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

Thesis (MScFoodSc)--Stellenbosch University, 2019.

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.

AFRIKAANSE OPSOMMING: Besproeiingswater is geïdentifiseer as een manier waarop vars landbouprodukte met patogene besmet kan word. Dit is van belang in Suid-Afrika waar sommige riviere wat vir besproeiing van vars produkte aangewend word dikwels patogene bevat. In hierdie verband is dit belangrik om rivierwater voor besproeiing te behandel om sodoende die moontlike risiko van voedseloordraagbare siektes wat met die inname van besmette produkte geassosieer word, te verminder. Ultraviolet (UV) bestraling kan gebruik word om water te ontsmet en daar is gevind dat dit effektief is teen watergedraagde patogene. Kennisgapings bestaan egter steeds ten opsigte van die minimum effektiewe UV dosis wat benodig word om verskillende watergedraagde patogene effektief te inaktiveer. Die doel van hierdie studie was dus om die ontsmettingseffektiwiteit van lae druk (LP) UV bestraling op rivierwater te evalueer. Escherichia coli (E. coli) isolate, insluitend drie Shiga-toksien produserende Escherichia coli (STEC) isolate, is blootgestel aan vyf verskillende UV dosisse (20, 30, 40, 50 en 60 mJ.cm-2) in steriele Ringer’s oplossing. Variasie in UV sensitiwiteit is tussen isolate waargeneem teen alle dosisse. Log reduksies het gewissel tussen 3.6 - 4.4 log vir die laagste dosis (20 mJ.cm -2). Omgewingsisolate was meer UV bestand as die ATCC isolaat. Gebaseer op hierdie resultate is die invloed van waterkwaliteit op UV bestralingseffektiwiteit ondersoek deur ‘n UV weerstandbiedende omgewing-STEC isolaat (STEC 210) in beide geoutoklafeerde rivierwater en Ringer’s oplossing te inokuleer. Die resultate het gewys dat waterkwaliteit parameters nie UV ontsmettingseffektiwiteit negatief beïnvloed nie mits die regte dosis toegepas is. Die ontsmettingseffektiwiteit van UV (40 en 60 mJ.cm-2) bestraling is daarna ondersoek in rivierwater (Eerste, Krom en Plankenburg riviere). Die Eerste rivier het E. coli vlakke getoon wat voldoen aan die aanbevole besproeiingsriglyn limiete . Die Krom rivier het ook lae E. coli vlakke getoon, maar ‘n eenmalige STEC teenwoordigheid, sowel as die konsekwente teenwoordigheid van ESBL produserende Enterobacteriaceae is waargeneem. Die Plankenburg rivier het die hoogste E. coli vlakke getoon asookkonsekwente teenwoordigheid van STEC en ESBL produserende Enterobacteriaceae. UV bestraling van die Eerste en Krom Riviere het gelei tot onopspoorbare lae vlakke van Enterobacteriaceae, totale kolivorme en E. coli. ‘n Geteikende drie log reduksie is behaal na behandeling van die Plankenburg Rivier water met beide UV dosisse. Dit het daartoe gelei dat die water binne aanvaarbare besproeiing standaarde val. ‘n Dosis van 40 mJ.cm-2 was egter oneffektief teen STEC. Die herstel van alle indikatorpopulasies is waargeneem en was minder as 1% by beide UV dosisse (40 en 60 mJ.cm-2), maar was minder by die hoër dosis (60 mJ.cm-2). Laastens is die effek van ‘n kombinasie van denneboom “biochar” filtrasie met UV bestraling op rivierwater ondersoek. Die kombinasie-behandeling het gelei tot ‘n verbeterde UVT% en beter UV bestralingseffektiwiteit teen mikroörganismes. In geheel toon UV bestraling die potensiaal om water van ‘n aanvaarbare standaard vir vars produk besproeiing, in terme van die E. coli lading, te produseer. UV bestralingseffektiwiteit teenoor ander belangrike watergedraagde voedselpatogene, soos Salmonella, is nie ondersoek nie. Daar word aanbeveel dat die ontsmettingseffektiwiteit van UV teenoor hierdie patogene in die toekoms ondersoek word. Aanvaarbare vlakke van hierdie patogene in besproeiingswater moet ook bepaal word om sodoende riglyn-aanbevelings te maak.