The influence of disinfection processes on heterotrophic bacterial growth in distribution systems.

There is increasing interest in the use of ozone as a disinfectant for drinking water because it is effective against a wide range of pathogens and it oxidizes some precursors of disinfection by-products (DBPs). However it is well known that ozonation increases bacterial nutrients by converting some nonbiodegradable organic matter to more biodegradable compounds and ozonation itself produces DBPs. The City of Ann Arbor switched its primary disinfectant from chloramine to ozone in June, 1996. This provided a unique opportunity to examine the change of bacterial populations before and after ozonation. Studies were performed at the water treatment plant, and in the distribution system. The first part of this thesis focused on the relative resistance of indigenous heterotrophic bacteria to each step of the water treatment process. Each treatment step exerted a different selection pressure on the microbial distribution. The surviving bacteria after ozonation were mostly spore-forming gram-positives (Mycobacterium, Bacillus spp., Corynebacterium). The second part provided information on the change of the bacterial populations and the chemical parameters in the distribution system following ozonation. A new parameter, GP (growth potential), was developed to assess the potential of the water to sustain growth of heterotrophic bacteria. The literature suggests that ozonation will result in a higher organic carbon concentration and will therefore stimulate bacterial growth. Our findings were that the organic carbon concentration indeed increased after ozonation, but the bacterial growth was suppressed efficiently even with a lower chloramine residual. The GP also decreased after ozonation. Overall, the predominant species shifted from Acinetobacter, Pseudomonas, Xanthomonas, Moraxella (no-ozone period) to Moraxella, Acinetobacter, Aeromonas, Pseudomonas (ozone period). There was also a difference in the predominant bacteria after ozonation depending upon the distance from the water plant. Since biological processes can be easily upset, near real-time estimation of bacteria in both the treatment plant and the distribution system is helpful for the daily operation of the plant and the distribution system. A novel application of the bioluminescence method using microfiltration proved to be very sensitive, fast, and practical in estimating bacterial counts. Early intervention is now possible leading to improved drinking water quality.Ph.D.Applied SciencesBiological SciencesEngineering, Sanitary and MunicipalHealth and Environmental SciencesMicrobiologyPublic healthUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130758/2/9811122.pd