The use of bayesian networks and bootstrap to evaluate risks linked to the microbial contamination of leafy greens irrigated with reclaimed water in Southeast Spain

Abstract

The use of reclaimed water for irrigation is one of the most common strategies to address water scarcity in many regions of the world, and many of the most intensive production areas of fruits and vegetables rely on these water sources to produce high quality fresh produce. However, there are still concerns regarding the microbiological quality and safety of products irrigated with reclaimed water. In this study, we propose an innovative approach to evaluate factors affecting this potential risk. Using the concentration of Escherichia coli as a proxy (an indicator) for bacterial pathogens, we define a probabilistic model divided in two parts. The variation in bacterial concentration during water reclamation and distribution is described by a Bayesian Network, where variability and uncertainty are included by data augmentation using non-parametric bootstrap. The second part, is a stochastic model that predicts the microbial concentration on the plant accounting for cross-contamination and bacterial survival.

The novel approach is used to evaluate the factors affecting the contamination and potential risk associated with the consumption of leafy greens irrigated with reclaimed water from two wastewater treatment plants (WWTP) in several growing fields located in the south-east of Spain. According to the model, the microbial concentration in the outlet of the WWTP has a relatively low impact on the probability of E. coli concentrations on the plant to exceed 2 log CFU/g (a common threshold), and the impact of the irrigation system (overhead, drip or irrigation) would be insignificant. Instead, the probability of exceedance would be dominated by soil-to-plant contamination due to splashing, when organic amendments are used as fertilizers. Therefore, provided every step in water reclamation from water generation to point of use is kept safe, current reclamation treatments from WWTPs would be effective in reducing microbial concentrations in reclaimed water.