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