Slide 1
Cl2 vs. ClO2
Slide 2
ClO2 vs. Cl2 for Biofilm Control:
What the literature says about Cl2 for control of biofilm.
Cl2 for Biofilm Control
Slide 3
Hypochlorite ion is so reactive with the biofilm constituents that it is effectively
removed by reaction before it can penetrate the biofilm
Boivin, J. W., Shapka, R., Khoury, A. E., Blenkinsopp, S., and Costerton, J. W., "An Old and a New Method of Control for Biofilm Bacteria," Paper No 171, Corrosion 92.
Cl2 for Biofilm Control
Slide 4
Chlorine does not penetrate well into a biofilm
Chen, X., and Stewart, P., “Chlorine Penetration into Artificial Biofilm is Limited by a Reaction-Diffusion Interaction,” Environmental Science & Technology 30(6), 2078(1996).
Cl2 for Biofilm Control
Slide 5
Cells in biofilms are protected from biocide action and are killed only at
biocide concentrations orders of magnitude higher than that necessary
to kill suspended cells
de Beer, D., Srinivasan, R., and Stewart, P., “Direct Measurement of Chlorine Penetration into Biofilms during Disinfection,” Applied Environmental Microbiology, 60(12), 4339(1994).
Cl2 for Biofilm Control
Slide 6
1 - 2 ppm free chlorine does not prevent the growth of biofilms
Nagy, L. A. et. al., “Biofilm Composition, Formation and Control in the Los Angeles Aqueduct System,” Proc. 1982 AWWA WQTC, Nashville, Tenn.
Cl2 for Biofilm Control
Slide 7
Coliforms were recovered from a potable water supply one week after scrubbing a redwood tank with a 200
ppm chlorine solution
Seidler, R., Morrow, J., and Bagley, S., “Klebsielleae in Drinking Water Emanating From Redwood Tanks,” Applied Environmental Microbiology, 33, 893(1977).
Cl2 for Biofilm Control
Slide 8
It is not possible to clean up a biofouled system simply by resuming a discontinued
microbiological control program. A fully developed biofilm can be removed only through
intensified on-line or off-line treatment
Kozelski, K. J., “Field Experience with a Simple Cooling Water Biofilm Monitoring Device,” TP IWC-83-46, International Water Conference, Pittsburgh, PA, October 24-26, 1983.
Cl2 for Biofilm Control
Slide 9
Conclusions:—Chlorine does not penetrate biofilm well - it can’t get
through to inactivate biofilm bacteria.—Chloramine inactivates biofilm better than chlorine
because it can penetrate the biofilm, however, It does not remove the biofilm.
Therefore, a biocide which both removes biofilm and inactivates bacteria, in the presence of organics and ammonia, is needed.
Cl2 for Biofilm Control
Slide 10
ClO2 and Biofilm
Chlorine works better if applied at a low continuous level, while ClO2 works
better if applied intermittently at higher dosages
Mayack, L. A., Soracco, R. J., Wilde, E. W, and Pope, D. H., “Comparative Effectiveness of Chlorine and Chlorine Dioxide Regimes for Biofouling Control,” Water Res., 18(5),593(1984).
Slide 11
Although chlorite is a poor disinfectant, its presence retards the rate of bacterial aftergrowth in the distribution system
Masschelein, W. J., "Chlorine Dioxide," Chemical Oxidation, Technology for the Nineties, First International Symposium, Vanderbilt University, Nashville, Tenn., February 20-22, 1991.
ClO2 and Biofilm
Slide 12
Under the right circumstances, chlorite ion can be converted to ClO2 and the biofilm will be removed as a result.
Clark, J. B., and Langley, D. E., "Biofilm Control," United States Patent 4,929,365. May 29, 1990.
ClO2 and Biofilm
Slide 13
Bernarde, M. A., Israel, B. M., Olivieri, V. P., and Grandstrom, M. L., "Efficiency of Chlorine Dioxide as a Bactericide," Appl. Microbiol., 13, 776(Sept. 1965).
TIME (sec) TO ACHIEVE 99% KILL
DOSAGE - ppm
60 120 180
0.8
0.6
0.4
0.2 E. Coli (15,000 cells/ml)
Cl2 ClO2
Cl2
ClO2
pH 6.5
pH 8.5
How chemistry impacts
disinfection
ClO2 vs. Cl2 vs. pH
Slide 14
Cl2 + organics
TOX (chlorinatedorganics)
THM (trihalomethanes, CHCl3, CHCl2BrCHClBr2, CHBr3)
ClO2 + organicsMinimal TOX formation
Minimal THM formation
Oxidation products
Reaction Characteristics of ClO2
Slide 15
How chemistry impacts dosage
requirements
Amount consumed by organics and
ammonia Cl2 ClO2
Amount ‘inactivated’ by pH effects
Amount available for disinfection
Reaction Characteristics of ClO2
Slide 16
Removes biofilm much better than Cl2
Disinfection unaffected by pH
More selective - I.e., doesn’t react with many organics, therefore, - lower dosage to get good results
More environmentally friendly (fewer THMs, TOX, etc.)
Less corrosive than bleach
Does not form elemental sulfur if fed at the proper dosage, unlike bleach
ClO2 vs. Cl2
Slide 17
Must be made on site
Generally requires 2 or 3 precursors
Requires a special reactor called a ‘generator’
Economics generally do not favor ClO2, but it may in highly contaminated water.
Disadvantages of ClO2