If you haven’t read the first part of our series on Chlorine Dioxide and Reverse Osmosis (RO) membranes, read about biofilm and chlorine dioxide first! This post reviews literature prior to 1990 and up through 2010, as there have been a few reports concerning the effect Chlorine dioxide has on RO membranes in the literature. We at PureLine believe it is important to understand the scientific literature and rationale behind water treatment solutions, and the literature surveyed below provides helpful background when it comes to chlorine dioxide and RO.
Literature Review of Chlorine Dioxide and Its Impact on RO Membranes Prior to 1990
One group reported on the effect of chlorine on cellulose acetate (henceforth CA) membranes.[i],2 Above 50 mg/L chlorine was shown to attack CA membranes after one week of continuous exposure. At 10 mg/L for 15 days, no detectable change in performance was noted. Chlorine dioxide was shown to be unreactive towards CA membranes.[ii]
Most of the early work reported in the literature was done at the California Water Resources Center.[iii] While this work was important, several important factors should be considered.
- First, in the early 1980s, generating chlorine-free chlorine dioxide was difficult, as the method for speciating the oxychlorine species had not yet been fully developed.[iv]
- The second factor was that the method used to produce chlorine dioxide was to acidify sodium chlorite (HCl + NaClO2), which, at that time, was thought to produce chlorine-free chlorine dioxide. The authors noted that the chlorine dioxide was prepared and used with the assumption that no free chlorine was present, but the solution was not tested.[v] It has subsequently been suggested that some free chlorine may be produced as an intermediate by this method.[vi]
- The third factor is that acidifying sodium chlorite produces chlorous acid (HClO2), which disproportionates to form chlorine dioxide. The disproportionation reaction can take a relatively long time (i.e., days)6 which means that it may not have been chlorine dioxide alone (E° = 0.95 V), but a mixture of chlorine dioxide and chlorous acid (HClO2; E° = 1.56 V) that was investigated.[vii] Given the much stronger oxidizing power of chlorous acid over chlorine dioxide, it is not unreasonable to assume that chlorous acid would be much more aggressive towards an RO membrane than would chlorine dioxide.
- The last factor was the relative concentrations studied. In several reports the following oxidants and dosages were compared:5,[viii]
|Table 1. Oxidant and Dosage Studied|
Apparently, the dosages were selected based on relative molecular weight to the dosage of chlorine. That is, 3.0 mg/L chlorine was considered to be equivalent to 3.0 mg/L chlorine dioxide, whereas in the real world, the highest concentration of chlorine dioxide that would be equivalent to 3.0 mg/L chlorine would be about 20—25 % of that amount, or 0.6—0.7 mg/L. (In systems with a high chlorine demand, this spread would widen.) Similarly, bromine equivalency would also be less than that shown.
In these reports, several membranes were tested at concentrations of oxidant and at arbitrary pHs of 8.6, 5.6, and 3.0. Membranes tested included those shown below:
|Table 2. Key to Membrane Manufacturer and Polymer Type|
|A1||UOP||Aromatic Polyamide (composite)|
|C2||Environgenics||CA-CTA 50/50 (homogeneous)|
|C5||Environgenics||CA-CTA (composition unknown)|
|V1||Hydranautics||Homogeneous CA (coated with vinyl acetate)|
|X1||FilmTec||Composition unknown (composite)|
The CA membranes were resistant to all of the oxidants tested.
The U-1 membrane appeared to be sensitive to all oxidants tested, being very sensitive to bromine and to chlorine dioxide.
The other non-CA membranes tested, A-2 and X-2, showed good oxidant stability with chlorine dioxide, except at pH 8.6. The reasons for this are not clear, except that the disinfection ability of chlorine dioxide appears to improve as the pH increases. One possible explanation may be that as the pH increases, chlorine dioxide tends to adsorb more strongly to surfaces than at lower pHs.[ix]
In summary, the lack of analytical methods capable of differentiating chlorine dioxide from disinfection byproducts, including chlorine, in generated chlorine dioxide solutions, has raised serious questions regarding the aggressiveness of generated chlorine dioxide toward RO membranes. This was the most significant issue regarding use of chlorine dioxide in disinfecting RO membranes during this period.
