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sur le développement des zones arides et semi-arides

Titre : The quest for safe irrigation water : investigating uv irradiation treatment of river water to reduce microbial loads

Auteur : Oosthuizen, Marco

Université de soutenance : Stellenbosch University

Grade : Master of Science (MScFoodSc) 2022

Résumé partiel
Several studies have investigated the microbiological and physico-chemical characteristics of some Western Cape rivers used as sources of irrigation water for fresh produce. During this study, four Western Cape rivers were studied. The findings have shown that some of the rivers may pose a public health risk for consumers and jeopardise fresh produce safety, as tests indicated that Escherichia coli (E. coli) counts often exceeded the recommended irrigation water guidelines. As a water disinfection treatment, ultraviolet (UV) irradiation has proven to be effective and environmentally friendly, however, the application is still relatively novel in South Africa. Therefore, the aim of this study was to investigate UV irradiation treatment of river water to reduce microbial loads, for improved fresh produce safety. In the first research chapter (chapter 3), the variation in microbial and physico-chemical characteristics of a Western Cape river system over a longer distance (including sites that were after confluence points with other rivers) was investigated. The results showed that the water quality of one river system varies at different sampling sites, often exceeding guideline limits. Ultraviolet transmission (UVT %) and E. coli counts ranged from 28.00 to 90.40% and 2.322 to 3.913 log CFU.mL ̄¹, respectively. Several point and non-point pollution sources along the river could have resulted in the variations observed. Shiga toxin-producing Escherichia coli (STEC) and Extended spectrum beta- lactamase (ESBL) – producing Enterobacteriaceae were detected at certain water sites. These results suggested that, left untreated, water from this river could affect fresh produce safety as a result of microbial transfer that can occur during irrigation. The second research chapter investigated the effect of low-pressure, lab-scale UV doses (20, 40 and 60 mJ.cm ̄²) on the Heterotrophic Plate count (HPC) and Total Psychrotrophic Aerobic Bacteria Count (TPAC) populations. Results indicated that these populations showed UV resistance, and certain pathogens such as Bacillus cereus and Aeromonas hydrophila were identified from the surviving populations. It was also observed that UV irradiation eliminated most STEC and ESBL-producing strains, which is an important observation considering the advantages UV disinfection can have for fresh produce safety. In the third research chapter larger volumes of river water (1 000L) were treated in a medium- pressure UV disinfection pilot plant. Four different bag filters (5, 20, 50 & 100 μm) were evaluated as a pre-treatment step prior to UV disinfection, with the purpose of improving the water quality. Results showed slight improvements in suspended solids, with minimal reductions in dissolved and microbial content. However, bag filters with the smallest pore size of 5 μm showed best results. Medium-pressure UV treatment at pilot-scale was tested on large volumes (1 000 L) of water from three rivers.

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