Chlorine is a relatively ineffective method of controlling Legionella-containing biofilm in cooling water towers. It is still used as a short-term solution during outbreaks, but does not have lasting effects because it only works on the surface removing layers of the biofilm at a time and does not penetrate the slime layer. When bacteria production rates are high, however, chlorine is made ineffective because growth is occurring more quickly than the rate of biofilm removal by chlorine treatment. Chlorine treatment works best when cooling towers are relatively clean and there is not a high bacterial growth rate.
Chlorine dioxide, on the other hand, when applied correctly, has been known to be very effective in removing biofilm in difficult-to-treat cooling towers. Chlorine dioxide is “a gas produced by activating sodium chlorite with an oxidizing agent or an acid source. Sodium chlorite is converted to chlorine dioxide through a chlorine dioxide generator and applied as a dilute solution.” It is able to penetrate biofilm and inactivate bacteria found in these slime layers. Another benefit includes efficacy over a broader water pH range compared to chlorine; it also does not react with organic compounds or ammonia-nitrogen.
You may also read more about Advantages and Disadvantages of Various Legionella Disinfection Methods at the Journal of Legionella and the Law.