| Popis: |
Biological contamination with pathogens, whether intentional or unintentional, is a potential problem in drinking water distribution systems. This study assesses the possibility of spore-forming pathogens retention on internal drinking water distribution system pipe surfaces. In the first study, polyvinyl chloride (PVC), cement-lined ductile iron, and ductile iron pipe coupons (3 cm x 14 cm) cut from new water main piping were conditioned for three months in dechlorinated Cincinnati, Ohio tap water. Bare and conditioned coupons were spiked with Bacillus atrophaeus subsp. globigii, a surrogate for Bacillus anthracis. Spore retention differed between pipe materials and the presence of established biofilm from conditioning also had an impact. Conditioned PVC coupons had significantly lower spore retention (31±11%) than conditioned cement-lined coupons (61±14%) and conditioned iron coupons (71±8%). In addition to measuring spore retention, two sampling techniques, brushing and scraping, were tested for their ability to recover the inoculated spores from the coupons. Mean recoveries for all materials ranged from 37±30% to 43±20% for brushing vs. 24±10% to 51±29% for scraping. On cement-lined pipe, brushing yielded a significantly higher recovery than scraping. No differences were seen between brushing and scraping the PVC and iron pipe coupons. The second phase of experiments involved comparisons of the three pipe materials with different surface preparations – bare (new), conditioned (exposed to flowing city tap water for 13 months), and coated with artificial “biofilm” (agarose). It should be noted that in this case, the conditioning tap water was used as delivered from the tap and was not dechlorinated. To assess spore retention on the various surfaces, a suspension of 2x104 spores in 2 mL of dechlorinated tap water was applied to the coupon surfaces for 20 minutes followed by rinsing with dechlorinated tap water to collect spores to determine the number of spores released from the surface. Membrane filter plate count numbers were used to enumerate spores released from the coupon. Due to non-normal distributions, all data were analyzed using non-parametric statistics. Conditioned ductile iron retained significantly more spores than the other pipe materials as seen by the significant differences between conditioned PVC, conditioned cement, bare iron, and agarose-coated bare iron. Significant differences were also seen between conditioned cement and conditioned PVC. A second study was conducted to evaluate spore retention on artificial “biofilm” vs. a natural, but nutrient augmented “natural” biofilm grown in a recirculation tank. The second study was done at three different spore concentrations. Conditioned ductile iron retained significantly more spores than the other pipe materials as seen by the significant differences between the condition iron and: conditioned PVC, conditioned cement, bare iron, and agarose-coated bare iron. Significant differences were also seen between conditioned cement and conditioned PVC. Tests comparing spore retention between agarose “biofilm” and a nutrient-augmented biofilm also showed that for all three concentrations, augmented “natural” biofilm significantly retained more spores than agarose “biofilm”. Augmented “natural” biofilm on PVC coupons retained more spores than agarose “biofilm” on PVC. In contrast, augmented compared to agarose biofilm on cement-lined coupons showed no significant differences leading us to believe that the agarose “biofilm” simulates spore retention as long as the natural biofilm covers the pipe surface at a certain thickness. |
