There have been serious questions and apparent discrepancies in both laboratory and field work regarding whether chlorine dioxide is harmful to thin film composite RO membranes.
The presence of metals, specifically iron, as part of the fouling deposit found on RO membranes during autopsy is well known. In some cases, the amount of iron in the water entering the RO unit was very low, and yet iron was found to be concentrated in the fouling deposit.
Flux increased from 22 gpm to 35 gpm in about 120 hours. Their conclusion was that impure chlorine dioxide (chlorine dioxide with some free chlorine present) was destroying the membrane. This led to Phase 2, where chlorine dioxide free from chlorine was used.
Further investigation led to the HCl: chlorite process, which is not thought to produce free chlorine, except as a short-lived intermediate. Still, under these conditions, there was a rapid decrease in salt rejection. Further investigation revealed the presence of iron. This resulted in Phase 3, which employed continued use of HCl: chlorite generation method and improved iron removal in the pretreatment process.
In Phase 3, the flux was stable and salt rejection was nearly identical to results obtained without use of chlorine dioxide. Their conclusions were as follows:
- Chlorine gas generated chlorine dioxide rapidly degraded RO membranes.
- HCl: chlorite generation did not degrade membranes.
- Membrane degradation appeared to be due to iron.
- Chlorine dioxide did not degrade membrane performance in the absence of metals, specifically reduced iron.
These results suggest that the presence of metal ions in feedwater may catalyze the reaction of chlorine dioxide or chlorite ion with RO membranes and may explain the controversy surrounding the degradation of RO membranes when chlorine dioxide is used.
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