We hear from our clients that there are handfuls of instances when chlorine does not control microbiological growth effectively. In fact, a very large percentage of the early reports of the successful use of chlorine dioxide in cooling systems have been in systems where chlorine just didn’t perform.
Why does chlorine fail in some systems, and why does chlorine dioxide work when chlorine doesn’t?
There appear to be three primary reasons: the ability of the oxidant to penetrate biofilm, the rate of biofilm growth, and the secondary biological inhibiting effect of the chlorite ion.
Reason 1 – The Ability of an Oxidant to Penetrate Biofilm
Chlorine penetrates a bacterial biofilm very slowly, if at all. Chlorine reacts with biofilm fragments at the surface of the biofilm and is inactivated well before it can penetrate and disinfect the bacteria inside.[i]
To investigate the reasons for the failure of chlorine, it is instructive to look at chlorine. Free Available Chlorine (FAC) is the combination of hypochlorous acid (HOCl) and the hypochlorite ion (OCl–). In terms of disinfection, it is the hypochlorous acid, which is the primary disinfectant species, as the hypochlorite ion has been stated to have 1/20—1/300th the disinfecting power of hypochlorous acid,[ii] depending upon which species of bacteria is tested. (Different bacteria are known to have different levels of resistance to inactivation by oxidants). Hypochlorous acid is a very reactive, polar, uncharged molecule, while the hypochlorite ion is charged. This combination of characteristics prevents chlorine from penetrating biofilm, which can be thought of as a layer of organic covering a metal surface.
Were one to take a beaker of several hundred mg/L of chlorine, and pour a layer of hexane, a non-miscible solvent into the beaker, no chlorine would partition itself into the hexane, no matter how long the beaker is allowed to sit or how much the solutions are mixed.
Chlorine dioxide is less polar, less reactive (E° = 0.95V vs. 1.47V for chlorine), and uncharged. So if one were to take an equivalent concentration of aqueous chlorine dioxide, and repeat the experiment with hexane, after a short period of time, the chlorine dioxide will partition into the organic phase, which will become brightly yellow. Chlorine dioxide can even be prepared in non-aqueous solvents.[iii] This is one reason why chlorine dioxide is thought to be so effective at the control, removal, and prevention of bacterial biofilm.
As supporting evidence, there are several case histories in the literature where chlorine dioxide was able to bring a badly biofouled cooling system that was being ineffectively treated with chlorine under control.[iv]
Reason 2 – The Rate of Biofilm Growth vs. The Rate of Biofilm Removal
Several well-known factors are involved in biofilm development. Of these, the amount and nature of the nutrients available to the bacteria are two of the most important. Systems where leaks are likely to be contaminants which bacteria can readily metabolize as a food source are the most commonly affected. This would include food plant cooling systems or alcohol-producing facilities, among others. In cooling towers contaminated by a high concentration of easily metabolizable nutrients, the rate of biofilm formation may be much greater than the rate at which chlorine can remove the biofilm. In such systems, where there is a high loading of nutrient, oxidants such as chlorine and bromine are ineffective.[v]
So, in any water system, the effectiveness of the disinfectant is the difference between the rate at which the disinfectant removes the biofilm and the rate at which biofilm is produced by bacteria. In highly contaminated systems treated with chlorine or bromine, bacteria win the race.
Chlorine dioxide is effective because, once again, it can readily penetrate the biofilm and inactivate the bacteria that are producing the biofilm, stopping the production of biofilm at the source. Chlorine or bromine must strip off the biofilm in order to reach and inactivate the biofilm producing organisms.
Reason 3 – Secondary Impact of the Chlorite Ion
During numerous field trials, bacterial recovery or rebound was slower than expected. This had been attributed to a complete removal of bacterial biofilm, making it more difficult for the bacterial population to reestablish. Subsequent inspections revealed that while much of the biofilm had been removed, some biofilm was still intact. Questions remained about the slow bacterial recovery.
Several observations have led to the conclusion that chlorite has a significant effect on bacterial recovery and biofilm growth. Masschelein noted that while chlorite is not very effective as a disinfectant, it has bacteriostatic properties and retards the rate of bacterial rebound in potable water systems.[vi] In this way, the chlorite slows the regrowth of biofilm.
One group was issued a patent for biofilm removal.[vii] Their method included adding “stabilized chlorine dioxide” to a system and then adding a bacterial nutrient. This nutrient resulted in an increase in bacterial activity that in turn resulted in a higher acid production in the biofilm. As the stabilized chlorine dioxide moved through the biofilm, chlorine dioxide was released by the action of acid on the chlorite. Biofilm removal resulted.
In another report, chlorine dioxide in potable water was consumed in the distribution system, the residual disappearing. Yet the residual reappeared at the end of the distribution system.[viii] Reasons were not known, although one recent explanation was that traces of residual chlorine reacted with the chlorite to produce a measurable residual of chlorine dioxide.[ix]
Based on the above observations, one can conclude that it may have been the result of biofilm action that regenerated the chlorine dioxide.
