Frequently Asked Questions about Reverse Osmosis Membranes and Ion Exchange Resins (11-20)

11. Why is the pH of RO permeate lower than that of the feed water?

Understanding the balance between CO2, HCO3−, and CO3＝ provides the best answer to this question. In a closed system, the relative amounts of CO2, HCO3−, and CO3＝ change with pH. At low pH, CO2 is dominant; at medium pH, HCO3− is dominant; and at high pH, CO3＝ is dominant.

Because RO membranes remove dissolved ions but not dissolved gases, the CO2 content in RO permeate is generally the same as that in the RO feed water. However, HCO3− and CO3＝ are often reduced by one to two orders of magnitude. This disrupts the balance between CO2, HCO3−, and CO3＝ in the feed water. In a series of reactions, CO2 combines with H2O, resulting in a shift in equilibrium until a new equilibrium is established.

CO2 + H2O  HCO3－ + H+

If the feed water contains CO2, the pH value of the RO permeate will always decrease. For most RO systems, the pH value of the reverse osmosis permeate will decrease by 1-2 pH values. When the feed water alkalinity and HCO3－ are high, the pH value of the permeate will decrease even more.

A very small number of feed waters contain less CO2, HCO3－, or CO3＝, resulting in less noticeable changes in the permeate pH value. Some countries and regions have regulations on the pH value of drinking water, generally between 6.5 and 9.0. Our understanding is that this is to prevent corrosion of water supply pipes. Drinking water with a low pH value itself will not cause any health problems. As is well known, many commercially available carbonated beverages have a pH value between 2 and 4.

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

To achieve the best cleaning results, choosing the right cleaning agents and procedures is crucial. Incorrect cleaning can actually worsen system performance. Generally, acidic cleaning solutions are recommended for inorganic scale contaminants, while alkaline cleaning solutions are recommended for microbial or organic contaminants.

13. Which cleaning agent supplier should I recommend?

We do not recommend specific cleaning agent suppliers. However, we provide commonly used cleaning solutions and information on how to determine the compatibility of commercial cleaning formulations with membrane systems on our website or in this manual.

14. How do I identify which membrane elements need replacement?

If the permeate from a particular series or unit of pressure vessel in a reverse osmosis or nanofiltration system shows an abnormally high salinity (conductivity), this clearly indicates a leak in the O-rings or a malfunction in that component. The key to fault identification lies in the design of the membrane system, which should allow for easy diagnosis and identification of any membrane elements or system components exhibiting abnormal performance. Each pressure vessel should be equipped with a sampling port, and the permeate should be segmented to facilitate tracing the problematic pressure vessel from the total permeate flow. Furthermore, each pressure vessel should allow for the insertion of a sampling tube into the permeate pipe to probe the permeate conductivity and pinpoint the exact location of the fault.

O-ring leaks are the most common cause of water quality degradation. However, if a faulty component has been identified, we recommend disassembling it to pinpoint the problem. Dow Chemical also offers various membrane fault analysis services for a fee.

15. Why does the ROSA software report an error message: "Unable to complete calculation after 200 iterations"?

Some countries use commas as decimal points, while ROSA software defaults to using dots as decimal points. Please change your computer settings to resolve this issue. Go to Start -> Settings -> Control Panel -> Regional Settings -> Number, and set the dot symbol to the decimal point.

16. When will ROSA software be updated?

To stay updated on ROSA software updates, please subscribe to our newsletter or visit the ROSA webpage at http://www.filmtec.com/.

17. Does Dow offer membrane analysis services?

Dow offers this service called DIRECTOR Service. You can find information about this service on the Dow Liquid Separation website.

18. How can membranes be applied to new fields?

For new applications, membrane technology has the potential to significantly save energy and even help develop new products or create new industries. Sometimes membrane-based processes are simple, while others are complex. In such cases, small-scale feasibility studies should be conducted to obtain the parameters required for scale-up design. For end-users of membrane systems, we recommend collaborating with membrane system suppliers and research institutions on small-scale but challenging application research.

19. Should a degassing device be installed between the first and second RO stages?

Degassing is usually not necessary. However, for specific requirements and feed water composition, a degassing device may be selected. The main reason for installing a degassing device is the need to reduce CO2 or H2S content. When the water contains H2S or there is potential for dust pollution in the surrounding environment, vacuum degassing should be used.

20. How to remove iron contamination?

Sodium bisulfite is the most effective method for removing iron contamination.