This document discusses the effectiveness of chlorine (Cl2) and chlorine dioxide (ClO2) for controlling biofilms. It summarizes literature findings that Cl2 does not penetrate or remove biofilms well, while ClO2 is more effective at penetrating and removing biofilms. ClO2 is also less affected by pH and forms fewer disinfection byproducts than Cl2. However, ClO2 has some disadvantages in that it must be made on-site using a generator and multiple precursors, which can increase costs compared to Cl2 in some situations.

1. Slide 1
Cl2 vs. ClO2

2. Slide 2
ClO2 vs. Cl2 for Biofilm Control:
What the literature says about
Cl2 for control of biofilm.
Cl2 for Biofilm Control

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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).

11. 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

12. 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

13. 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

14. Slide 14
Cl2 + organics
TOX (chlorinated
organics)
THM (trihalomethanes,
CHCl3, CHCl2Br
CHClBr2, CHBr3)
ClO2 + organics
Minimal TOX formation
Minimal THM formation
Oxidation products
Reaction Characteristics of ClO2

15. 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

16. 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

17. 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