Literature Review of Chlorine Dioxide and Its Impact on RO Membranes 1990 – 2000
A later researcher made similar observations and performed experiments on various RO membranes under more real-world conditions.[x] Membranes he tested included DuPont B-15, FilmTec FT-30, Desalination System 2B, and UOP MP and L membranes.
In general, Adams stated that the common practice of using the mg/L oxidant x hrs of operation (analogous to the Ct factor used in potable water to measure disinfection efficiency) to look at membrane life when exposed to oxidants is misleading. He also stated that high concentrations of chlorine dioxide are aggressive to membranes. However, he also stated that at concentrations < 1.0 mg/L, membranes were found to have a greater tolerance for chlorine dioxide than for chlorine. This is important because chlorine dioxide is known to retain its excellent biocidal characteristics at that concentration. In addition, because chlorine dioxide is a gas, it can permeate through the membrane and can provide biocidal protection downstream of the RO unit.
Other researchers investigated the use of acidified chlorite to clean polysulfone membranes.[xi] They found that chlorine dioxide can be used to disinfect RO membranes, although chlorine dioxide was not found to be effective at removing inorganic deposits.
These researchers later reported on the use of chlorine dioxide as a sanitizer in ultrafiltration systems, commonly found in dairies.[xii] In particular they were interested in determining the corrosivity various formulations containing chlorine dioxide would be if used intermittently. They found acidified chlorite (acidified with HCl) was very aggressive to stainless (low pH and high chlorides), but that chlorine dioxide made from acid/bleach/chlorite was not corrosive. Their work concluded that chlorine dioxide can be used safely for sanitation of UF systems at a concentration of 15 mg/L and near neutral pH.
In a later report, one author said,
“One of the most promising alternatives to chlorine is chlorine dioxide. Studies show the great potential of chlorine dioxide due to its biocidal effectiveness, lack of harmful by-products, and its relatively mild effect on polymeric membrane structures. Removal of chlorine dioxide may not even be needed ahead of the membranes, providing a continuous disinfection throughout the entire membrane system at low dosages (1—2 mg/L).”[xiii]
In summary, as use of RO increased and biofouling of those membranes became more of an issue, the use of chlorine dioxide was explored, more and more with a significant improvement in biofouling control, during this time period.
Other Work – 2000 – 2010
In a 2004 study, results of a six-month long test with up to 100 mg/L chlorine dioxide on thin film polyamide membrane samples were reported.[xiv] The chlorine dioxide was prepared by acidification of chlorite, but the acid used was sulfuric and not hydrochloric, thus ensuring that the potential for formation of chlorine was reduced. A very low change in the salt passage resulted.
In a 2005 study to investigate the tendency of chlorine dioxide to degrade membranes during long-term exposure, laboratory experiments using 0.05—0.9 mg/L chlorine-free chlorine dioxide were conducted over about 1000 hours.[xv] Dow FilmTec FT-30/BW-30 membranes were exposed to continuous chlorine dioxide concentrations between 0.05 and 0.1 mg/L for up to 600 hours with a slight decline in permeate flowrate (although the permeate flowrate was still equal to or greater than untreated reference membranes) but with no increase in salt passage. The chlorine dioxide concentration was increased to 0.5 mg/L for about 150 hours and then to 0.9 mg/L for the last 250 hours. During the entire period, there was no increase in salt passage, and the flux, which appeared to decline during the first 600 hours, began to increase over the last 400 hours.
The work suggests that chlorine dioxide, applied continuously at levels at or below ~ 1 mg/L, maintains flux with stable salt passage, and that with such use, not only are the TFC RO membranes undamaged, but the performance of these membranes may actually be enhanced with use of chlorine dioxide [Pitochelli et al. 2005].[xvi]
[i] Vos, K., Burris, F., and Riley, R., “Kinetic Study of the Hydrolysis of Cellulose Acetate in the pH Range of 2-10,” Journal of Applied Polymer Science, 10, (May 1966).
