Analysis of Common Problems in Reverse Osmosis Systems (Part 2)

14. How long can the system be shut down without performing a system flush?

If the system uses scale inhibitors, when the water temperature is between 20～38℃, approximately 4 hours; below 20℃, approximately 8 hours; if the system does not use scale inhibitors, approximately 1 day.

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

The membrane system is designed for continuous operation, but in actual operation, there will always be a certain frequency of start-up and shutdown.

When the membrane system is shut down, it must be flushed at low pressure with its own product water or pre-treated qualified water to displace the high-concentration but scale-inhibitor-containing concentrate from the membrane elements.

Measures should also be taken to prevent water leakage from the system and the introduction of air, because if the elements dry out, irreversible product water flux loss may occur.

If the shutdown is less than 24 hours, no measures need to be taken to prevent microbial growth. However, if the shutdown time exceeds the above regulations, a protective solution should be used for system preservation or the membrane system should be flushed regularly.

16. How is the direction of the brine seal on the membrane element determined?

The brine seal on the membrane element must be installed at the inlet end of the element, with the opening facing the inlet direction. When water enters the pressure vessel, the opening (lip) will further open, completely sealing the bypass 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: reactive silicon (monomeric silicon) and colloidal silicon (polymeric silicon): Colloidal silicon does not have ionic characteristics, but its size is relatively large. Colloidal silicon can be retained by fine physical filtration processes, such as reverse osmosis, and its content in water can also be reduced by coagulation techniques, such as coagulation clarification pools. However, separation techniques that rely on ionic charge characteristics, such as ion exchange resins and continuous electrodialysis ( CDI), are very limited in removing colloidal silicon.

Reactive silicon is much smaller than colloidal silicon, so most physical filtration techniques such as coagulation clarification, filtration, and flotation cannot remove reactive silicon. Processes that can effectively remove reactive silicon are reverse osmosis, ion exchange, and continuous electrodialysis.

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

Reverse osmosis membrane products correspond to a pH range of generally 2～11. The 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 at low pH, they are mainly in a non-ionic state, while at high pH values, they dissociate and become ionic. Since the higher the charge of the same ion, the higher the removal rate of the membrane, and the lower or non-charged, the lower the removal rate of the membrane, therefore, the pH has a significant impact on the removal rate of certain impurities.

19. Influent What is the relationship between TDS and conductivity?

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

20. How do you know if the membrane is fouled?

The following are common symptoms of fouling:

Water production decreases at standard pressure

Operating pressure must be increased to achieve standard water production v

Increased pressure drop between influent and concentrate v

Increased weight of membrane element v

Significant change in membrane rejection rate (increase or decrease)

When the element is removed from the pressure vessel, water is poured onto the inlet side of the upright membrane element, and the water cannot flow through the membrane element, only overflowing from the end face (indicating that the inlet flow path is completely blocked)

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

If the preservative solution becomes cloudy, it is likely due to microbial growth. Membrane elements protected with sodium bisulfite should be checked every three months.

If the preservative solution becomes cloudy, the element should be removed from the storage bag and re-immersed in fresh preservative solution at a concentration of 1% (weight) food-grade sodium bisulfite (not cobalt-activated), soaked for about 1 hour, and re-sealed. The element should be drained before repackaging.

22. What are the influent requirements for RO membrane elements and IX ion exchange resins?

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

1. Suspended solids;

2. Colloids;

3. Calcium sulfate;

4. Algae;

5. Bacteria;

6. Oxidants, such as residual chlorine;

7. Oil or grease (must be below the detection limit of the instrument);

8. Organic matter and iron-organic complexes;

9. Metal oxides such as iron, copper, and aluminum corrosion products.

Influent water quality will have a significant impact on the life and performance of RO elements and IX resins.

Twenty-three, What impurities can RO membranes remove?

RO membranes can effectively remove ions and organic matter, with a higher removal rate than nanofiltration membranes. Reverse osmosis typically removes 99% of salts from the water, and the removal rate of organic matter in the feed water is ≥99%.

Twenty-four, How do you know what cleaning method to use for your membrane system?

To achieve the best cleaning results, it is crucial to select the appropriate cleaning agents and steps. Incorrect cleaning can actually worsen system performance. Generally, for inorganic scaling contaminants, acidic cleaning solutions are recommended; for microbial or organic contaminants, alkaline cleaning solutions are recommended.

Twenty-five, Why is the pH of RO product water lower than the pH of the feed water?

In a closed system, the relative content of CO2, HCO3-, and CO32- changes with pH. Under low pH conditions, CO2 is the main component; in the medium pH range, it is mainly HCO3-; and in the high pH range, it is mainly CO32-.

Because RO membranes can remove dissolved ions but not dissolved gases. The CO2 content in RO product water is basically the same as that in RO feed water, but HCO3- and CO32- can often be reduced by 1-2 orders of magnitude. This will disrupt the equilibrium between CO2, HCO3-, and CO32- in the feed water. In a series of reactions, CO2 will combine with H2O to cause the following reaction equilibrium transfer until a new equilibrium is established.

CO2 + H2O ---HCO3- + H+

If the feed water contains CO2, the pH of the product water from the RO membrane element will always decrease. For most RO systems, the pH of the reverse osmosis product water will decrease by 1-2 pH units. When the feed water alkalinity and HCO3- are high, the pH of the product water will decrease even more. In very few feed waters with less CO2, HCO3-, or CO32-, the change in product water pH will be less noticeable.

Reverse osmosis effluent has a low pH. A metering pump is used to add NaOH to adjust the pH to alkaline, because when the pH is between 7.5 and 8, the desalination effect of reverse osmosis can reach its optimum.

Twenty-six, How can the energy consumption of the membrane system be reduced?

Low-energy membrane elements can be used, but it should be noted that their desalination rate is slightly lower than that of standard membrane elements.