect. Therefore, the concentration of chromium should be reduced or at least reduced to Cr3+ in the pretreatment section.

10. What kind of pretreatment is generally required for the RO system?

The usual pretreatment system consists of the following coarse filtration (~80 micron) to remove large particles, adding oxidant such as sodium hypochlorite, and then fine filtration through a multi-media filter or clarifier, and then adding sodium sulfite to reduce residual oxidizing agent such as residual chlorine. After the zui, install the security filter before the high pressure pump inlet. The role of the security filter, as its name implies, is the ultimate insurance measure for zui to prevent accidental large particles from damaging the high pressure pump impeller and membrane elements. A water source containing a large amount of suspended solids usually requires a higher degree of pretreatment to achieve the specified influent requirements; for water sources with a high hardness content, it is recommended to use softening or acid addition and scale inhibitors for microbial and organic matter content. High water sources also require the use of activated carbon or anti-contamination membrane components.

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, phage and bacteria, at least above 3log (removal rate >99.9%). However, it should also be noted that in many cases, microbial re-growth may still occur on the water side of the membrane, depending on the way of assembly, monitoring and maintenance, that is, the ability of a system to remove microorganisms depends on the key. Whether the system is designed, operated, and managed properly, 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 at higher temperature conditions, the operating pressure should be lowered to keep the water production constant, and vice versa. Please refer to the relevant section for the temperature correction factor TCF for changes in water production.

13. What is particle and colloidal contamination? How to determine?

In the case of reverse osmosis or nanofiltration systems, fouling of particles and colloids can seriously affect the water production of the membrane and sometimes reduce the salt rejection. The early symptoms of colloidal fouling are the increase of system pressure difference. The source of particles or colloids in the membrane inlet water source varies from place to place, often including bacteria, sludge, colloidal silicon, iron corrosion products, etc. Aluminum and ferric chloride or cationic polyelectrolytes can also cause fouling if they are not effectively removed in the clarifier or media filter. In addition, the cationic polyelectrolyte will also react with the anionic scale inhibitor, and the precipitate will foul the membrane element. The tendency of such fouling in the water or the pretreatment is evaluated by SDI15. Please refer to the relevant chapter for details.

14. Without system flushing, how long does Zui allow to stop?

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

15. Can the reverse osmosis pure water system start and stop frequently?

The membrane system is designed as a 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 with low pressure with its produced water or pretreated water to displace high concentration of scale inhibitor-containing concentrated water from the membrane element. Measures should also be taken to prevent water from leaking out of the system and introducing air, as the component loses water and can cause irreversible loss of water production flux. If the shutdown is less than 24 hours, there is no need to take measures to prevent microbial growth. However, if the downtime exceeds the above requirements, the protective liquid should be used for system preservation or timed rinsing of the membrane system.

16. How is the direction of the brine seal installed on the membrane element determined?

It is required that the brine sealing ring on the membrane element is installed at the water inlet end of the element, and the opening faces the water inlet direction. When the pressure vessel is filled with water, the opening (lip) is further opened to completely seal the inlet water from the membrane element and A bypass flow between the inner walls 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 has no ion characteristics, but the scale is relatively large, and colloidal silicon can be retained by fine physical filtration process, such as Infiltration, it is also possible to reduce the content of water by coagulation technology, such as coagulation clarifiers, but those separation techniques that rely on ion charge characteristics, such as ion exchange resins and continuous electrodeionization processes (CDI), are very effective in removing colloidal silicon. limited.

The size of active silicon is much smaller than that of colloidal silicon, so most physical filtration techniques such as coagulation clarification, filtration and air flotation cannot remove active silicon. The process of effectively removing active silicon is reverse osmosis, ion exchange and continuous. Electrodeionization process.

