How often should the RO system be cleaned

Generally, when the standardized flux decreases by 10-15%, or the desalination rate of the system decreases by 10-15%, or the operating pressure and the pressure difference between sections increase by 10-15%, RO system should be cleaned. The cleaning frequency is directly related to the pretreatment degree of the system. When SDI15 < 3, the cleaning frequency may be 4 times a year; When SDI15 is around 5, the cleaning frequency may be doubled, but the cleaning frequency depends on the actual situation of each project site.

  What is SDI?

At present, the best technology to evalsuate colloidal pollution in ro/nf system is to measure the sedimentation density index (SDI) of the inflow water (also known as pollution plugging index), which is an important parameter that must be determined before ro design. During ro/nf operation, it is necessary to measure regularly (2-3 times daily measurement of surface water), and ASTM d4189-82 specifies the standard of this test. The water inlet regulation of membrane system is that SDI15 value must be ≤ 5. The effective technologies to reduce SDI pretreatment include multi-media filter, ultrafiltration, microfiltration, etc. Adding a polymer before filtering can sometimes enhance the above physical filtration and reduce the SDI value.

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

In many water inflow conditions, it is feasible to use ion exchange resin or reverse osmosis. The choice of process is determined by economic comparison. Generally, the higher the salt content, the more economical the reverse osmosis is, the lower the salt content, the more economical the ion exchange will be. Due to the popularity 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 more reasonable water treatment scheme. For further understanding, please consult the representative of water treatment engineering company.

How many years can reverse osmosis membrane elements normally take?

The service life of the membrane depends on the chemical stability of the membrane, the physical stability of the components, the washability, the water source, pretreatment, cleaning frequency, operation and management level. It is usually more than five years based on economic analysis.

What is the difference between RO and nanofiltration?

Nanofiltration is a membrane liquid separation technology between ro contract ultrafiltration. Ro can remove the smallest solute, with molecular weight less than 0.0001 μ m, and nanofiltration can remove the solute with molecular weight of about 0.001 μ M. Nanofiltration is a kind of low pressure reverse osmosis in essence, which is used in the situation where the purity of produced water after treatment is not very strict. Nanofiltration is suitable for the treatment of well water and surface water. Nanofiltration is suitable for water treatment system without high desalination rate like reverse osmosis, but it has high removal ability for hardness components, sometimes called "softening film". The nanofiltration system has low operating pressure and lower energy consumption than the corresponding reverse osmosis system.

What separation capability does membrane technology have?

Reverse osmosis is the most precise liquid filtration technology. The reverse osmosis membrane can intercept the soluble salts and other inorganic molecules and organic matter with molecular weight greater than 100. On the other hand, water molecules can freely pass through the reverse osmosis membrane, and the removal rate of typical soluble salt is more than 95-99%. The operating pressure is from 7bar (100psi) when the water is brackish to 69bar (1000PSI) at sea water. Nanofiltration can remove impurities and organic matter with molecular weight greater than 200-400 at 1nm (10 Angstrom), the removal rate of soluble solid is 20-98%, and that of salt containing monovalent anion (such as NaCl or CaCl2) is 20-80%, while that of salt containing divalent anion (such as MgSO4) is higher, 90-98%. Ultrafiltration has the separation effect on macromolecules larger than 100-1000 angstrom (0.01-0.1 μ m). All soluble salts and small molecules can be removed through ultrafiltration membrane, including colloid, protein, microorganism and macromolecular organic matter. The molecular weight of most ultrafiltration membranes is 1000-100000. The range of removal of particles by microfiltration is about 0.1-1 μ M. generally, suspended matter and colloid of large particles can be intercepted, while macromolecules and dissolved salts can freely pass through microfiltration membrane. Microfiltration membrane is used to remove bacteria, micro flocculants or total suspended solids TSS. The pressure on both sides of typical membrane is 1-3bar

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

The maximum allowable silica concentration depends on temperature, pH value and scale inhibitor. Generally, the maximum allowable concentration at the concentrated water end is 100ppm without scale inhibitor, and the maximum allowable concentration of silica in concentrated water can be allowed by some scale inhibitors is 240ppm. Please consult the scale inhibitor supplier.

What is the effect of chromium on RO film?

