Impact des organismes superieurs sur la qualite microbiologique de l'eau potable

Internalization and transport of Cryptosporidium and Giardia (oo)cysts in granular filtration were studied in the first phase of this thesis. A study was conducted at pilot scale by seeding high concentrations of Cryptosporidium and Giardia (oo)cysts previously inactivated with UV in granular activated carbon (GAC) filtration columns naturally colonized by zooplankton. An experimental protocol has been developed to allow the release and the enumeration of internalized (oo)cysts following a complete disruption of zooplankton organisms by sonication. Samples of granular media (GAC) were collected at different depths of the filter bed after the seeding. Zooplankton was isolated from the GAC samples and from the filtered effluent in order to extract and enumerate internalized (oo)cysts. Under such conditions, it was demonstrated that predation by zooplankton results in the internalization of a limited fraction of (oo)cysts in the filter bed, followed by the transport of a portion of the internalized (oo)cysts to the effluent. An increased concentration of internalized (oo)cysts in the filtered effluent was observed 3 weeks after seeding in the absence of filter backwashing. Rotifers are suggested as the main zooplankton group responsible for the transport of internalized (oo)cysts to the effluent water.;Following this work, a comparative evaluation of microbial protective mechanisms in UV (UVC 254 nm) and solar (UVA) disinfection was conducted. Assays on internalization were performed using the protocol developed for UVC tests within this thesis, with E. coli as a disinfection target internalized by the nematode C. elegans. UVA irradiation was simulated using a lamp (365 nm) in the laboratory. A reduction in the UVA inactivation rate of total coliforms due to aggregation was demonstrated, thus a protective effect similar to that reported against UVC disinfection. However, the removal of particles by membrane filtration (8 mum) showed no impact on the efficiency of inactivation by UVA, in contrast to the raise in inactivation rates found when using an identical protocol to with UVC inactivation. Internalization by nematodes showed a similar protective effect against UVA and UVC disinfection. Approximately 24% of a 5.60 J/cm2 UVA fluence was estimated to be effective in inactivating E. coli bacteria located within the nematode C. elegans. Like UVC disinfection (254 nm), a less pronounced protection effect was observed at a lower fluence.;Finally, as a final stage of this thesis, a quantitative microbial risk assessment was performed to quantify the annual probability of infection among drinking water consumers due to pathogens internalized by zooplankton organisms. Predation on protozoan (oo)cysts by rotifers in granular filtration has been targeted as the source of the risk of microorganisms' internalization in drinking water. This risk analysis allowed estimating that the annual risk of infection among drinking water consumers due to internalized (oo)cysts is lower than the targeted risk of one infection in 10,000 people annually. The application of a UV disinfection process following granular filtration in drinking water treatment plants was estimated to potentially reduce by about two orders of magnitude the probability of infection associated with the internalized (oo)cysts in the treated water. (Abstract shortened by UMI.);In the second phase of this project, the impact of the internalization of microorganisms on the effectiveness of UV disinfection was evaluated using the nematode C. elegans as a predator for E. coli bacteria and spores of B. subtilis. A protocol was developed to (i) allow predation and internalization of both target microorganisms by nematodes, (ii) expose the co-suspension to UV irradiation (254 nm) and (iii) extract the internalized bacteria using a sonication protocol allowing the disruption of nematodes and enumeration of bacteria by standard culture methods. These assays have demonstrated that microorganisms internalized by higher organisms can be partially protected against UV disinfection (254 nm). Approximately 15-16% of the 40 mJ/cm2 fluence, typically applied in drinking water treatment plants, reached the E. coli bacteria and B. subtilis spores internalized by nematode C. elegans. This protective effect was less pronounced at a lower fluence. Thus, UV disinfection presents a potential for inactivation of microorganisms internalized by higher organisms.