[ii] Vos, K., Nusbaum, I., Hatcher, A., and Burris, F., “Storage, Disinfection, and Life of Cellulose Acetate Reverse Osmosis Membranes,” Desalination, 5, 157(1968).
[iii] Glater, J., McCutchan, J., McCray, S., and Zachariah, M., “The Effect of Halogens on the Performance and Durability of Reverse Osmosis Membranes,” ACS Symposium Series, 153(1}, 173 (1981).
Glater, J., Zachariah, M., McCray, S., McCutchan, J., “Reverse Osmosis Membrane Sensitivity to Ozone and Halogen Disinfectants,” Desalination, 48, 1(1983).
Glater, J., McCutchan, J., McCray, S., and Zachariah, M. “The Effect of Water Pretreatment Chemicals on the Performance and Durability of Reverse Osmosis Membranes.” UCLA Report No. 831 (1983)
Zachariah, M. and Glater, J., “A Mechanistic Study of Halogen Interactions with Polyamide Reverse Osmosis Membranes,” Reverse Osmosis and Ultrafiltration, S. Sourirajan, ed., 345(1985).
[iv] Aieta, E.M., Roberts, P.V., and Hernandez, M., “Determination of Chlorine Dioxide, Chlorine, Chlorite, and Chlorate in Water,” Journal of the American Water Works Association, 76, 64(January 1984).
[v] McCutchan, J., and Glater, J., “The Effect of Feedwater Pretreatment Chemicals on the Performance and Durability of Reverse Osmosis Membranes,” UCLA-ENG-8004, Water Resource Center 71, January 1980.
[vi] Pitochelli, A., “Chlorine Dioxide Generation Chemistry,” 3rd International Symposium: Chlorine Dioxide: Drinking Water, Process Water, and Wastewater Issues, New Orleans, LA., September 14 – 15, 1995.
[vii] Simpson, G., “Analytical,” Chapter 8, Practical Chlorine Dioxide: Volume I – Foundations,” Greg D. Simpson & Associates, Publishers, 2005.
[viii] Glater, J., McCutchan, J., McCray, S., and Zachariah, M., “Halogen Interactions with Typical Reverse Osmosis Membranes, “ AWWA Water Reuse Symposium II, Washington, DC, 2, 1339(August 1981).
McCray, S., Glater, J., and McCutchan, J., “The Effect of pH and Halogens on the Stability of Reverse Osmosis Membranes,” UCLA-ENG-8115, WRC 73, July, 1981.
McCutchan, J., Glater, J., McCray, S., Nobe, K., and Zachariah, M., “Saline Water Demineralization by Means of a Semipermeable Membrane,” Water Resources Center, Desalination Report No 75, July 1982.
McCutchan, J., Glater, J., McCray, S., Zacharian, M., and Nobe, K., “The Effect of Water Pretreatment Chemicals on the Performance and Durability of Reverse Osmosis Membranes,” Office of Water Research and Technology Report, April, 1983.
[ix] Simpson, G., unpublished results, 1997.
[x] Adams, W., “The Effects of Chlorine Dioxide on Reverse-Osmosis Membranes,” Desalination, 78(3), 439 (1990).
[xi] Bohner, H. and Bradley, R., “Effective Control of Microbial Populations in Polysulfone Ultrafiltration Membrane Systems,” Journal of Dairy Science, 73, 2309(1990).
[xii] Bohner, H., and Bradley, R., “Corrosivity of Chlorine Dioxide Used as a Sanitizer in Ultrafiltration Systems,” Journal of Dairy Science, 74, 3348(1991).
[xiii] Saad, M., “Biofouling Prevention in RO Polymeric Membrane Systems,” Desalination, 2(88), 85(1992).
[xiv] Wise, B., Marker, L., and Muellere, P., “Effectiveness of Chlorine Dioxide in Sanitizing Thin-Film Membrane Systems,” Ultrapure Water, 13 (September 2004).
[xv] Pitochelli, A., Private Communication, October 2005.
[xvi] Pitochelli, A., Mainz, E., and Griffith, D., “Continuous Chlorine Dioxide Use to Prevent Biofilm Formation on RO Membranes,” Ultrapure Water, 40(May/June 2005).