Some heavy metals such as chromium can catalyze the oxidation of chlorine, which will cause irreversible property attenuation of the membrane. This is because cr6+ is less stable than cr3+ in water. It seems that the metal ions with high oxidation price will have stronger destructive effect. Therefore, the chromium concentration should be reduced in the pretreatment section or at least cr6+ should be reduced to cr3+.

What pretreatment is generally required for RO system?

The general pre-treatment system consists of the following: coarse filtration (~ 80 microns) to remove large particles, adding oxidants such as sodium hypochlorite, and then through multi-media filter or clarifier for precise filtration, adding sodium bisulfite to reduce residual chlorine and other oxidants, and finally installing security filter before the inlet of high-pressure pump. As the name implies, the safety filter is the ultimate insurance measure to prevent the accidental damage of large particles on the impeller and membrane components of high-pressure pump. Water source with more suspended particles usually needs higher pretreatment to meet the requirements of water inflow; For water source with high hardness content, it is recommended to use soften or add acid and scale inhibitor. For water 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?

Ro is very dense, and has a very high removal rate for viruses, phages and bacteria, at least more than 3log (removal rate is more than 99.9%). However, it should be noted that in many cases, microorganisms may appear on the water producing side of membrane, which mainly depends on the way of assembly, monitoring and maintenance. That is, the ability of a system to remove microorganisms depends on whether the system design, operation and management are appropriate rather than the nature of membrane elements.

What is the effect of temperature on water yield?

The higher the temperature, the higher the water yield, and vice versa. When operating at a higher temperature, the operating pressure should be lowered to keep the water yield unchanged, and vice versa. For temperature correction coefficient TCF of water yield change, please refer to relevant chapter.

What are particulate and colloidal pollution? How to measure?

Once the fouling of particles and colloids occurs in reverse osmosis or nanofiltration system, the water production of membrane will be seriously affected, and the desalination rate will be reduced sometimes. The early symptom of colloidal pollution plugging is the increase of system pressure difference. The source of particles or colloids in the water source of membrane water varies from place to place, and often includes bacteria, silt, colloidal silicon, iron corrosion products, etc. the drugs used in the pretreatment part, such as polyaluminium and ferric chloride or cationic polymerizer, may cause pollution plugging if they cannot be effectively removed in the clarifier or medium filter. In addition, the cationic polyelectrolyte will react with anionic scale inhibitor. The sediment will contaminate the membrane blocking element. SDI15 is used to evalsuate the pollution blocking tendency or pretreatment in water. Please refer to the detailed introduction in relevant chapters.

How long is the maximum shutdown allowed without system flushing?

If the system uses the hindrance agent, the water temperature is between 20 ℃ and 38 ℃, about 4 hours; About 8 hours at 20 ℃; If the system is not using scale inhibitor, about one day. 15. how can the energy consumption of membrane system be reduced?

It is easy to use low energy consumption membrane elements, but it should be noted that their desalination rate is slightly lower than that of standard membrane elements.

Can the RO pure water system start and stop frequently?

Membrane system is designed according to continuous operation, but in practice, there will always be a certain frequency of start-up and shutdown. When the membrane system is shut down, it is necessary to wash the membrane system with its produced water or qualified water after pretreatment, and replace the concentrated water with high concentration but containing scale inhibitor from the membrane element. Measures should also be taken to prevent the air from leaking out of the system, because if the component is lost and dried, it may cause irreversible loss of water production flux. If the shutdown is less than 24 hours, no measures are required to prevent microbial growth. However, if the shutdown time exceeds the above requirements, the protective liquid shall be used as the system to save or regularly wash the membrane system.

How to determine the direction of installing brine sealing ring on membrane element?

The brine sealing ring on the membrane element is required to be installed at the inlet end of the element, and the opening is facing the direction of water inflow. When the pressure vessel is fed water, its opening (LIP) will be further opened to completely seal 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 (monomer silicon) and colloidal silicon (polysilicon): colloidal silicon has no ion characteristics, but the scale is relatively large. Colloidal silicon can be intercepted by fine physical filtration process, such as reverse osmosis, and the content of water can be reduced by coagulation technology, such as coagulation clarifier, but those separation technologies which rely on the characteristics of ion charge can be used, For example, ion exchange resin and continuous electrodeionization process (CDI) have limited effect on the removal of colloidal silicon.

