Evaluation of the efficacy of chemical, ultraviolet (UV) and combination treatments on reducing microbial loads in water prior to irrigation

Van Rooyen, Brandon Burger (2018-03)

Thesis (MSc Food Sc)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Die belangrikheid van die implimentering van koste effektiewe, plaasvlak onstmetting-behandlings oplossings is tydens die ondersoek van die Wes-Kaapse riviere uitgelig. Onbehandelde besproeiingswater wat gebruik word het die potensiaal om ernstige gesondheidsriskos te verhoog, omdat fekale kolivorm (FK) vlakke dikwels die toelaatbare limiet van 1 000 FK per 100 mL oorskry. Chloor (Cl), Perasynsuur (PAA) en Waterstofperoksied (H2O2) is gewilde chemiese ontsmettingsmiddels, wat oor jare al gebruik word vir ontsmetting van water. Plaasvlak ultraviolet (UV) bestraling, ‘n minder konvensionele keuse om water te behandel, is ook bewys om voordelig te wees in die ontsmetting van water. Aanvanklik vereis die behandeling van Cl, PAA en H2O2 in kombinasie met laedruk (LD) UV (Cl+UV; PAA+UV; H2O2+UV) eers die evaluering van elke behandeling se doeltreffendheid op sy eie. Omgewings Escherichia coli (E. coli) isolate F11.2 en MJ58 het meer weerstand teen ontsmetting getoon wanneer dit aan Cl (6 mg.L-1) blootgestel is. Isolaat F11.2 wys ‘n hoër sensitiwiteit teenoor PAA (4 mg.L-1), waar log verminderings van > 3 log vir beide 15 en 25 min kontaktyd waargeneem is. LP-UV dosisse van 13mJ.cm-2 was egter meer doeltreffend as enige van die ander chemiese ontsmettingsmiddels gebruik vir E. coli isolate. Gekombineerde behandelings het nie meer bewyse getoon op die inisiasie van gevordered oksidasie prosesse (GOPs) nie, aangesien die som van die individuele behandelings die log reduksies beter aangedui het. ‘n Addisionele studie het die impak van die rivier op ontsmetting behandelings ondersoek, terwyl die chemiese en UV dosisse dieselfde gehou is as die eerste studie. In ag genome die variëring in die fisies-chemiese eienskappe van die rivierwater het Cl die FK en TK groepe die effektiefste verminder, waar nie minder as 2.9 log reduksie vir TK en vêr oor 3 log reduksie vir FK aangeteken is. PAA en H2O2 het hoogs gekompromitteerde ontsmetting aangedui en was nie in staat, as ‘n losstaande behandeling, om voldoende beskerming teen die natuurlik teenwoordige mikro-organismes in die rivierwater te bied nie. Alhoewel residuele Cl vlakke van > 1 mg.L-1 gemeet is, is die post-behandeling ‘n bekommernis, omdat die vorming van ontsmetting bymiddels onwelkom is. UV behandelings is sterk beïnvloed deur swak ultraviolet oordrag persentasies (UVO%) en troebelheid, wat dus die effektiwiteit in ‘n groot mate laat afneem. Die evaluering van die voordele van kombinasiebehandelings, indien enige, deur die aanvang van GOP was oorbodig aangesien UV-behandelings so effektief was. Die doeltreffendheid van medium-druk (MP) UV-bestraling (25 - 30 mJ.cm-2) op loodskaal was in sommige gevalle in staat om die FK-vlakke suksesvol te verminder met meer as 3 log. Daar is egter aansienlik swakker (p <0.05) ontsmetting gerapporteer vir al die chemiese behandelings. UV-bestraling is weer direk beïnvloed deur swak optiese water eienskappe wat gemeet vir die rivierwater. Cl ontsmetting gedoseer teen 3 mg.L-1, die helfte van die dosis wat in vorige proewe gebruik is, blyk steeds die mees doeltreffendste van die chemiese behandelings wat ondersoek is. Ongeag daarvan kon Cl net FK op die beste met 1,58 log verminder, wat onvoldoende was in die lig van die aanvanklike > 6.0 log FK-vlakke. Positief, wanneer die Cl by 3 mg.L-1 toegedien word, het residuele vlakke nooit 0,50 mg L-1 oorskry nie. In die meeste gevalle is geen beduidende verskille (p> 0.05) waargeneem tussen alleenstaande UV-behandelings en kombinasiebehandelings nie, dus is onbeduidende bydraes deur gevorderde oksidasieprosesse (GOP's) gemaak. Ondersoek na die effekte van fotoreparasie het tot 13,72% en 15,86% fotoherwinning vir onderskeidelik TK en FK gewys, na UV-bestraalde rivierwater vir 3 uur lank blootgestel aan 3,5 kLux-intensiteit. Met die oog op die belangrikheid van UV-bestraling vir die mikrobiese reduksie in kombinasiebehandelings in hierdie studie, sal 'n 15,86% herstelvermoë vir FK in baie gevalle veroorsaak dat die teiken van 1 000 cfu 100 mL-1 nie bereik word nie. Aangesien die doeltreffendheid van die ontsmettingsbehandelings beïnvloed is deur wisselende mikrobiese en fisies-chemiese eienskappe van rivierwater, is die vermoë van ‘biochar’ om die aanvanklike mikrobiese en fisies-chemiese kwaliteit van rivierwater te verbeter, ondersoek. Aansienlike verbeterings (p <0.05) tot die rivierwaterkwaliteit is waargeneem vir die ‘biochar’ filterkolomme van bloekom, met aansienlik minder doeltreffende filtrasie aangeteken vir pynappel biochar filterkolomme. Geen mikrobiologiese groei is waargeneem ná die bloekom ‘biochar’ filtrasie nie, en met beduidende verbeteringe aan UVT% van 49,60% tot 88,00% na filtrasie. Maar, voorheen 'ebruikte bloekom filterkolomme was oneffektief as dit vir > 48 uur gelaat, met ‘n uitwassing van 'n> 3 log aangetekne vir beide TK en FK. Met die huidige studie het kombinasiebehandelings nie besproeiingswater nie konsekwente, aanvaarbare standaarde vir vars produkte gelewer nie. Dit was omdat UV-bestraling die belangrikste bydraer tot ontsmetting vir die kombinasiebehandelings was, en uitermate beïnvloed word deur swak en wisselende watergehalte. Tweedens het die swak bydraes deur chemiese ontsmettingsmiddels tot die algehele ontsmetting, gelei tot die afhanklikheid van UV-bestraling vir aanvaarbare waterdesinfeksie. Meer effektiewe filtrasieprosesse, gekombineer met verhoogde chemiese en UV dosisse, moet ondersoek word om UV-ontsmetting, en uiteindelik kombinasiebehandelings verder te optimaliseer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103733
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