Persistence of Spore Forming Bacteria on Drinking Water Biofilm and Evaluation of Decontamination Methods
This research used a surrogate organism, Bacillus globigii, to better understand the response of a corroded concrete distribution system containing an established biofilm to different chlorine concentrations following a bioterrorism event. Biofilm was cultivated on concrete coated slides in a biofilm annular reactor using microorganisms found in Cincinnati, OH tap water. After one month of accumulation, biofilm HPC values of approximately 6.0 x 10^3 CFU/cm^2 were measured on the slides. Three phases of experimentation were conducted. The first phase of experimentation examined the persistence of adhered and planktonic spores in the absence of a chlorine residual, while the second and third phases were identical and examined spore persistence with different chlorine concentrations. The experimental target concentrations were 1 and 5 mg/l in reactors 1 and 2, respectively. During the first phase, 0.1 ml of 10^9 cells/ml spore suspension was injected into both reactors. Spore counts of 250-300 CFU/cm^2 were seen initially on the slides, and by 225 hours a steady state existence of approximately 125 CFU/cm^2 developed in both reactors. Initial spore counts in the bulk water dropped more than 2 logs within 12 hours, but average counts of 185 and 95 CFU/ml were measured in reactors 1 and 2, respectively, up through hour 126. At 389 and 442 hours, a Clorox/DI water mixture was injected into the reactors. After 90 hours, adhered spore counts in reactor 1 had dropped to undetectable levels, while the same result occurred in reactor 2 after 50 hours. The results of this phase suggest that adhered and planktonic spores are inactivated by free chlorine, but that they persist in a non-chlorinated network. During the second and third phases of experimentation, 1 ml of the spore suspension was injected into both reactors. Addition of the chlorine also began at the time of injection. Initial spore counts in the bulk water ranged from 3 x 10^4 to 4 x 10^4 CFU/ml. All bulk measurements dropped to undetectable levels within 24 hours. A comparison of initial adhered spore counts in both reactors for each phase showed that the counts in reactor 2 were significantly lower than in reactor 1. In all reactors, the adhered spore counts rapidly decreased with time, and within 48 hours all adhered spores had disappeared. The results of phases 2 and 3 suggest that the presence of a chlorine residual at the time of injection causes the inactivation of a significant number of spores, that an increased chlorine concentration acts more quickly to inactivate the spores and can help limit adherence to biofouled pipe walls, and that both 1 and 5 mg/l chlorine concentrations were equally as effective at inactivating spores over time.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:biofilm bioterrorism concrete distribution pipes chlorine disinfection bacillus anthracis globigii annular reactor
Date of Publication:01/01/2008