Several questions you need to know about reverse osmosis in sewage treatment

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 double, but the cleaning frequency depends on the actual situation at each project site.

2. What is SDI?

The current effective evalsuation of possible colloidal pollution in the inflow of RO/NF systems

A good technique is to measure the sedimentation density index (SDI) of incoming water, which is an important parameter that must be determined before RO design. During RO/NF operation, regular measurements must be conducted (2-3 daily measurements 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. The effective technologies for reducing SDI pretreatment include multi media filters, ultrafiltration, microfiltration, etc. Adding a polyelectrolyte 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 influent 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. 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 better technical and economically reasonable water treatment solution. For further understanding, please consult a representative of the water treatment engineering company. How many years can reverse osmosis membrane components be used in general? The service life of a membrane depends on its chemical stability, physical stability of components, washability, water source, pre-treatment, cleaning frequency, and operational management level. According to economic analysis, it is usually over 5 years.

How many years can reverse osmosis membrane components be used in general?

The service life of a membrane depends on its chemical stability, physical stability of components, washability, water source, pre-treatment, cleaning frequency, and operational management level. According to economic analysis, it is usually over 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 smaller solutes with a molecular weight of 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 corresponding reverse osmosis systems.

6. What is the separation ability of membrane technology?

Reverse osmosis is currently a more precise liquid filtration technology. Reverse osmosis membranes retain inorganic molecules such as soluble salts and organic compounds with molecular weights greater than 100. On the other hand, water molecules can freely penetrate the reverse osmosis membrane, with a typical removal rate of soluble salts ranging from 95% to 99%. The operating pressure ranges from 7 bar (100 psi) when the inlet water is brackish water to 69 bar (1000 psi) when the seawater is used. 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%, and the removal rate of salts containing univalent 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 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 the ultrafiltration membrane, and the substances that can be removed include colloids, proteins, microorganisms, and macromolecular organic matter. The retention molecular weight of most ultrafiltration membranes is between 1000 and 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 macromolecules and soluble salts can freely pass through the microfiltration membrane. The microfiltration membrane is used to remove bacteria, microflocules, or total suspended solids (TSS), with a typical pressure of 1-3bar on both sides of the membrane

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 allowable concentration of silica in the inlet of the reverse osmosis membrane?

The allowable concentration of silicon dioxide depends on temperature, pH value, and scale inhibitor. Usually, the allowable concentration at the concentrated water end is 100ppm without the addition of scale inhibitor. Some scale inhibitors can allow the concentration of silicon dioxide in concentrated water to be 240ppm. Please consult the scale inhibitor supplier.

9. What is the effect of chromium on RO membranes?

Some heavy metals such as chromium can catalyze the oxidation of chlorine, leading to irreversible degradation of the membrane. This is because Cr6+has poorer stability than Cr3+in water. It seems that metal ions with high 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 usual pre-treatment system consists of coarse filtration (~80 microns) to remove large particles, adding oxidants such as sodium hypochlorite, and then precision filtration through a multi medium filter or clarification tank. Then, adding oxidants such as sodium bisulfite to reduce residual chlorine, and installing a security filter before the high-pressure pump inlet. The function of the safety filter, as the name suggests, is to serve as a final safety measure to prevent accidental damage of large particles to the high-pressure pump impeller and membrane components. Water sources with high levels of particulate suspended solids usually 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 components are also required.

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 over 3 logs (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 method of assembly, monitoring, and maintenance. In other words, the ability of a system to remove microorganisms depends critically on the appropriate system design, operation, and management rather than the nature of the membrane components themselves.

12. How does temperature affect water production?

The higher the temperature, the higher the water yield, and vice versa. When operating at higher temperature conditions, the operating pressure should be lowered to maintain the same water yield, 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 clarifier or medium filter. In addition, cationic polydielectrics 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 it allowed to shut down without system flushing?

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

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

Can the reverse osmosis pure water system be frequently started and stopped?

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, and replace high concentration concentrated water containing scale inhibitors from the membrane components. Measures should also be taken to prevent the introduction of air due to water leakage in the system, 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 as the system for preservation or regular 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.

17. How to remove silicon from water?

Silicon in water exists in two forms, active silicon (monomer 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 are reverse osmosis, ion exchange, and continuous electrodeionization.

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

The pH range corresponding to reverse osmosis membrane products is generally 2-11, and the effect of pH on the membrane performance itself is very small, 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, weak acids such as citric acid are mainly non ionic at low pH conditions, while dissociation occurs at high pH values and becomes ionic. Due to the high degree of charge of the same ion, the removal rate of the membrane is high. If the degree of charge 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 influent water and conductivity?

When obtaining the inlet conductivity value, it is necessary to convert it 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 to obtain a good approximate conversion rate.

How do I know if the membrane has been contaminated?

The following are common symptoms of pollution:

Under standard pressure, the water production rate decreases. In order to achieve standard water production, it is necessary to increase the operating pressure v, increase the pressure drop between the inlet and concentrated water v, and increase the weight of membrane components v. There is a significant change 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 that the inlet channel is completely blocked).

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

When the protective solution becomes turbid, 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 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), 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, colloids, calcium sulfate, algae, bacteria, oxidants, such as residual chlorine, oil or lipid substances (which must be below the detection limit of the instrument), organic and iron organic complexes, metal oxides such as iron, copper, aluminum corrosion products, and influent water quality will have a significant impact on the lifespan and performance of RO components and IX resins.

23. What impurities can RO membrane remove?

RO membrane can effectively remove ions and organic matter, and reverse osmosis membrane has a higher removal rate than nanofiltration membrane. Reverse osmosis usually can 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 which cleaning method to use for your membrane system?

In order to achieve better cleaning results, it is very important to choose targeted cleaning agents and cleaning steps. Incorrect cleaning can actually worsen system performance. Generally speaking, for inorganic fouling pollutants, it is recommended to use acidic cleaning solutions, microbial or organic pollutants, and alkaline cleaning solutions are recommended.

25. 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, a good 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 dominates, with HCO3- being the main component in the medium pH range and CO3=in the high pH range. Due to the RO membrane being able to remove soluble ions but not dissolved gases, the CO2 content in the RO production water is basically the same as that in the RO influent. However, HCO3- and CO3=can often be reduced by 1-2 orders of magnitude, which 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 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 pH values. When the alkalinity and HCO3- of the influent are high, the pH value of the produced water will decrease even more.

A very small amount of influent water, containing less CO2, HCO3- or CO3=so that there is less change in the pH value of the produced water. Some countries and regions have regulations for the pH value of drinking water, usually 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 pH values between 2-4.

The above is what Hongjie Water has brought to you about "a few questions you need to know about reverse osmosis in sewage treatment". I hope it can help you!

Shenzhen Hongjie Water Technology Co., Ltd. is an integrated high-tech enterprise specializing in product research and development, production, sales, engineering design, installation and debugging, technical consulting, and sales of supporting materials in the fields of industrial water treatment and drinking water treatment. Our main products include the design, manufacturing, installation, debugging, and maintenance of water treatment equipment, including pure water equipment, ultra pure water equipment, reverse osmosis equipment, GMP purified water equipment, EDI deionized water equipment, domestic sewage equipment, industrial wastewater equipment, softened water equipment, and reclaimed water reuse equipment, all of which are integrated into an environmentally friendly high-tech manufacturer.