Direct and indirect QMRA of infectious Cryptosporidium oocysts in reclaimed water

Water scarcity leads to an increased use of reclaimed water, which in turn calls for an improvement in water reclamation procedures to ensure adequate quality of the final effluent. The presence of infectious Cryptosporidium oocysts (IOO) in reclaimed water is a health hazard for users of this resource. Here, we gathered information on Cryptosporidium (concentrations, infectivity and genotype) in order to perform quantitative microbial risk assessment (QMRA). Moreover, data concerning the spores of sulphite-reducing clostridia (SRC) were used to undertake QMRA at a screening level. Our results show that the probability of infection (PI) by Cryptosporidium depends on the tertiary treatment type. The mean PI using the exponential dose-response model was 3.69 × 10⁻⁶ in tertiary effluents (TE) treated with UV light, whereas it was 3 log₁₀ units higher, 1.89 × 10⁻³, in TE not treated with this disinfection method. With the β-Poisson model, the mean PI was 1.56 × 10⁻⁴ in UV-treated TE and 2 log₁₀ units higher, 4.37 × 10⁻², in TE not treated with UV. The use of SRC to perform QMRA of Cryptosporidium showed higher PI than when using directly IOO data. This observation suggests the former technique is a conservative method of QMRA.