20 common problems and solutions of reverse osmosis membrane

How often should the reverse osmosis system be cleaned?

In general, when the standardized flux decreases by 10-15%, or the system desalination rate decreases by 10-15%, or the operating pressure and differential pressure between segments increases by 10-15%, the RO system should be cleaned. 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 have to be doubled but the cleaning frequency depends on the actual situation of each project site.

What is SDI?

The best technique currently available to evaluate the potential for colloidal contamination in RO/NF system feed water is to measure the silt density index (SDI, also known as the fouling index) of the feed water, which is an important parameter that must be determined prior to RO design and must be measured periodically during RO/NF operation (2 to 3 times daily for surface water), ASTM D4189-82 specifies the test standard. The feed water regulations for membrane systems are that the SDI15 value must be ≤ 5. Effective technologies for SDI reduction pretreatment are multi-media filters, ultrafiltration, microfiltration, etc. The addition of polyelectric media prior to filtration can sometimes enhance the above physical filtration and reduce SDI values.

Should I use reverse osmosis or ion exchange process for general feed water?

In many feed water conditions, the use of ion exchange resin or reverse osmosis is technically feasible, the choice of process should be determined by economic comparison, in general, the higher the salt content, the more economical reverse osmosis, the lower the salt content, the more economical ion exchange. Due to the popularity of reverse osmosis technology, the use of reverse osmosis + ion exchange process or multi-stage reverse osmosis or reverse osmosis + other depth desalination technology combination process has become a recognized technical and economic more reasonable water treatment program.

How many years do reverse osmosis membrane elements generally last?

The service life of a membrane depends on the chemical stability of the membrane, physical stability of the element, cleanability, water source, pretreatment, cleaning frequency, and operation management level. Based on 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 solutes with molecular weight less than 0.0001 microns, while nanofiltration can remove solutes with molecular weight around 0.001 microns. Nanofiltration is essentially a low-pressure reverse osmosis for applications where the purity of the treated produced water is not particularly stringent. Nanofiltration is suitable for treating well water and surface water. Nanofiltration is suitable for water treatment systems where high desalination rates like reverse osmosis are not necessary, but where the ability to remove hardness components is high, sometimes referred to as "softening membranes", and where nanofiltration systems operate at lower pressures and consume less energy than the corresponding reverse osmosis systems.

What are the separation capabilities of membrane technology?

Reverse osmosis is currently the most sophisticated liquid filtration technology. Reverse osmosis membranes retain inorganic molecules such as dissolved salts and organic matter with molecular weight greater than 100, while 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 (1,000 psi) for seawater. Nanofiltration can remove impurities with particles of 1 nm (10Å) and organic matter with molecular weight greater than 200 to 400. The removal rate of dissolved solids is 20 to 98%, and the removal rate of salts containing monovalent anions (such as NaCl or CaCl2) is 20 to 80%, while the removal rate of salts containing divalent anions (such as MgSO4) is higher at 90 to 98%. Ultrafiltration has a separation effect for macromolecules larger than 100 to 1,000 Å (0.01 to 0.1 µm). All dissolved salts and small molecules can pass through ultrafiltration membranes, and substances that can be removed include colloids, proteins, microorganisms, and large organic molecules. Most ultrafiltration membranes have a MWCO of 1,000 to 100,000. microfiltration removes particles in the range of 0.1 to 1 micron, and typically, suspended matter and large colloids are retained while macromolecules and dissolved salts pass freely through microfiltration membranes, which are used to remove bacteria, microflocs, or total suspended solids TSS, typically at pressures of 1 to 3 bar on both sides of the membrane.

What kind of pretreatment is generally required for RO system?

The usual pretreatment system is composed as follows: coarse filtration (~80 microns) to remove large particles, addition of oxidants such as sodium hypochlorite, followed by precision filtration through a multi-media filter or clarifier, then addition of oxidants such as sodium bisulfite to reduce residual chlorine, and finally installation of a security filter before the high-pressure pump inlet. The role of the security filter, as the name implies, is as the final insurance measure to prevent the occasional large particles on the high-pressure pump impeller and membrane elements of the destructive effect. Water sources containing high levels of suspended particles usually require a higher degree of pretreatment to meet the required influent requirements; water sources with high hardness content are recommended to be softened or acid and scale inhibitor added, etc. For water sources with high microbial and organic content, activated carbon or anti-pollution membrane elements 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, phages and bacteria, at least 3log or higher (>99.9% removal rate). However, it is important to note that in many cases, microbial re-growth on the produced water side of the membrane can still occur, depending on how it is assembled, monitored and maintained, meaning that the ability of a system to remove microorganisms depends critically on the design, operation and management of the system rather than the nature of the membrane elements themselves.

How does temperature affect water production?

The higher the temperature, the higher the water production and vice versa. When operating at higher temperatures, the operating pressure should be adjusted down to keep the water production constant and vice versa.

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

The membrane system is designed for continuous operation, but in actual operation, there is always a certain frequency of start-up and shutdown. When the membrane system is shut down, it must be flushed at low pressure with its produced water or pretreated water to replace the high concentration of concentrated water containing scale inhibitor from the membrane element. Measures should also be taken to prevent the introduction of air due to water leakage from the system, as loss of water from the element may result in irreversible loss of produced water flux. If the shutdown is less than 24 hours, there is no need to take measures to prevent the growth of microorganisms. However, if the downtime exceeds the above, a protective solution should be used to preserve the system or flush the membrane system regularly.

