Factors Affecting the Transport of Pathogens & Pathogen Surrogates in Saturated Porous Media: Implications for Natural & Engineered Drinking Water Filters

[摘要] Three tiers of bench-scale experiments were conducted to evaluate the use of laboratory columninvestigations for studying the transport and removal of pathogenic microorganisms (i.e. disease causingviruses, bacteria and protozoa) and pathogen-surrogates (i.e. (bio)colloids) in saturated porous media(filtration). These experiments were used to explore the effects of individual and concurrent factors on thetransport and removal of a suite of (bio)colloids at a range of environmentally relevant conditions typicalof natural riverbank filtration and engineered drinking water filters. Several bench-scale column designswere investigated to elucidate laboratory-scale column size factors that may affect reproducibility of(bio)colloid passage through granular media filtration. The physical and chemical factors investigated fortheir individual and concurrent effects on the transport of a suite of (bio)colloids included: media grainsize, media uniformity coefficient, ionic strength, and the presence of natural organic matter. The suite ofpathogens and (bio)colloids utilized in this study included PR772 bacteriophage, Escherichia coli RS2gbacteria, Salmonella typhimurium bacterial pathogen, and two sizes of fluorescent polycarbonatemicrospheres (1.1 μm and 4.5 μm). In addition to S. typhimurium, pathogenic bacterial strains of E. coliand Pseudomonas aeruginosa were isolated and used in an experiment to investigate the effects ofbacterial exposure to different environmental water matrices (impacted by various land-uses) on thetransport of pathogenic bacteria. Additionally, the effects of bacterial exposure to the different watermatrices on cell size and surface EPS composition of the suite of bacterial pathogens were investigated.Pathogen and (bio)colloid removal was assessed for the three experiments by plotting breakthroughcurves and/or removal value from each trial, followed by ANOVA to determine the statisticalsignificance of the effect of each parameter studied on (bio)colloid removal. The outcomes of this workhave several implications for the use of bench-scale column studies in (bio)colloid transportinvestigations to improve the understanding of natural and engineered filter performance.Laboratory bench-scale experiments using replicate glass columns proved to be a useful tool ininvestigating factors that affect (bio)colloid transport in saturated porous media. In contrasts to commonrecommendations for experimental design (e.g., column diameter (D) to collector diameter (d) ratio > 50),column and collector media designs with D/d between 15 and 116 did not have a significant effect on thereproducibility and removal of a suite of (bio)colloids in transport investigations using varying ionicstrengths and flow velocities representative of natural subsurface environments. Accordingly, small scalecolumn studies of (bio)colloid removal by filtration that are conducted at D/d < 50 should not beuniversally disregarded because of wall effects concerns.ivObservations of (bio)colloid removal by granular media filtration were generally consistent with colloidfiltration theory. Grain size, ionic strength and the presence of natural organic matter significantlyaffected the removal of a suite of (bio)colloids at values representative of natural field conditions.Interaction effects were also identified between the chemical factors of ionic strength and natural organicmatter, as well as between physical media characteristics of grain size and uniformity coefficient. Theseresults suggest that synergistic effects within physical and chemical factors known to effect pathogentransport in saturated porous media should be considered when assessing pilot- and full-scale filterperformance demonstrations.Differences in removal between the suite of bacterial pathogens investigated at conditions representativeof subsurface filtration were small (<0.5 log), suggesting that nuances between the removal of variousstrains of bacteria that are present at the micro-scale may not be substantial at the macro- or field-scale.The effects of bacterial EPS on (bio)colloid transport may be more important in environments withprofuse biofilm formation (unlike the ;;clean-bed” environments used in this study). Established andstandardised methods for EPS extraction and characterization for a range of applications are necessary toimprove our understanding of bacterial EPS production, and the effects of these compounds in a range ofsaturated porous media environments. A conceptual model was developed to encompass the current stateof knowledge on bacterial EPS effects on bacterial removal and the results presented herein.

[发布机构] University of Waterloo

[关键词] Civil Engineering