Browsing by Author "Van Rooyen, Brandon Burger"

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    Evaluation of the efficacy of chemical, ultraviolet (UV) and combination treatments on reducing microbial loads in water prior to irrigation
    (Stellenbosch : Stellenbosch University, 2018-03) Van Rooyen, Brandon Burger; Sigge, G. O.; Lamprecht, Corne; Stellenbosch University. Faculty of AgriSciences. Dept. of Food Science.
    ENGLISH ABSTRACT: The investigation of Western Cape Rivers has highlighted the importance of the implementation of cost-effective, on-farm disinfection treatments solutions. Irrigation water, if used untreated, has the potential to be a serious health hazard as faecal coliform (FC) levels often far exceed the allowable limit of 1 000 FC per 100 mL water. Chlorine (Cl), peracetic acid (PAA) and hydrogen peroxide (H2O2) are popular chemical disinfectants that have been used in water disinfection over the years. On-farm ultraviolet (UV) irradiation, a less conventional water treatments option, can also prove to be advantageous for water treatment. The aim of this study, therefore, was to investigate the application of chemical treatments in combination with UV irradiation in the disinfection of river water used for irrigation. Initially, the efficacy of Cl, PAA and H2O2 in combination with low-pressure (LP) UV (Cl+UV; PAA+UV; H2O2+UV) required evaluating the stand-alone efficacy of each treatment first. Environmental Escherichia coli (E.coli) strains, F11.2 and MJ58 when exposed to Cl (6 mg.L-1) and H2O2 (2.5 mg.L-1) showed much resistance to disinfection. Strain F11.2, showed much greater sensitivity to PAA (4 mg.L-1), recording > 3 log reductions for both 15 and 25 min contact times. However, LP-UV doses of 13 mJ.cm-2 proved more effective than any of the chemical disinfectants for the E. coli strains. Combination treatments did not show much evidence on the initiation of advanced oxidation processes (AOPs) as the sum of the individual treatments more clearly justified the log reductions recorded. An additional study investigated the impact of river on disinfection treatments whilst keeping the chemical and UV doses the same as in the first study. Considering the variability in the physico-chemical properties of the river water, Cl most effectively reduced the TC and FC groups, recording no less than 2.9 log reduction for TC and well over 3 log reduction for FC. PAA and H2O2 showed highly compromised disinfection and were unable, as stand-alone treatments, to offer adequate defence against the naturally present microorganisms in the river water. However, residual Cl levels of > 1 mg.L-1 measured, post-treatments is of concern, as the formation of disinfection by-products (DBPs) is unwelcomed. UV treatments showed to be greatly influenced by poor ultraviolet transmission percentages (UVT%) and turbidity, which greatly decreased its effectiveness. Assessing the benefits of combination treatments, if any, through the initiation of AOP proved redundant as UV treatments were so effective. The efficiency of medium-pressure (MP) UV irradiation (25 – 30 mJ.cm-2) at pilot-scale, was able to, in some instances, successfully reduced FC levels by over 3 log. However, significantly poorer (p<0.05) disinfection was reported for all the chemical treatments. UV irradiation was again directly affected by poor optical water characteristics measured for the river water. Cl disinfection, dosed at 3 mg.L-1, half that of the dose used in previous trials, still proved to be the most effective of the chemical treatments investigated. Regardless thereof, Cl was only able to reduce FC by 1.58 log at best, which was insufficient, considering the > 6.0 log initial FC levels. Positively, when dosing Cl at 3 mg.L-1, residual levels never exceeded 0.50 mg.L-1. In most instances, no significant differences (p>0.05) were observed between stand-alone UV treatments and combination treatments, thus, insignificant contributions were made by advanced oxidation processes (AOPs). Investigating the effects of photo-repair revealed up to 13.72% and 15.86% photo-recovery for TC and FC, respectively, after UV irradiated river water was subjected to visible light at 3.5 kLux intensity for 3 h. Considering the importance of UV irradiation for the microbial reduction in combination treatments in this study, a 15.86% recovery rate for FC would, in many instances, result in the target 1 000 colony forming units (cfu). 100 mL-1 not being met. As the efficacy of the disinfection treatments was influenced by varying microbial and physico-chemical properties of river water, the ability of biochar to improve the initial microbial and physico-chemical quality of river water was investigated. Significant improvements (p<0.05) to river water quality were observed for the eucalyptus biochar filter columns, with significantly less effective filtration recorded for pine biochar filter columns. No microbiological growth was detected after eucalyptus biochar filtration. And with significant improvements to UVT% from 49.60% to 88.00% after filtration. However, previously ‘used’ eucalyptus filter columns proved to be ineffective if left unused for > 48 h, recording a > 3 log washout for TC and FC. From the current study, combination treatments did not produce irrigation water of consistent acceptable standards for fresh produce. This was a results of UV irradiation being the main contributor to disinfection for the combination treatments and being greatly influenced by poor and varying water quality. Secondly, the poor contributions made by chemical disinfectants to the overall disinfection resulted in the dependence on UV irradiation for acceptable water disinfection. More effective filtration processes, combined with increased chemical and UV doses should be investigated to further optimise UV disinfection and ultimately combination treatments.