14. How long is the maximum allowable downtime without system flushing?
If the system uses scale inhibitor, when the water temperature is between 20℃ and 38℃, about 4 hours; when it is below 20℃, about 8 hours; if the system does not use scale inhibitor, about 1 day.
15. Can the reverse osmosis pure water system start and stop 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, the produced water or pretreated water must be used for low-pressure flushing to replace the concentrated water with high concentration but 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 provision, protective liquid should be used to save the system or flush the membrane system regularly.
16. 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 water inlet end of the element, while the opening faces the water inlet direction, when feeding the pressure vessel, the opening (lip) will be further opened to completely seal the water inlet from the membrane element and the pressure vessel wall between the side flow.
17. How to remove silicon from water?
Silicon in water exists in two forms, active silicon (monomeric silicon) and colloidal silicon (polysilicon): colloidal silicon has no ionic characteristics, but the scale is relatively large, colloidal silicon 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 process (CDI), have very limited effect on the removal of colloidal silica.
The size of active silica is much smaller than colloidal silica, so 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.
18. 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 membrane performance itself is very small, which is one of the significant features different from other membrane products, but the characteristics of many ions in water itself are greatly affected by pH, for example, when citric acid and other weak acids are mainly in non-ionic state under low pH conditions, while they appear to be dissociated and in ionic state under high pH value. Since the same ion, high degree of charge, the membrane removal rate is high, low or no charge, the membrane removal rate is low, so pH has a huge impact on the removal rate of certain impurities.
19. What is the relationship between influent TDS and conductivity?
When the influent 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, seawater is converted to a ratio of 1.4 while brackish water is converted to a ratio of 1.3, which usually gives a good approximation of the conversion rate.
20. 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 standard water yield.
An increase in the pressure drop between the influent and concentrated water.
An increase in the weight of the membrane element.
A significant change (increase or decrease) in the membrane removal rate.
When the element is removed from the pressure vessel, 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 surface (indicating that the inlet water channel is completely blocked)
21. How to prevent microorganisms from growing in the original package of membrane element?
When the protection liquid is cloudy, it is probably due to the growth of microorganisms. The membrane element protected by sodium bisulfite should be checked once every three months.
When the protective solution appears cloudy, the element should be removed from the preservation seal bag, re-soaked in fresh protective solution with 1% (by weight) food grade sodium bisulfite (not activated by cobalt), soaked for about 1 hour, and resealed and sealed, and the element should be drained before repackaging.
22.What are the inlet water requirements for RO membrane elements and IX ion exchange resin?
Theoretically, the incoming RO and IX systems should be free of the following impurities.
Suspended matter.
Colloids.
Calcium sulfate.
Algae.
Bacteria.
Oxidizing agents, such as residual chlorine.
Oils or lipids (which must be below the lower detection limit of the instrument).
Organic substances and iron-organic complexes.
Iron, copper, aluminum corrosion products and other metal oxides
The inlet water quality will have a huge impact on the life and performance of the RO element and IX resin.
23.What impurities can be removed by RO membrane?
RO membrane can remove ions and organics very well, RO membrane has higher removal rate than nanofiltration membrane, RO can usually remove 99% of salts in feed water, and the removal rate of organics in feed water is ≥99%.
24. How do I know which cleaning method to use to clean your membrane system?
In order to get the best cleaning effect, it is very important to choose the right cleaning agent and cleaning procedure, the wrong cleaning will actually deteriorate the system performance.
25. Why is the pH of the RO produced water lower than the pH of the feed water?
In a closed system, the relative content of CO2, HCO3- and CO32- varies with pH. Under low pH conditions, CO2 accounts for the major part, in the medium pH range, it is mainly HCO3-, and in the high pH range, it is mainly CO32-.
Since RO membrane can remove dissolved ions but not dissolved gases, the CO2 content in RO produced water is basically the same as the CO2 content in 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 reaction, CO2 will combine with H2O The following shift of reaction equilibrium occurs until a new equilibrium is established.
CO2 + H2O → HCO3- + H+
If the feed water contains CO2, the pH of the produced water from the RO membrane element will always decrease. For most RO systems the pH of the produced water will have a pH drop of 1 to 2 pH values, and when the feed water alkalinity and HCO3- are high, the pH drop of the produced water will be even greater. A very small number of feed water, containing less CO2, HCO3- or CO32- will see less change in the pH of the produced water.
If the pH of reverse osmosis effluent is low, add metering pump to adjust the pH to alkaline by adding NaOH, because when the pH is between 7.5~8, the desalination effect of reverse osmosis can reach the best.
26. How can the energy consumption of membrane system be reduced?
Low energy consumption membrane elements can be used, but it should be noted that their desalination rate is slightly lower than that of standard membrane elements.
