Evaluating the disinfection efficacy of low-pressure ultraviolet irradiation on river water

[摘要] ENGLISH ABSTRACT: Irrigation water has been identified as one way by which fresh produce can become contaminated with pathogens. This is a concern in South Africa, where some rivers used for the irrigation of fresh produce often carry pathogens. In this regard, treating river water prior to irrigation is important to reduce the possible risk of foodborne disease outbreaks associated with the consumption of contaminated produce. Ultraviolet (UV) irradiation can be used to decontaminate water and has been shown to be effective against waterborne pathogens. Knowledge gaps, however, still exist regarding the minimum effective UV dose required to effectively inactivate different waterborne pathogens. The aim of this study was, therefore, to evaluate the disinfection efficacy of low-pressure (LP) UV on river water.Escherichia coli (E. coli) strains including three Shiga-toxin producing Escherichia coli (STEC) strains were exposed to five different UV doses (20, 30, 40, 50 and 60 mJ.cm-2) in sterile Ringer's solution. Variation in UV sensitivity among strains was observed at all doses. Log reductions ranged between 3.6 - 4.4 log for the lowest dose (20 mJ.cm -2). Environmental strains were more UV resistant than the ATCC strain. Based on these results, the influence of water quality on UV irradiation efficacy was investigated by inoculating a resistant environmental STEC strain (STEC 210) into both autoclaved river water and Ringer's solution. Results showed that water quality parameters did not negatively influence UV disinfection efficacy provided the same dose was applied.The disinfection efficacy of UV (40 and 60 mJ.cm-2) irradiation was then investigated against river water (Eerste, Krom and Plankenburg rivers). The Eerste River showed E. coli levels falling below the recommended irrigation guideline limit. The Krom River also showed low E. coli levels, but a once-off STEC detection, as well as the consistent occurrence of ESBL producing Enterobacteriaceae was observed. The Plankenburg River showed the highest E. coli levels and consistent detection of STEC and ESBL producing Enterobacteriaceae. UV irradiation of the Eerste and Krom rivers resulted in undetectable levels of Enterobacteriaceae, total coliforms and E. coli. A targeted three log reduction was achieved following disinfection of the Plankenburg River water at both UV doses. This resulted in water within acceptable irrigation standards. A dose of 40 mJ.cm-2 was, however, ineffective against STEC. Repair of all indicator populations was observed and was less than 1% at both UV doses (40 and 60 mJ.cm-2) but, was less at the higher dose (60 mJ.cm-2). Lastly, the effects of combining pine biochar filtration with UV irradiation was investigated on river water. The combination treatment resulted in improved UVT% and better UV irradiation efficacy of microorganisms.Overall, UV irradiation showed potential in producing water of acceptable standard for fresh produce irrigation in terms of the E. coli load. However, UV efficacy against other important waterborne pathogens such as Salmonella was not investigated. It is recommended that, the disinfection efficacy of UV against these pathogens, be investigated in future. Acceptable levels of these pathogens in irrigation water should also be explored, to make guideline recommendations.

[发布机构] Stellenbosch University