A risk model for Escherichia coli survival in a sequential sand-filter (SF) and turbulent flow annular-reactor with ultraviolet irradiation (UV) for potable water production

Abstract

The presence of suspended solids (SS) in raw feed-water can reduce the efficacy of Ultraviolet (UV) irradiation (Chem. Eng. Sci. 143 (2016) 55–62). Therefore, an integrated SS pre-treatment with rapid sand-filters (SF) is sometimes used. Here we synthesize for the first time a Fr 13 risk assessment of a two-step integrated SF with a UV reactor (SF-UV). The aim was to simulate how stochastic fluctuations in steady-state plant parameters could impact UV efficacy for potable water production. Failure of the SF-UV is defined as unwanted survival of viable Escherichia coli. Results show the overall failure of SF-UV operations is 40.4% which equates to 148 failures per annum. The mean reduction in SS in the SF was log10 −1.11 (90%), with a subsequent reduction in viable E. coli in the UV reactor of log10 −2.93 (99.9%). SF-UV is shown to be a combination of successful and failed operations and not all failed SF automatically result in failure in overall SF-UV efficacy. The second-tier simulation showed that the greater the safety tolerance the greater the loss of flexibility in SF-UV reactor. A precision feed-water flow control is needed to minimize underlying vulnerability to unexpected failure. This work will be of immediate interest to risk analysts, benefiting operators and managers responsible for producing potable water using UV irradiation.