Full Description

Water distribution system biofilms can be important sources of microbialcontamination into the water supply. The growth and composition of these biofilmsdepend on a variety of factors including pipe material and the water matrix. In an effortto advance microbial water quality, and/or reduce the formation of disinfectionbyproducts, many water utilities are changing their treatment practices. These changescan include adding ultraviolet (UV) treatment, and/or a change in the disinfectant residual. Both ofthese could alter biofilm composition, and are being addressed in this investigation(AwwaRF project #3087).By using a flow-through laboratory model and molecular techniques, the studycharacterized and compared the composition of biofilms formed under various chemicaltreatments (chlorine, monochloramine, and chlorine dioxide) at typical concentrations,with and without upstream UV treatment, using the same source. Tap water from a softsurface water source (Halifax, Canada), was first passed through granular activatedcarbon filters to remove any residual disinfectant before entering the annular reactors.Annular reactor coupons made of cast iron or polycarbonate, were used to represent andcompare pipe materials. Controls without any additional disinfectant residual were alsoincluded.Biofilm samples were removed from the coupons at defined intervals, and totalDNA was extracted. Samples were also cultured on R2A medium for heterotrophic platecount, and full sweeps were harvested to extract DNA from cultivable bacteria forcomparison to direct DNA isolation. The DNA samples from both sources were then usedto identify the composition of the biofilm bacterial community using the PCR-DGGE(denaturing gradient gel electrophoresis) technique. Universal primers for a variableregion of the 16S rRNA gene were used to amplify bacterial DNA. These fragments werethen separated on a DGGE gel which separates same size fragments on the basis of theirsequence. Identification is achieved by carefully excising bands from the gel, cloning,sequencing and comparing sequences with libraries from online databases (BLAST andRibosomal Database Project II).Differences and similarities were clearly seen and identified in the band patternsobtained on the DGGE gels for each of the variables examined. Following sequencingand identification, some microorganisms were found to be common between treatmentsbut in other cases microorganisms appearing in one treatment were apparently absentfrom the other. At this stage, the significance of such differences remains to bedetermined. One of the most obvious differences was the different banding patternobserved when the disinfectant treatment was varied. A large number of differences inbanding pattern have also been observed between direct DNA isolations and cultivablebacteria on R2A plates. Includes 14 references, tables, figures.

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Edition: Vol. - No. Published: 11/01/2006 Number of Pages: 8File Size: 1 file , 2.4 MB