Declining water supplies around the country have forced cities to look at recycled municipal wastewater as a major source of water for certain commercial – institutional and light industrial applications. The City of San Antonio, Texas implemented a program of using well-disinfected tertiary treated municipal wastewater as makeup to the cooling towers of buildings, hospitals, universities, and small industrial facilities.[i] While work remains to be done, approximately one third of the possible end users have opted to use this recycled wastewater. Others are more reluctant for a variety of reasons, including concerns over receiving water that may be of lesser quality than potable water. As corrosion and deposit formation potential is generally lower at lower cycles of concentration (COC), some equipment owners are also reluctant to risk increasing the potential for corrosion and deposit formation in their system which would be higher with higher COC. Despite these concerns, some cities are promoting the use of wastewater for industrial use.
Heavy industry has also been impacted. The California Department of Health Service, in Administrative Code Title 22, requires industrial facilities located within a specified distance of a municipal wastewater plant to use the treated wastewater as plant process water.[ii]
The reuse of municipal wastewater will only increase. An item of absolutely critical importance is that the wastewater be treated in such a way as to assure that the quality of the water is sufficient for the desired goal. Various technologies will be employed to achieve these goals. Reverse osmosis (RO) technology will be an integral part of many of these applications.
RO technology is used extensively in a number of applications. In addition to water reuse, another major use of RO is for pretreatment of water used to produce the high purity feedwater of power plant steam generation systems. Generally, RO is used to take the load off of demineralizers which polish the water for further use in these high-pressure steam generators.
The Achilles Heel of all RO units is biofouling.[iii] Bacteria that survive any disinfection process can find a home in the membrane. Bacterial survivors that reach the membrane may attach and begin to produce a polymer coating that generally protects the bacteria from external threats such as bacterial toxins or oxidants. This polymer is composed primarily of polysaccharides and effectively acts as “glue,” providing a sticky place where other bacteria and any dirt, debris, or scale that gets by the prefilter can lodge. The biofilm can also promote the formation of scale or deposits.
As biological material builds up on the RO membrane, the differential pressure increases, and RO effluent water quality generally suffers. Run lengths between required cleanings decrease, and this results in an increase in the cost of operation. Some have suggested that the costs of membrane cleaning lie somewhere between 5—20% of the operating costs.[iv]
Common cleaning chemistries include various acids, bases, surfactants, detergents, chelants, oxidants, enzymes, and various blends of these chemicals.[v] The optimum cleaning practice is likely membrane and site specific, as the type of foulant is a function of the composition of the influent water.
Although these cleaning methodologies are somewhat effective, none are completely effective at removing the biofilm and none are 100% effective at inactivating the bacteria that produce the biofilm.[vi] Concerns over the potential for degradation of membranes by oxidants have limited the use of oxidizing disinfectants for membrane cleaning. Cellulose acetate membranes appear to be the exception, as these membranes are reportedly quite resistant to oxidation in the neutral pH range.[vii]
The search for a chlorine-resistant membrane has been a major undertaking which continues today.[viii] Alternatively, an effective biocide that is less aggressive towards the membrane might be found. Such a biocide should be able to penetrate and remove the biofilm at a relatively low dose. The biocide should be able to inactivate bacteria in the bulk water, be easy to apply, and be cost-effective. Chlorine dioxide stands alone when these criteria are selected.
This post is the beginning of a series surveying chlorine dioxide and reverse osmosis membranes.
Chlorine Dioxide and Biofilm
Most common chlorine dioxide-generating chemistries produce some free chlorine. The generator can be tweaked, to provide a slight excess of chlorite, which, for a well-designed generator, should eliminate free chlorine in the effluent. However, there is still the potential for production of some free chlorine, which is known to be very aggressive to certain types of RO membranes. Only a few types of generators are available commercially that produce no free chlorine by design.
