25 common problems and solutions of reverse osmosis membrane treatment technology in sewage treatment

In general, when the standardized flux decreases by 10-15%, or the desalination rate of the system decreases by 10-15%, or the operating pressure and inter section pressure difference 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 a year; When SDI15 is around 5, the cleaning frequency may need to be doubled, but the cleaning frequency depends on the actual situation at each project site.

How often should the reverse osmosis system be cleaned?

In general, when the standardized flux decreases by 10-15%, or the desalination rate of the system decreases by 10-15%, or the operating pressure and inter section pressure difference 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 a year; When SDI15 is around 5, the cleaning frequency may need to be doubled, but the cleaning frequency depends on the actual situation at each project site.

2. What is SDI?

The best effective technique for evalsuating colloid pollution in the inflow of RO/NF systems is currently to measure the sedimentation density index (SDI) of the inflow, which is an important parameter that must be determined before RO design. During the operation of RO/NF, regular measurements must be taken (2-3 times a day for surface water). ASTM D4189-82 specifies the standard for this test. The inlet water requirement for the membrane system is that the SDI15 value must be ≤ 5. Effective technologies for reducing SDI pretreatment include multimedia filters, ultrafiltration, microfiltration, etc. Adding a polymeric dielectric before filtration can sometimes enhance the aforementioned physical filtration and reduce the SDI value.

3. Should reverse osmosis process or ion exchange process be used for general inflow?

Under many inflow conditions, the use of ion exchange resin or reverse osmosis is technically feasible, and the choice of process should be determined by economic comparison. Generally, the higher the salt content, the more economical reverse osmosis is, while the lower the salt content, the more economical ion exchange is. Due to the widespread use of reverse osmosis technology, the combination of reverse osmosis+ion exchange process or multi-stage reverse osmosis or reverse osmosis+other deep desalination technologies has become a recognized technology and more economically reasonable water treatment solution. If you need further understanding, please consult a representative of a water treatment engineering company. How many years can reverse osmosis membrane components generally be used? The service life of a membrane depends on its chemical stability, physical stability of components, washability, inlet water source, pre-treatment, cleaning frequency, and operational management level. According to economic analysis, it is usually more than 5 years.

How many years can reverse osmosis membrane components generally be used?

The service life of a membrane depends on its chemical stability, physical stability of components, washability, inlet water source, pre-treatment, cleaning frequency, and operational management level. 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 and ultrafiltration. Reverse osmosis can remove the smallest solute with a molecular weight less than 0.0001 micrometers, while nanofiltration can remove solutes with a molecular weight of around 0.001 micrometers. Nanofiltration is essentially a type of low-pressure reverse osmosis, used in situations where the purity of 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 high desalination rates like reverse osmosis, but have a high ability to remove hardness components, sometimes referred to as "softening membranes." Nanofiltration systems have low operating pressure and lower energy consumption than their corresponding reverse osmosis systems.

What is the separation ability of membrane technology?

Reverse osmosis is currently the most precise liquid filtration technology. Reverse osmosis membranes retain 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 reverse osmosis membranes, and 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 (1000 psi) for seawater. Nanofiltration can remove impurities in particles 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 20-80%, while salts containing divalent anions (such as MgSO4) have a higher removal rate of 90-98%. Ultrafiltration has a separation effect on macromolecules larger than 100-1000 angstroms (0.01-0.1 micrometers). All soluble salts and small molecules can pass through ultrafiltration membranes, and the substances that can be removed include colloids, proteins, microorganisms, and macromolecular organic matter. The retention molecular weight of most ultrafiltration membranes ranges from 1000 to 100000. The range of particle removal by microfiltration is about 0.1-1 micrometers. Generally, suspended solids and large particle colloids can be intercepted, while large molecules and soluble salts can be retained.

7. Who sells membrane cleaning agents or provides cleaning services?

Water treatment companies can provide specialized membrane cleaning agents and cleaning services, and users can purchase cleaning agents for membrane cleaning according to the suggestions of membrane companies or equipment suppliers.

8. What is the maximum allowable concentration of silica in the inlet of the reverse osmosis membrane?

The maximum allowable concentration of silica depends on temperature, pH value, and scale inhibitor. Generally, the maximum allowable concentration in concentrated water is 100ppm without the addition of scale inhibitor. Some scale inhibitors can allow the maximum concentration of silica in concentrated water to be 240ppm. Please consult the scale inhibitor 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 the stability of Cr6+in water is worse than that of Cr3+. It seems that metal ions with higher oxidation valence have a stronger destructive effect. Therefore, the concentration of chromium should be reduced or at least Cr6+should be reduced to Cr3+in the pre-treatment section.

