Frequently Asked Questions about Reverse Osmosis Membranes and Ion Exchange Resins (41-50)

41. Which compounds are stable to Dow FILMTEC membranes?

Membrane stability depends on many factors, such as temperature, pressure, concentration, pH, trace organic minerals (oils), bacteria, oxidants, and heavy metals. Therefore, specific stability data cannot be provided. The most effective approach is to identify these potential harmful factors before starting the system and prevent their occurrence through pretreatment.

42. What are the advantages of a short-blade, multi-element membrane structure design?

Within membrane elements with short blades and many blades, the flux across the membrane surface is more uniform, resulting in higher permeate efficiency and significantly reducing the possibility of fouling. However, due to production technology limitations, many membrane element suppliers are still using older element designs and have not been able to achieve breakthroughs in this area.

43. If two systems have the same feedwater volume but different operating temperatures, how will the permeate distribution differ between the two systems?

The higher the water temperature, the lower the operating pressure. Despite the reduced operating pressure, the first element in the reference system operating at lower temperatures will still exhibit a higher permeate flow rate. At high water temperatures, the system still maintains the same osmotic pressure at the end, but the operating pressure decreases. This results in a lower membrane surface velocity at the final membrane element compared to a low-temperature system. Consequently, the flux distribution unevenness between upstream and downstream elements increases significantly in systems operating at high water temperatures; that is, the flux is higher at the upstream elements and lower at the downstream elements. This situation is detrimental to the balanced and stable operation of the system (similar situations exist when using low-pressure and ultra-low-pressure membrane elements).

44. What are the harmful effects of silicon in boiler feedwater?

In high-pressure boiler feedwater, the issue of silicon content must be addressed. This is because silicon exhibits a significant ability to volatilize with steam. Subsequently, silicon contained in the steam will deposit in a glassy state on the turbine blades in the low-pressure section of the turbine due to the decrease in steam pressure, affecting turbine efficiency. Both active and colloidal forms of silicon can cause this failure because, under the temperature conditions of a high-pressure boiler, colloidal silicon will decompose into silicates and then evaporate into the steam phase.

45. Can membrane systems be frequently started and stopped? Membrane systems are designed for continuous operation, but in practice, there will always be a certain frequency of start-ups and shutdowns. When the membrane system is shut down, it must be flushed at low pressure with its permeate water or pretreated water to replace the high-concentration concentrate containing scale inhibitors from the membrane elements. Measures should also be taken to prevent water leakage and the introduction of air, as element dehydration can lead to irreversible permeate flux loss. If the shutdown is less than 24 hours, no measures to prevent microbial growth are necessary. However, if the shutdown time exceeds the above limits, a protective solution should be used to preserve the system or the membrane system should be flushed periodically.

46. Does ROSA5 include an automatic design function?

ROSA5 currently does not have an automatic design function; this function is planned to be added in future ROSA upgrades.

47. In a three-stage system design, can the permeate pressure be used in the first stage and a pressure booster pump be used before the third stage?

Yes, it can.

48. How to Design a Permeate Backflow Tank?

In seawater desalination systems, a permeate backflow tank is sometimes required. Its required volume is usually the volume of the pressure vessel minus the volume of the membrane element (considering the element's porosity).

49. How to Determine the Orientation of the Brine Seal Ring on the Membrane Element?

The brine seal ring on the membrane element should be installed at the inlet end of the element, with the opening facing the inlet direction. When water is fed into the pressure vessel, its opening (lip) will further open, completely sealing off the bypass flow of inlet water between the membrane element and the inner wall of the pressure vessel.

50. Can Chlorine Dioxide (ClO2) be Used as a Bactericide in Membrane Systems?

Generally, chlorine dioxide is not permitted as a bactericide for membrane elements. While its bactericidal effect is similar to chlorine, its bactericidal efficacy is equal to or slightly higher than chlorine. Unlike dissolved chlorine, ClO2 only acts as an oxidant and does not participate in the addition reaction of chlorine, thus having a lower potential for membrane damage. Some users have successfully applied this technology in Dow FILMTEC membrane systems, but extreme caution should be exercised when using it, as the presence of metal ions in the solution can catalyze the degradation reaction of the membrane by ClO2. Only the bactericide DBNPA is considered safer and more effective, and using ClO2 may result in damage to membrane performance.