25 common problems and solutions of reverse osmosis!

Published in:2020-04-06    Category:Common problem  News Center  

Reverse osmosis, also known as reverse osmosis, is a membrane separation operation that uses the pressure difference as the driving force to separate the solvent from the solution. Because it is opposite to the direction of natural penetration, it is called reverse osmosis. According to different osmotic pressures of various materials, reverse osmosis pressure greater than osmotic pressure, that is, reverse osmosis method, can be used to achieve the purpose of separation, extraction, purification and concentration. This article introduces 25 common problems and solutions of reverse osmosis membranes in water treatment applications!

How often should the reverse osmosis system be cleaned?

In general, when the standardized flux drops by 10-15%, or the system desalination rate drops 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 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 project The actual situation on site.

What is SDI?

At present, the best technique for evaluating the possible colloidal pollution of RO / NF system influent is to measure the sedimentation density index (SDI, also known as fouling index) of influent, which is an important parameter that must be determined before RO design During the operation of RO / NF, periodic measurements must be made (for surface water measured 2 to 3 times a day). ASTM D4189-82 specifies the test standards.

The water inlet regulation of the membrane system is that the SDI15 value must be ≤5. Effective techniques for reducing SDI pretreatment include multi-media filters, ultrafiltration, and microfiltration. Adding a polydielectric before filtering can sometimes enhance the above-mentioned physical filtering and reduce the SDI value.

Should the reverse osmosis process or the ion exchange process be used for influent?

Under many water inlet conditions, it is technically feasible to use ion exchange resin or reverse osmosis. The choice of process should be determined by economic comparison. In general, the higher the salt content, the more economical the reverse osmosis. The lower the amount, the more economical the ion exchange.

Due to the widespread popularity of reverse osmosis technology, the combined process of reverse osmosis + ion exchange process or multi-stage reverse osmosis or reverse osmosis + other deep desalination technology has become a more technically and economically more reasonable water treatment program. To understand, please consult a water treatment engineering company representative.

Reverse osmosis membrane components can generally be used for several years? The service life of the membrane depends on the chemical stability of the membrane, the physical stability of the element, the cleanability, the water source, the pretreatment, the cleaning frequency, the operation management level, etc. According to economic analysis, it is usually more than 5 years.

How many years can reverse osmosis membrane elements 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 source of influent water, pretreatment, cleaning frequency, operation management level, etc. According to economic analysis, it is usually more than 5 years.

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 with a molecular weight of less than 0.0001 microns. Nanofiltration can remove solutes with a molecular weight of about 0.001 microns.

Nanofiltration is essentially a low-pressure reverse osmosis, which is used in the occasions where the purity of produced water after treatment 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 a high desalination rate like reverse osmosis, but it has a high removal capacity for hardness components, sometimes referred to as “softened membranes”. The nanofiltration system has low operating pressure and energy consumption is lower than Corresponding reverse osmosis system.

What is the separation ability of membrane technology?

Reverse osmosis is currently the most sophisticated liquid filtration technology. Reverse osmosis membranes trap inorganic molecules such as soluble salts and organic substances with a molecular weight greater than 100. On the other hand, water molecules can freely pass through the reverse osmosis membrane, which is typically soluble. The salt removal rate is> 95-99%.

The operating pressure ranges from 7 bar (100 psi) when the inlet water is brackish water to 69 bar (1,000 psi) when it is seawater. Nanofiltration can remove impurities at 1nm (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 ~ 80%, and the removal rate of salt containing dianion (such as MgSO4) is relatively high, 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. Removable substances include colloids, proteins, microorganisms and macromolecular organic matter. The molecular weight cutoff of most ultrafiltration membranes is 1,000-100,000. The range of microfiltration to remove particles is about 0.1 to 1 micron. Generally, suspended solids and large particle colloids can be trapped while macromolecules and soluble salts can freely pass through the microfiltration membrane. The microfiltration membrane is used to remove bacteria and For flocs or total suspended solids TSS, the typical pressure on both sides of the membrane is 1 to 3 bar.

Who sells membrane cleaning agents or provides cleaning services?

Water treatment companies can provide special membrane cleaning agents and cleaning services.Users can purchase cleaning agents for membrane cleaning according to the recommendations of membrane companies or equipment suppliers.

What is the maximum allowable silica concentration in the reverse osmosis membrane inlet water?

The maximum allowable concentration of silica depends on temperature, pH value and scale inhibitor. Generally, the maximum allowable concentration in concentrated water end is 100ppm when no scale inhibitor is added, some scale inhibitors can allow the highest silica concentration in concentrated water For 240ppm, please consult the scale inhibitor supplier.

What effect does chromium have on RO membranes?

Certain heavy metals such as chromium will catalyze the oxidation of chlorine, which in turn causes the irreversible performance of the diaphragm to decay. This is because Cr6 + is less stable than Cr3 + in water. It seems that the oxidation of metal ions with high valences is more damaging. Therefore, the concentration of chromium should be reduced in the pretreatment section or at least Cr6 + should be reduced to Cr3 +.

What kind of pretreatment is generally required for RO systems?

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

The role of the security filter, as the name suggests, is to act as the ultimate insurance measure to prevent the accidental large particles from damaging the high-pressure pump impeller 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 adding acid and scale inhibitors, etc., for microbial and organic content High water source also requires the use of activated carbon or anti-fouling membrane elements.

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 above 3log (removal rate> 99.9%). However, it should also be noted that in many cases, microorganisms may still re-grow on the water production side of the membrane, which mainly depends 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 design, operation and management are appropriate is not the nature of the membrane element itself.

How does temperature affect water production?