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

The brine seal on the membrane element is required to be installed at the inlet end of the element, with the opening facing the inlet direction, so that when feeding the pressure vessel, the opening (lip) will be further opened to completely seal the inlet water from the membrane element to the pressure vessel wall.

How to remove silicon from water?

Silica exists in water in two forms, active silica (monomeric silica) and colloidal silica (polysilicon): colloidal silica has no ionic characteristics but is relatively large in scale. Colloidal silica can be retained by fine physical filtration processes, such as reverse osmosis, and can also be reduced in water by coagulation techniques, such as coagulation clarifiers, but those separation techniques that rely on ionic charge characteristics, such as ion exchange resins and continuous electrodeionization processes (CDI), are very limited in their effectiveness in removing colloidal silica.

The size of active silica is much smaller than colloidal silica, so that most physical filtration techniques such as coagulation clarification, filtration and air flotation are unable to remove active silica, and the processes that can effectively remove active silica are reverse osmosis, ion exchange and continuous electrodeionization processes.

What is the effect of pH on removal rate, water yield and membrane life?

The pH range of reverse osmosis membrane products is generally from 2 to 11. The effect of pH on the membrane performance itself is very small, which is one of the distinctive features different from other membrane products, but the characteristics of many ions in water itself are greatly affected by pH, for example, when weak acids such as citric acid are mainly in non-ionic state under low pH conditions, while they appear to be dissociated and in ionic state under high pH. Since the same ion, with high charge, has high membrane removal rate, and with low or no charge, has low membrane removal rate, the effect of pH on the removal rate of certain impurities is huge.

What is the relationship between influent TDS and conductivity?

When a feed water conductivity value is obtained, it must be converted to a TDS value so that it can be entered in the software design. For most water sources, the conductivity/TDS ratio is between 1.2 and 1.7. For ROSA design, a conversion ratio of 1.4 for seawater and 1.3 for brackish water is used, which usually gives a good approximation of the conversion ratio.

How do I know if a membrane is contaminated?

The following are common symptoms of contamination.

A decrease in water production at standard pressure.

An increase in operating pressure necessary to achieve the standard water yield.

An increase in the pressure drop between the influent and concentrated water.

An increase in the weight of the membrane element.

Significant change (increase or decrease) in membrane removal rate.

When the element is removed from the pressure vessel and water is poured on the inlet side of the vertical membrane element, water cannot flow through the membrane element and only overflows from the end face (indicating a complete blockage of the inlet water channel).

How to prevent microorganism growth in the original package of membrane element?

When the protective fluid is cloudy, it is most 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 element should be removed from the preservation seal, re-soaked in fresh protective solution at a concentration of 1% by weight food grade sodium bisulfite (not cobalt activated) for approximately 1 hour, resealed and sealed, and the element should be drained before repackaging.

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

Theoretically, the incoming RO and IX systems should be free of the following impurities.

Suspended matter, colloids, calcium sulfate, algae, bacteria, oxidants such as residual chlorine and other oil or lipid substances (must be below the lower detection limit of the instrument) organic matter and iron-organic complexes; iron, copper, aluminum corrosion products and other metal oxides.

The feed water quality will have a huge impact on the life and performance of the RO element and IX resin.

What impurities can RO membranes remove?

RO membranes can remove ions and organics very well. RO membranes have higher removal rates than nanofiltration membranes, and RO can usually remove 99% of the salts in feed water and ≥99% of the organics in feed water.

How do I know which cleaning method to use for my membrane system?

In order to get the best cleaning results, it is important to choose the right cleaning agent and cleaning procedure, as the wrong cleaning can actually deteriorate the system performance.

Why is the pH of RO produced water lower than the pH of feed water?

The best answer to this question can be found when the balance between CO2, HCO3- and CO32- is understood. In a closed system, the relative content of CO2, HCO3- and CO32- varies with pH, with CO2 taking the major part under low pH conditions, HCO3- in the medium pH range, and CO32- in the high pH range. -. Since RO membrane can remove dissolved ions but not dissolved gases, the CO2 content of RO produced water is basically the same as the CO2 content of RO feed water, but HCO3- and CO32- can often be reduced by 1 to 2 orders of magnitude, which will break the balance between CO2, HCO3- and CO32- in feed water, and in a series of reactions, CO2 will combine with H2O The following shift of reaction equilibrium occurs until a new equilibrium is established.

HCO3- + H+ → H2O + CO2 

If the feed water contains CO2, the pH of RO produced water will always decrease. For most RO systems, the pH of RO produced water will have a pH drop of 1 to 2 pH, and when the feed water alkalinity and high, the pH drop of produced water will be even greater.

For a very small number of incoming water, containing less CO2, HCO3- or CO32- so as to see less changes in the pH of produced water, certain countries and regions, there are regulations for drinking water pH, generally 6.5 to 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 known that many commercially available carbonated The pH value of many commercially available carbonated beverages is between 2 and 4