Influence of bacterial biofilms on Bacillus globigii spore viability in model chlorinated water distribution systems

Viability of Bacillus globigii spores in chlorinated polyvinyl chloride (PVC) pipe loop systems were examined under oligotrophic conditions. Three 2.5 cm × 10 m pipe loops having poised free chlorine concentrations of 0.0, 0.5, and 1.0 mg/L were seeded with 3.0 × 10⁸B. globigii spores each and viability was assessed over a 21 day period in both the recirculating waters and within the biofilms associated with pipe wall surfaces. After 10 min of exposure, viable spores were found to be associated within the pipe biofilms. In the untreated pipe loop spore counts remained statistically consistent in both the bulk water and biofilm until 1.0 mg/L free chlorine was introduced, then spores were completely inactivated in less than seven days. Spores within the pipe loop poised at 0.5 mg/L free chlorine showed a 7.6-log₁₀ inactivation in the bulk water phase, but only a 2.7-log₁₀ inactivation was observed within the biofilm after 14 days of treatment. Complete inactivation was observed in the 1.0 mg/L free chlorine system in both the biofilm and the bulk water phase in less than 10 min. These data demonstrated that B. globigii spores were readily incorporated into PVC pipe biofilms, which decreased spore inactivation nearly five orders of magnitude under moderate free chlorine concentrations.