10. What kind of preprocessing is generally required for RO systems?

The typical pre-treatment system consists of coarse filtration (~80 microns) to remove large particles, adding oxidants such as sodium hypochlorite, then precision filtration through a multimedia filter or clarification tank, adding oxidants such as sodium bisulfite to reduce residual chlorine, and finally installing a safety filter before the inlet of the high-pressure pump. The function of the security filter, as the name suggests, is to serve as the ultimate safety measure to prevent accidental large particles from damaging the impeller and membrane components of the high-pressure pump. Water sources with high levels of particulate suspended solids typically require higher levels of pre-treatment to meet the specified inflow requirements; For water sources with high hardness content, it is recommended to use softening or adding acid and scale inhibitors. For water sources with high microbial and organic content, activated carbon or anti fouling membrane elements are also needed.

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 logs or more (removal rate>99.9%). However, it should be noted that in many cases, microorganisms may still reproduce on the water producing side of the membrane, which mainly depends on the assembly, monitoring, and maintenance methods. In other words, the ability of a system to remove microorganisms depends crucially on whether the system design, operation, and management are appropriate rather than the properties of the membrane components themselves.

12. What is the impact 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 lowered to maintain the same water production, and vice versa. Please refer to the relevant chapters for the temperature correction factor TCF for changes in water production.

13. What are particle and colloid pollution? How to measure?

Once particles and colloids become clogged in reverse osmosis or nanofiltration systems, it can seriously affect the membrane's water production and sometimes reduce the desalination rate. The early symptoms of colloidal fouling are an increase in system pressure difference, and the sources of particles or colloids in the membrane inlet water source vary from place to place, often including bacteria, sludge, colloidal silicon, iron corrosion products, etc. The drugs used in the pre-treatment part, such as polyaluminum and ferric chloride or cationic polyelectrolyte, may also cause fouling if they cannot be effectively removed in the clarification tank or medium filter. In addition, cationic polymeric dielectrics can also react with anionic scale inhibitors, and their precipitates can contaminate and block membrane components. SDI15 is used to evalsuate the tendency of such fouling or pre-treatment in water. Please refer to the detailed introduction in relevant chapters.

14. How long is the maximum allowed shutdown without system flushing?

If the system uses a blocking agent, when the water temperature is between 20-38 ℃, it takes about 4 hours; Approximately 8 hours below 20 ℃; If the system is not using scale inhibitors, it will take about 1 day. 15. How can the energy consumption of the membrane system be reduced?

Low energy consumption membrane elements are sufficient, but it should be noted that their desalination rate is slightly lower than that of standard membrane elements.

It can freely pass through the microfiltration membrane, which is used to remove bacteria, micro flocs, or total suspended solids (TSS). The typical pressure on both sides of the membrane is 1-3 bar

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

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 is shut down, it is necessary to use its produced water or pre treated qualified water for low-pressure flushing to replace high concentration concentrated water containing scale inhibitors from the membrane components. Measures should also be taken to prevent water leakage in the system and the introduction of air, as if the components lose water and dry up, irreversible loss of water production flux may occur. If the shutdown is less than 24 hours, there is no need to take measures to prevent microbial growth. But if the shutdown time exceeds the above regulations, protective liquid should be used for system preservation or timed flushing of the membrane system.

How to determine the direction of installing salt water sealing rings on membrane components?

The salt water sealing ring on the membrane element is required to be installed at the inlet end of the element, with the opening facing the inlet direction. When the pressure vessel is filled with water, its opening (lip edge) will further open, completely sealing the side flow of water from the membrane element to the inner wall of the pressure vessel.

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 the characteristics of ions, but has a relatively large scale. Colloidal silicon can be intercepted by fine physical filtration processes, such as reverse osmosis, and the content of water can be reduced through coagulation technology, such as coagulation clarification tanks. However, separation technologies that rely on ion charge characteristics, such as ion exchange resins and continuous electrodeionization processes (CDI), The effect on removing colloidal silicon is very limited.

The size of activated silicon is much smaller than that of colloidal silicon, so most physical filtration technologies such as coagulation clarification, filtration, and air flotation cannot remove activated silicon. The processes that can effectively remove activated silicon include reverse osmosis, ion exchange, and continuous electrodeionization.

