Efficacy of Chlorine Dioxide as a Disinfectant for Bacillus Spores in Drinking-Water Biofilms

This paper presents results describing the effectiveness of chlorine dioxide penetration into a drinking-water distribution system biofilm/corrosion matrix and decontamination of adhered Bacillus globigii spores, a surrogate for Bacillus anthracis. Biofilm and corrosion products were developed using biofilm annular reactors containing oxidized scaled, iron coupons. Reactors were inoculated with B. globigii spores after biofilm development, and decontamination was undertaken with bulk-phase chlorine dioxide concentrations of 5, 10, 15, and $25\mathrm{mg}/\mathrm{L}$. Initial biofilm viable B. globigii spore densities of ${10}^6\mathrm{CFU}/{\mathrm{cm}}^2$ were reduced to 50 to $300\mathrm{CFU}/{\mathrm{cm}}^2$ at chlorine dioxide concentrations of 25 and $15\mathrm{mg}/\mathrm{L}$, respectively, within 6 days. B. globigii spore distribution throughout the biofilm/corrosion matrix depth and the change in viable spore count during chlorine dioxide disinfection were examined using a microslicing technique. Four layers of 360 μm thickness were sliced, and these showed that B. globigii spores were equally distributed throughout the biofilm/corrosion matrix depth. Furthermore, chlorine dioxide acted on all layers simultaneously, but spores still persisted in the deepest layer of the biofilm/corrosion matrix after 6 days of disinfection at 15 and $25\mathrm{mg}/\mathrm{L}$ chlorine dioxide.