Factors Affecting the Transport of Pathogens & Pathogen Surrogates in Saturated Porous Media: Implications for Natural & Engineered Drinking Water Filters
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Three tiers of bench-scale experiments were conducted to evaluate the use of laboratory column investigations for studying the transport and removal of pathogenic microorganisms (i.e. disease causing viruses, 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 the transport and removal of a suite of (bio)colloids at a range of environmentally relevant conditions typical of natural riverbank filtration and engineered drinking water filters. Several bench-scale column designs were 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 for their individual and concurrent effects on the transport of a suite of (bio)colloids included: media grain size, media uniformity coefficient, ionic strength, and the presence of natural organic matter. The suite of pathogens and (bio)colloids utilized in this study included PR772 bacteriophage, Escherichia coli RS2g bacteria, Salmonella typhimurium bacterial pathogen, and two sizes of fluorescent polycarbonate microspheres (1.1 μm and 4.5 μm). In addition to S. typhimurium, pathogenic bacterial strains of E. coli and Pseudomonas aeruginosa were isolated and used in an experiment to investigate the effects of bacterial exposure to different environmental water matrices (impacted by various land-uses) on the transport of pathogenic bacteria. Additionally, the effects of bacterial exposure to the different water matrices 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 breakthrough curves and/or removal value from each trial, followed by ANOVA to determine the statistical significance of the effect of each parameter studied on (bio)colloid removal. The outcomes of this work have several implications for the use of bench-scale column studies in (bio)colloid transport investigations to improve the understanding of natural and engineered filter performance. Laboratory bench-scale experiments using replicate glass columns proved to be a useful tool in investigating factors that affect (bio)colloid transport in saturated porous media. In contrasts to common recommendations 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 the reproducibility and removal of a suite of (bio)colloids in transport investigations using varying ionic strengths and flow velocities representative of natural subsurface environments. Accordingly, small scale column studies of (bio)colloid removal by filtration that are conducted at D/d < 50 should not be universally disregarded because of wall effects concerns. iv Observations of (bio)colloid removal by granular media filtration were generally consistent with colloid filtration theory. Grain size, ionic strength and the presence of natural organic matter significantly affected 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 organic matter, as well as between physical media characteristics of grain size and uniformity coefficient. These results suggest that synergistic effects within physical and chemical factors known to effect pathogen transport in saturated porous media should be considered when assessing pilot- and full-scale filter performance demonstrations. Differences in removal between the suite of bacterial pathogens investigated at conditions representative of subsurface filtration were small (<0.5 log), suggesting that nuances between the removal of various strains 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 with profuse biofilm formation (unlike the “clean-bed” environments used in this study). Established and standardised methods for EPS extraction and characterization for a range of applications are necessary to improve our understanding of bacterial EPS production, and the effects of these compounds in a range of saturated porous media environments. A conceptual model was developed to encompass the current state of knowledge on bacterial EPS effects on bacterial removal and the results presented herein.