See chlorine and chlorine dioxide’s effect on biofilm in a 53 second video!
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[i] Chen, X., and Stewart, P., “Chlorine Penetration into Artificial Biofilm is Limited by a Reaction-Diffusion Interaction,” Environmental Science and Technology, 30(6), 2078(1996).
de Beer, D., Srinivasan, R., and Stewart, P., (1994), “Direct Measurement of Chlorine Penetration into Biofilms during Disinfection,” Applied and Environmental Microbiology, 60(12), 4339.
Xu, X., Stewart, P., and Chen, X., “Transport Limitation of Chlorine Disinfection of Pseudomonas aeruginosa Entrapped in Alginate Beads,” Biotechnology and Bioengineering, 49, 93(1996).
[ii] Robeck, G. G., “Chlorine, Is There a Better Alternative?” Science of the Total Environment, 18 235-43(1981).
White, G. C., Handbook of Chlorination, Van Nostrand Reinhold Company, New York, 1986.
[iii] 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.
[iv] Sussman, S., and Ward, W. J., “Microbiological Control with Chlorine Dioxide Helps Save Energy,” Materials Performance, 16(7), 24(1977).
Ward, W. J., Lee, J. W., and Freymark, S. G., “Advantages of Chlorine Dioxide as Biocide,” Ammonia Plant Safety, 20, 64-9(1978).
McGuire, L., and Dishinger, T., “Chlorine Dioxide Solves Biofouling Problems in a Refinery Cooling Tower Used for Phenol Destruction,” TP 84-06, CTI, Houston, Tx, February 1984.
Rauh, J. S., “Chlorine Dioxide: A Cooling Water Microbiocide,” Proceedings of the Annual Industrial Pollution Conference, WWEMA, Water Wastewater Equip. Manuf. Assoc., 4th, McLean, Va., 20(1976).
Freund, J., Booker, J., and Ward, W., “Biocontrol – Key to Heat Exchanger Efficiency in Grain Alcohol Plant,” Proceedings International Water Conference, 1984.
Pacheco, A. M., Durham, H. E., Dhillon, R., and Edward, C., “The Use of Chlorine Dioxide to Control Microbiological Growth in an Ethylene Glycol – Contaminated Cooling Tower… A Case History,” CTI Annual Conference, 1989, TP89-14.
Simpson, G., Laxton, G., Miller, R., and Clements, W., “A Focus on Chlorine Dioxide: The ‘Ideal’ Biocide,” Paper No. 472, Corrosion 93, New Orleans, La., March 8-12, 1993.
Simpson, G. D., Laxton, G. D., Miller, R. F., and Clements, W. R., “A Focus on Chlorine Dioxide: Biocide of Choice for ‘Stressed’ Cooling Water Systems,” presented at WaterTech 93, Houston, Texas, November 10-12, 1993.
Simpson, G. D., “Biofilm: Removal and Prevention with Chlorine Dioxide,” The Third International Symposium on Chlorine Dioxide: Process Water, Drinking Water and Water Waste Issues, New Orleans, La., September 14 – 15, 1995.
Laxton, G. D., and Simpson, G. D., “Chlorine Dioxide: The Versatile Oxidizer,” Water Management International (The Annual Review of the Water and Wastewater Industry) 35(1997).
Pickrell, W., Germer, M., Miller, J., and Simpson, G., “Plants Increase Production Through Use of Chlorine Dioxide,” Practical Industrial Water Treatment Technology for the New Millenium, Houston, TX, January 27-29, 1999.
Simpson, G., and Miller, J., “Control of Biofilm with ClO2,” Association of Water Technologies Annual Meeting, Honolulu, HA, October 28 – November 5, 2000.
Simpson, G., “Control of Biofilm in Cooling Towers with ClO2,” Fourth International Symposium on Chlorine Dioxide, Las Vegas, Nevada, February 15-16, 2001.
[v] Trulear, M. G., and Wiatr, C. L., (1988), “Recent Advances in Halogen-Based Biocontrol,” Technical Paper No. 19, Corrosion 88, St. Louis, MO, March 21-25, 1988.
[vi] Masschelein, W. J., “Chlorine Dioxide,” Chemical Oxidation, Technology for the Nineties, First International Symposium, Vanderbilt University, Nashville, TN, February 20-22, 1991.
[vii] Clark, J. B., and Langley, D. E., “Biofilm Control,” United States Patent 4,929,365, May 29, 1990.
[viii] Hoehn, R., Dietrich, A., Farmer, W., Orr, M., Lee, R., Aieta, E., Wood, D., and Gordon, G., “Household Odors Associated with the Use of Chlorine Dioxide,” Journal of the American Water Works Association, Research and Technology, 166, April 1990.
[ix] Hoehn, R. C., “Chlorine Dioxide Treatment of Drinking Water: Pros and Cons,” Association of Environmental Engineering Professors Lecture, The Annual AWWA Conference, Anaheim, California, June 19, 1995.