A Friday 13th risk assessment of failure of ultraviolet irradiation for potable water in turbulent flow

Abstract

Ultraviolet (UV) irradiation for potable water is an important alternative to widespread disinfection methods such as chlorine. Failure of UV irradiation to reduce levels of viable contaminants can lead to enduring health effects, with or without fatalities. Here a new risk assessment of failure of UV irradiation for potable water in turbulent flow in a series annular-reactor is presented using the Friday 13th (Fr 13) methodology of Davey and co-workers (Food Control 29(1), 248–254, 2013). The aim was to demonstrate the effects of random changes in UV parameters on plant failure. Failure is defined as unexpected levels of survival of Escherichia coli, a species of faecal bacteria widely used as an indicator for health risk. The assessment is based on a unit-operations model of UV irradiation together with extensive experimental data of Ye (2007). A failure factor (p) is defined in terms of the design reduction and actual reduction in viable E. coli contaminants. UV irradiation is simulated using a refined (Latin Hypercube) Monte Carlo (r-MC) sampling. Illustrative results show 16% of apparent successful operations, over the long term, can fail to achieve the design reduction in viable E. coli of 10−4.35 due to random effects. The analysis is shown to be an advance on current risk assessments because it produces all possible practical UV outcomes. Implications of Fr 13 methodology for practical re-design and targeted physical changes to UV plant for improved reliability and safety is discussed.