Chlorine dioxide offers some unique performance characteristics that make it suitable for further examination as a membrane cleaner or disinfectant for RO membrane biofilm control. Three of these crucial performance characteristics which merit further investigation of the use of chlorine dioxide on an RO membrane are:
Chlorine dioxide is a much weaker oxidant than other commonly used oxidants. The oxidation potential of chlorine dioxide in aqueous solution is 0.95V, compared to 1.33, 1.49, and 2.07 for bromine, chlorine, and ozone, respectively. Thus, chlorine dioxide is less reactive than any one of the three oxidants mentioned. A theory has been proposed that the weaker oxidant is better for biofilm control, as the weaker oxidant can more effectively penetrate the biofilm without reacting with biofilm constituents.[ix]
Unlike most other oxidants, chlorine dioxide can readily penetrate a biofilm. As an illustration, the mainly polysaccharide-composed biofilm can be considered as a layer of organic on the surface. This phenomenon can be easily demonstrated by taking a solution of aqueous chlorine dioxide, adding an equal amount of an immiscible organic (e.g., n-hexane) slowly so that the solution does not mix, and allowing the solution to stand unmixed for a few minutes. The chlorine dioxide will become distributed throughout both phases within a few minutes, as observed by both phases becoming yellow. This is further illustrated by the method of making chlorine dioxide in a non-aqueous solvent.[x]
When chlorine dioxide reacts, most of the time (50—80%) it accepts one electron and reverts to chlorite ion, chlorine dioxide, from which it was generated. This chlorite ion, when exposed to acids, can re-form chlorine dioxide. Given the fact that the polysaccharides found in biofilm can be acidic,[xi] the chlorite ion can still be effective at controlling and removing existing biofilm.[xii] An in-depth discussion of this phenomenon is given elsewhere [Simpson 1995].[xiii]
When selecting a biocide that is effective at controlling biofilm, chlorine dioxide is the superior selection.
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[i] Wilcut, E., Sims, J., and Tvedt, T., “San Antonio Water System’s Cooling Tower Audit Program Results in Significant Water Savings,” Cooling Technology Institute Annual Conference, Technical Paper, TP03-14, San Antonio, Texas, February 10 – 13, 2003.
[ii] Bresnahan, W., “Reuse of Treated Municipal Wastewater in Oil Refinery Cooling Towers,” Industrial Water Treatment, 22(May/June, 1996).
[iii] Flemming, H., Schaule, G., Grieve, T., Schmitt, J., and Tamachkiarowa, A., “Biofouling – the Achilles Heel of Membrane Processes,” Desalination, 113, 215(1997).
[iv] Fane, A., Proceedings, Symposium on Characterization of Polymers with Surface, Lappeenranta, Finland, 51(1997).
[v] Madaeni, S., Mohamamdi, R., and Moghadam, M., “Chemical Cleaning of Reverse Osmosis Membranes,” Desalination, 134, 77(2001).
[vi] Berg, J., Matin, A., and Roberts, P., “Growth of Disinfection Resistant Bacteria and Simulation of Natural Aquatic Environments in the Chemostat,” Water Chlorination, Environmental Impact and Health Effects, Jolley, et al., eds., 4(81), 1137(1981).
[vii] Vos, K., Nusbaum, I., Hatcher, A., and Burris, F., “Storage, Disinfection, and Life of Cellulose Acetate Reverse Osmosis Membranes,” Desalination, 5, 157(1968).
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).
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)
[viii] Glater, J., Hong, S., and Elimelech, M., “The Search for a Chlorine-Resistant Reverse Osmosis Membrane,” Desalination 95, 325(1994).
[ix] Costerton, W., private communication, September 1997.
[x] Pitochelli, A., “Generation of Chlorine Dioxide in a Non-Aqueous Medium,“ US Patent 5,405,549, April 11, 1995.
Pitochelli, A., “Generation of Chlorine Dioxide in a Non-Aqueous Medium,“ US Patent 5,693,252, December 2, 1997.
Pitochelli, A., “Generation and Storage of Chlorine Dioxide in a Non-Aqueous Medium,” US Patent 5,707,546, January 13, 1998.
[xi] Sutherland, I. W., Biotechnology of Microbial Exopolysaccharides, Cambridge University Press, .
Kanaani, Y., Adin, A., and Rav-Acha, Ch., “Biofilm Interactions in Water Reuse Systems: Adsorption of Polysaccharide to Kaolin,” Water Science and Technology, 26(3-4),673(1992).
[xii] Clark, J. and Langley, D., “Biofilm Control,” United States Patent 4,929,365. May 29, .
[xiii] Simpson, G., “Biofilm: Removal and Prevention with Chlorine Dioxide,” Third International Symposium: Chlorine Dioxide: Drinking Water, Process Water, and Wastewater Issues, New Orleans, LA., September 14 – 15, 1995.