The size of active silicon is much smaller than colloidal silicon, so most physical filtration technologies such as coagulation clarification, filtration and air flotation can not remove active silicon. The process of effective removal of active silicon is reverse osmosis, ion exchange and continuous electrodialization.

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

The pH range of reverse osmosis membrane products is generally 2-11, and the influence of pH on membrane properties is very small. This is one of the distinctive 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 non-ionic state at low pH, they are dissociated and ionic at high pH. Because of the same ion, the high charge, high removal rate and low charge or no charge, the removal rate of the membrane is low, so the pH has a great influence on the removal rate of some impurities.

What is the relationship between TDS and conductivity of water inlet?

When the conductivity value of water inlet is obtained, it must be converted into TDS value so that it can be input in software design. For most water sources, the ratio of conductivity to TDS is between 1.2 and 1.7. In order to design Rosa, the ratio of seawater is 1.4 and that of brackish water is 1.3, which can get a better approximate conversion rate.

How do I know if the membrane has been contaminated?

The following are common symptoms of pollution:

Under the standard pressure, the water yield decreased;

In order to meet the standard water yield, the operating pressure must be increased;

The pressure drop between the influent and the concentrated water increases;

The weight of membrane element increases;

The removal rate of membrane was significantly changed (increased or decreased).

When the element is taken out of the pressure vessel, the water is poured on the inlet side of the vertical membrane element, and the water cannot flow through the membrane element, and only overflows from the end surface (indicating that the inlet channel is completely blocked).

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

When the protective liquid appears to be turbid, it is likely that the microorganism is the reason. Membrane elements protected with sodium bisulfite shall be viewed every three months. When the protective solution is turbid, the components shall be taken out from the storage sealing bag and re soaked in the 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 sealed again. The components shall be drained before repacking.

What are the requirements for water inflow of RO membrane elements and IX exchange resin?

Theoretically, the entry into RO and IX systems should not contain the following impurities:

The complex of organic matter and iron organic matter of suspended substance, colloid, calcium sulfate, algae, bacteria, oxidant, such as residual chlorine, or lipid (which must be lower than the detection limit of the instrument); Metal oxides such as iron, copper, aluminum corrosion products.

The water quality of the water will have a great influence on the life and performance of RO components and IX resin.

What impurities can RO membrane remove?

RO membrane can remove ions and organic matter well. Reverse osmosis membrane has higher removal rate than nanofiltration membrane. Reverse osmosis usually can remove 99% salt in water supply and 99% of organic matter in water inlet.

How to know what cleaning method should membrane system use?

In order to get the best cleaning effect, it is very important to choose cleaning agents and cleaning steps that can be used for the disease. In fact, the wrong cleaning will worsen the system performance. Generally speaking, inorganic scaling pollutants are recommended to use acid cleaning solution, microorganism or organic pollutant, and alkaline cleaning solution is recommended.

Why is the pH of RO produced water lower than that of water inflow?

When we understand the balance between CO2, hco3- and co32-, we can find the best answer to this problem. In a closed system, the relative content of CO2, hco3- and co32- varies with the change of pH value. Under low pH, CO2 accounts for the main part. In the medium pH range, hco3-, and the high pH range, co32-. Because RO membrane can remove dissolved ions and cannot remove dissolved gas, the CO2 content in RO water is basically the same as that in RO water inflow, but hco3- and co32- can often reduce 1-2 order of magnitude, which will break the balance between CO2, hco3- and co32- in the influent. In series reactions, CO2 will be combined with H2O to transfer the following reaction equilibrium, Until a new balance is established.

HCO3-+ H+ → H2O + CO2  

If CO2 is contained in the water, the pH value of RO will always decrease. For most RO system, the pH value of RO system RO water will decrease by 1-2 pH values. When the alkalinity and high water inflow, the pH value of the water production will decrease more.

There are few water inflow, containing less CO2, hco3- or co32- so that the pH value of produced water will be less changed. In some countries and regions, the pH value of drinking water is generally 6.5-9.0. According to our understanding, it is to prevent the corrosion of water transmission pipeline, and drinking water with low pH value will not cause any health problems. It is well known that, Many commercial carbonated drinks have pH values between 2 and 4