Risk assessment for drinking water production: assessing the potential risk due to the presence of Cryptosporidium oocysts in water

This paper presents an approach for assessing the risk of producing non-compliant drinking water (i.e. one of the quality parameters exceeds the standards fixed by legislation), taking into account the quality parameters of raw water and the process line of the treatment plant (technology, different failure mode and corresponding failure rate). Firstly, nominal and degraded modes of each step of the treatment line are analysed, in order to obtain transfer functions (which give output concentration of parameters in function of the input concentration) for each step of the treatment and each quality parameter, in nominal and degraded functioning. The transfer function of the whole treatment process can thereby be obtained by combination of transfer function of each step, and failure conditions of the whole treatment process and corresponding degraded global transfer function could be determined. Secondly, an inversion of both global function (nominal and degraded) permits to estimate probability for the resource to exceed thresholds fixed by regulation (in that case, a scenario of non-compliant drinking water exists), and to obtain a compliant water availability. Finally, this paper presents a software tool realised to evaluate the risk of non-compliant produced water, using the described methodology. Finally, an approach of risk assessment for Cryptosporidium is also presented. This method allows to identification and puts priorities for utilities presenting the highest risk.