Evaluating the potential of ultraviolet irradiation for the disinfection of microbiologically polluted irrigation water

Fresh produce irrigation water from Western Cape Rivers carries faecal coliforms (FC) (Escherichia coli) at concentrations which often exceed the suggested limit of 1 000 FC per 100 mL and presents a health risk to consumers. On-farm ultraviolet (UV) irradiation presents several advantages for water disinfection but is an uncommon practice in South Africa. The aim of this study was to investigate the use of UV irradiation for river water disinfection prior to irrigation. Escherichia coli (E. coli) strains were exposed to low-pressure (LP) UV (4 mJ.cm) and UV/Hydrogen peroxide (H2O2) (4 mJ.cm/20 mg.L) treatments in Sterile Saline Solution (SSS). Strain variation in reductions was observed and ranged from 1.58 to 3.68 and 1.34 to 3.60 log for the UV and UV/H2O2 treatments, respectively. The UV/H2O2 treatment (4 mJ.cm/20 mg.L) was more effective, compared to UV singly, against some of the E. coli strains. Selected strains showed increased sensitivity at higher UV doses (8, 10 and 13 mJ.cm) and H2O2 concentrations (100 and 200 mg.L with 4 mJ.cm) but a 3 log target reduction was not always reached. For all UV and UV/H2O2 treatments maximum resistance was shown by an environmental strain. Reference strains should, therefore, not be used for the optimisation of UV based disinfection parameters. At 10 mJ.cm an American Type Culture Collection (ATCC) reference strain and an environmental strain (ATCC 25922 and F11.2) were both significantly less inactivated in sterilised river water compared to SSS. Enhanced water quality allowed for improved inactivation of the ATCC strain. Also, the efficiency of LP UV (5, 7 and 10 mJ.cm) and medium-pressure (MP) UV (13, 17 and 24 mJ.cm) radiation was investigated using water from the Plankenburg River. Water was sampled and treated on three respective days (Trials 1 to 3). Physico-chemical and microbiological water quality was always poor. The FC concentration reached a maximum of 6.41 log cfu.100 mL while UV transmission was always below 38%. For LP and MP UV irradiation increased doses resulted in increased disinfection but a 3 log reduction of FC was only attained when MP UV light was used in Trial 1. Disinfection efficiency was dependent on water quality and on the characteristics of the microbial population in the water. Since FC were never reduced to below 3 log cfu.100 mL, the water did not adhere to guidelines for produce irrigation. Photo-repair following irradiation was investigated in river water using MP UV doses of 13 and 24 mJ.cm and 3.5 kLux reactivating light, initially. Ultraviolet transmission was close to 50% and total coliform (TC) reduction exceeded 3 log, even at 13 mJ.cm. However, TC were reactivated from below 1 000 cfu.100. mL to 3.93 and 4.41 log cfu.100 mL for the 13 and 24 mJ.cm treatments, respectively. A higher MP dose (40 mJ.cm) and a different treatment regime (2 x 20 mJ.cm) inhibited photo-repair (compared to 13 and 24 mJ.cm) but TC were always recovered to a final concentration surpassing 3 log cfu.100 mL, even under lower light intensities (1.0 to 2.0 kLux). Stellenbosch University https://scholar.sun.ac.za