The higher the temperature, the higher the water production, and vice versa. When operating at a higher temperature, the operating pressure should be lowered to keep the water production unchanged, and vice versa.

What is particle and colloidal contamination? How to determine it?

Once the fouling of particles and colloids occurs in the reverse osmosis or nanofiltration system, it will seriously affect the water production of the membrane and sometimes reduce the desalination rate.

The early symptom of colloidal fouling is the increase of the system pressure difference. The source of particles or colloids in the water source of the membrane inlet varies from place to place, often including bacteria, sludge, colloidal silicon, iron corrosion products, etc. Aluminum and ferric chloride or cationic polydielectrics, if they cannot be effectively removed in clarifiers or media filters, may also cause fouling.

In addition, the cationic polydielectric will react with the anionic scale inhibitor, and its sediment will foul the membrane plugging components. SDI15 is used to evaluate the fouling tendency or pretreatment of water. Please refer to the relevant chapters for detailed introduction.

Without system flushing, how long is the longest allowable shutdown?

If the system uses after-inhibitors, when the water temperature is between 20 and 38 ° C, about 4 hours; when it is below 20 ° C, about 8 hours; if the system does not use scale inhibitors, about 1 day.

How can the energy consumption of the membrane system be reduced?

Membrane elements with low energy consumption can be used, but it should be noted that their desalination rate is slightly lower than standard membrane elements.

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

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

When the membrane system shuts down, it must use its produced water or pre-qualified water for low-pressure flushing to replace high-concentration concentrated water containing scale inhibitors from the membrane elements.

Measures should also be taken to prevent water from leaking into the system and introducing air, because if the components lose water and dry out, irreversible water loss may occur. If the shutdown is less than 24 hours, there is no need to take measures to prevent the growth of microorganisms. However, if the shutdown time exceeds the above-mentioned regulations, the protective liquid should be used for system preservation or regular flushing of the membrane system.

How to determine the direction of the brine seal ring installed on the membrane element?

The brine seal ring on the membrane element is required to be installed on the water inlet end of the element, and the opening faces the water inlet direction. When the pressure vessel is fed with water, its opening (lip) will be further opened to completely seal the inlet water from the membrane element and Bypass between the inner walls of the pressure vessel.

How to remove silicon from water?

There are two forms of silicon in water, active silicon (monomer silicon) and colloidal silicon (polysilicon): colloidal silicon has no ionic characteristics, but the scale is relatively large. Colloidal silicon can be intercepted by fine physical filtration processes, such as reverse Osmosis can also reduce the content of water through coagulation techniques, such as coagulation and clarification tanks, but those separation techniques that require ionic charge characteristics, such as ion exchange resins and continuous electrodeionization process (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 technologies 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.

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

Reverse osmosis membrane products correspond to the pH range, generally 2 to 11. The pH has little effect on the membrane performance itself, which is one of the distinguishing characteristics different from other membrane products, but the characteristics of many ions in water are greatly affected by pH. For example, when weak acids such as citric acid are mainly in a non-ionic state at low pH, they dissociate at a high pH and become ionic.

Due to the same ion, the degree of charge is high, the removal rate of the membrane is high, and the degree of charge is low or uncharged, the removal rate of the membrane is low, so pH has a great influence on the removal rate of certain impurities.

What is the relationship between influent TDS and conductivity?

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

How do I know if the membrane has been contaminated?

The following are the common symptoms of pollution:

Under standard pressure, water production decreases. In order to achieve standard water production, the operating pressure must be increased v. The pressure drop between inlet and concentrated water increases v. The weight of the membrane element increases v. The membrane removal rate changes significantly (increases or decreases)

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

How to prevent the growth of microorganisms in the original packaging of the membrane element?

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

When the protective solution becomes cloudy, the components should be removed from the sealed storage bag and re-soaked in fresh protective solution, the concentration of the protective solution is 1% (weight) food grade sodium bisulfite (not activated by cobalt), soak for about 1 hour , And reseal it, and drain the components before repackaging.

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

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

What impurities can be removed by RO membrane?

RO membranes can remove ions and organics very 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. The removal rate of organics in the influent water ≥99%.

How do you know which cleaning method should be used to clean your membrane system?

In order to obtain the best cleaning effect, it is very important to choose the cleaning agents and cleaning steps that can be symptomatic. Incorrect cleaning will actually deteriorate the system performance. Generally speaking, inorganic scale contaminants are recommended to use acidic cleaning solutions, microorganisms or organic For contaminants, alkaline cleaning solution is recommended.

Why is the pH value of RO product water lower than the pH value of inlet water?

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

Because RO membranes can remove soluble ions but not soluble gases, the CO2 content in RO produced water is basically the same as the CO2 content in RO inlet water, but HCO3- and CO3 = can often be reduced by 1 to 2 orders of magnitude This will break the balance between CO2, HCO3- and CO3 = in the influent. In the series of reactions, CO2 will combine with H2O to transfer the following reaction equilibrium until a new balance is established.

HCO3- + H + H2OàCO2 +

If the influent contains CO2, the pH of the RO product water will always decrease. For most RO system reverse osmosis, the pH value of the product water will have 1 to 2 pH drops. When the influent alkalinity and HCO3- are high At this time, the pH of the product water drops even more.

There are very few influent waters, containing less CO2, HCO3- or CO3 = so there is less change in the pH value of the produced water. In some countries and regions, there are regulations on the pH value of drinking water, generally 6.5 ~ 9.0, According to our understanding, this is to prevent the corrosion of the water pipeline, and drinking low-pH water itself will not cause any health problems. It is well known that many commercial carbonated beverages have a pH between 2 and 4.