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

The pH range corresponding to reverse osmosis membrane products is generally 2-11, and pH has little effect on the membrane performance itself, which is one of the significant characteristics different from other membrane products. However, the characteristics of many ions in water are greatly affected by pH. For example, when weak acids such as citric acid are in a non-ionic state under low pH conditions, they dissociate and become ionic at high pH values. Due to the high charge level of the same ion, the removal rate of the membrane is high. If the charge level is low or not, the removal rate of the membrane is low. Therefore, pH has a significant impact on the removal rate of certain impurities.

19. What is the relationship between TDS of incoming water and conductivity?

When obtaining the inlet conductivity value, it must be converted into TDS value so that it can be input during software design. For most water sources, the ratio of conductivity to TDS is between 1.2 and 1.7. For ROSA design, a ratio of 1.4 is used for seawater and a ratio of 1.3 is used for brackish water conversion, which usually yields a good approximate conversion rate.

How to know if the membrane has been contaminated?

The following are common symptoms of pollution:

Under standard pressure, water production decreases

In order to achieve standard water production, it is necessary to increase the operating pressure v

The pressure drop between inlet and concentrated water increases by v

The weight of membrane components increases by v

Significant changes in membrane removal rate (increase or decrease)

When the component is removed from the pressure vessel, pour water onto the inlet side of the vertical membrane component. Water cannot flow through the membrane component and only overflows from the end face (indicating complete blockage of the inlet channel).

21. How to prevent the growth of microorganisms inside the original packaging of membrane components?

When the protective solution becomes cloudy, it is likely due to the growth of microorganisms. Membrane components protected with sodium bisulfite should be inspected 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 with a concentration of 1% (by weight) food grade sodium bisulfite (not activated by cobalt), soaked for about 1 hour, and re sealed. The components should be drained before repackaging.

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

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

Suspended solids

colloid

calcium sulfate

algae

bacterium

Oxidants, such as residual chlorine, etc

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

Organic compounds and iron organic complexes

Metal oxides such as corrosion products of iron, copper, and aluminum

The inlet water quality will have a significant impact on the lifespan and performance of RO components and IX resins.

23. What impurities can RO membranes remove?

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

24. How do you know what cleaning method to use for your membrane system?

In order to achieve the best cleaning effect, it is very important to choose targeted cleaning agents and cleaning steps. Incorrect cleaning can actually deteriorate system performance. Generally speaking, for inorganic scaling pollutants, it is recommended to use acidic cleaning solutions, microbial or organic pollutants, and alkaline cleaning solutions are recommended.

Why is the pH value of RO produced water lower than the pH value of influent water?

When the balance between CO2, HCO3-, and CO3=is understood, the best answer to this question can be found. In a closed system, the relative content of CO2, HCO3-, and CO3=varies with pH value. Under low pH conditions, CO2 accounts for the majority, with HCO3- being the main component in the medium pH range and CO3=being the main component in the high pH range. Due to the ability of RO membrane to remove soluble ions but not soluble gases, the CO2 content in RO production water is basically the same as that in RO influent. However, HCO3- and CO3=can often be reduced by 1-2 orders of magnitude, which can break the balance between CO2, HCO3-, and CO3=in influent. In a series of reactions, CO2 will combine with H2O to undergo the following reaction equilibrium transfer until a new equilibrium is established.

HCO3-+H+H2O à CO2+

If the influent contains CO2, the pH value of the RO produced water will always decrease. For most RO systems, the pH value of the reverse osmosis produced water will decrease by 1-2 times. When the influent alkalinity and HCO3- are high, the pH value of the produced water will decrease even more.

A very small amount of incoming water, containing less CO2, HCO3- or CO3=, results in less variation in the pH value of the produced water. In some countries and regions, there are regulations for the pH value of drinking water, generally ranging from 6.5 to 9.0. According to our understanding, this is to prevent corrosion of the water supply pipeline. Drinking low pH water itself will not cause any health problems. As is well known, many commercially available carbonated beverages have a pH value between 2 and 4.

As for the cost of a set of two-stage purified water equipment, it depends on the customer's water production volume, the required configuration, and the price. Hongjie Water has 10 years of experience in producing purified water treatment equipment, rich production experience, professional technology, and after-sales service team. The equipment components are all from well-known brands, with guaranteed quality and after-sales service.