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

Reverse osmosis membrane products have a pH range of 2 to 11, and pH has little effect on membrane performance itself. This is one of the distinguishing features of other membrane products, but the properties of many ions in water are greatly affected by pH. For example, when a weak acid such as citric acid is in a low-pH condition, it is mainly in a non-ionic state, and at a high pH, ​​dissociation occurs and is in an ionic state. Due to the same ion, the degree of charge is high, the removal rate of the film is high, the degree of charge is low or not, the removal rate of the film is low, so the effect of pH on the removal rate of certain impurities is enormous.

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

When the influent conductivity value is obtained, it must be converted to 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 the ROSA design, the seawater is selected from the 1.4 ratio and the brackish water is converted to the 1.3 ratio. Usually, a good approximate conversion ratio can be obtained.

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

The following are common symptoms of contamination:

Under standard pressure, water production drops

In order to achieve the standard water production, the operating pressure must be increased v

Increased pressure drop between influent and concentrated water v

Increase in weight of membrane elements v

Significant change in membrane removal rate (increased or decreased)

When the component is removed from the pressure vessel, water is poured onto the water inlet side of the erected membrane element, water cannot flow through the membrane element, and only overflows from the end surface (indicating that the inlet flow passage is completely blocked).

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

When the protective liquid appears turbid, it is most likely due to microbial growth. Membrane elements protected with sodium bisulfite should be viewed every three months. When the protective liquid appears turbid, the component should be taken out from the storage sealed bag and re-soaked in a fresh protective solution with a protective solution concentration of 1% by weight of food grade sodium bisulfite (not activated by cobalt) and soaked for about 1 hour. And reseal the seal and drain the components before repackaging.

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

In theory, entering the RO and IX systems should not contain the following impurities:

Suspended matter

colloid

Calcium sulfate

Algae

bacterial

Oxidants, such as residual chlorine

Oil or lipid (must be below the lower detection limit of the instrument)

Complex of organic matter and iron-organic matter

Metal oxides such as iron, copper and aluminum corrosion products

Influent water quality will have a huge impact on the life and performance of RO components and IX resins.

23. What impurities can the RO membrane remove?

The RO membrane can remove ions and organic matter well. The reverse osmosis membrane has a higher removal rate than the nanofiltration membrane. The reverse osmosis can usually remove 99% of the salt in the feed water, and the removal rate of the organic matter in the influent water. ≥99%.

24. How do I know which cleaning method should be used to clean your membrane system?

In order to obtain a good cleaning effect, it is very important to choose a symptomatic cleaning agent and cleaning step. The wrong cleaning will actually deteriorate the system performance. Generally speaking, inorganic scaling pollutants, it is recommended to use acidic cleaning solution, microbe or organic. For contaminants, alkaline cleaning solutions are recommended.

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

When you understand the balance between CO2, HCO3- and CO3=, you can find a good answer to this question. In a closed system, the relative contents of CO2, HCO3- and CO3= vary with pH. Under low pH conditions, CO2 accounts for the majority, in the medium pH range, mainly HCO3-, and in the high pH range, mainly CO3=. Since the RO membrane can remove dissolved ions and cannot remove the dissolved gas, the CO2 content in the RO produced water is basically the same as the CO2 content in the RO influent, but HCO3- and CO3= can often be reduced by one to two orders of magnitude. This will break the balance between CO2, HCO3- and CO3= in the influent. In a series of reactions, CO2 will combine with H2O to undergo the following reaction equilibrium shift until a new equilibrium is established.

HCO3- + H+ H2OàCO2 +

If the influent contains CO2, the pH of the RO water will always decrease. For most RO systems, the pH of the reverse osmosis water will have a pH drop of 1-2, when the influent alkalinity and HCO3-high At the time, the pH of the produced water drops even more.

A very small amount of influent, containing less CO2, HCO3- or CO3 = so that the pH value of the produced water is less changed. In some countries and regions, the pH value of drinking water is specified, generally 6.5 to 9.0. According to our understanding, this is to prevent corrosion of the water pipeline, and drinking low pH water does not cause any health problems by itself. It is well known that many commercially available carbonated beverages have a pH between 2 and 4.