Colloid Filtration Theory Research Articles

Microbial risk assessment in heterogeneous aquifers: 2. Infection risk sensitivity

The entire chain of events of human disease transmitted through contaminated water, from pathogen introduction into the source (E. coli, rotavirus, and Hepatitis A), pathogen migration through the aquifer pathway, to ingestion via a supply well, and finally, the potential infection in the human host, is investigated. The health risk calculations are based on a relevant hazardous event with safe setback distances estimated by considering the infection risk from peak exposure in compliance with an acceptable level defined by a regulatory agency. A site‐specific hypothetical scenario is illustrated for an aquifer with similar characteristics as the Cape Cod site, Massachusetts (United States). Relatively large variation of safe distances for the three index pathogens is found; individually, none of the index pathogens could predict the safe distance under the wide range of conditions investigated. It is shown that colloid filtration theory (CFT) with spatially variable attachment‐detachment rates yields significantly different results from the effective CFT model (i.e., assuming spatially constant parameters).

Microbial risk assessment in heterogeneous aquifers: 1. Pathogen transport

Pathogen transport in heterogeneous aquifers is investigated for microbial risk assessment. A point source with time‐dependent input of pathogens is assumed, exemplified as a simple on‐site sanitation installation, intermingled with water supply wells. Any pathogen transmission pathway (realization) to the receptor from a postulated infection hazard is viewed as a random event, with the hydraulic conductivity varying spatially. For aquifers where VAR[lnK] < 1 and the integral scale is finite, we provide relatively simple semianalytical expressions for pathogen transport that incorporate the colloid filtration theory. We test a wide range of Damkohler numbers in order to assess the significance of rate limitations on the aquifer barrier function. Even slow immobile inactivation may notably affect the retention of pathogens. Analytical estimators for microbial peak discharge are evaluated and are shown to be applicable using parameters representative of rotavirus and Hepatitis A with input of 10–20 days duration.

Estimation of colloidal deposition from heterogeneous populations

This paper reports a study of size-heterogeneous colloid filtration in a new bed using different types of colloids under different conditions of flow and solution chemistry. Depth-wise variation of the particle-size-distributions f i ( x), and the total liquid-phase colloid concentration, C( x) are measured which are used to estimate the depth-wise variation of the liquid-phase concentration for each distinct section of the heterogeneous population, C i ( x). It is observed that log C i ( x) is linear with depth, for some systems, while it shows deviation from linearity, with the slope decreasing with depth, for others. Deposition-rates for these distinct sections of the heterogeneous population, k i, are estimated from the slopes of the log C i ( x) data. These deposition-rates were then compared with predicted homogeneous-population deposition-rates from Colloid Filtration Theory (CFT), which shows agreement between the CFT-based-deposition-rates and heterogeneous-population-data based deposition-rates, for low flow velocities. At higher flow velocities a gap between the CFT-based and Data-based deposition-rates is observed. Deposition-rates from CFT are then used in a heterogeneous-colloid-filtration model, to examine if heterogeneous colloid deposition can be expressed as the sum of its parts. It is observed that, the sum-of-parts model provides a reasonable estimate of colloidal deposition from heterogeneous populations. Based on these results, it is possible to make predictions of colloidal deposition from complex heterogeneous suspensions. A new method for studying heterogeneous colloid filtration is also proposed.

Removal efficiencies and attachment coefficients for Cryptosporidium in sandy alluvial riverbank sediment

Riverbank filtration has been shown to be effective for removing viable Cryptosporidium parvum oocysts. Drinking water systems that employ riverbank filtration may receive additional treatment credits beyond that which they can obtain using traditional engineering approaches. In order to develop guidance for removal effectiveness, screening level predictive modeling by colloid filtration theory combined with advection and dispersion modeling is potentially useful. Currently, only few studies have measured basic effective colloid filtration parameters for Cryptosporidium oocysts with naturally occurring riverbank sediments. In the focus of this study we conducted flow column experiments in triplicate and measured effective attachment rate coefficients for sandy river sediments of the Southern Great Plains which are low in organic matter. We found that for sediment sampled from these high-energy rivers there was no apparent dependency of C. parvum removal with carbon content, bacterial colony forming units, or with gross texture properties of the sands. The differences in particle size distribution for the sediments suggested that straining did not play a role in removal efficiency. First-order colloid attachment rate coefficients followed lognormal distribution functions. The coefficients also appeared to be unrelated to the differences in particle size distributions of the sediments, bacterial counts, or levels of total carbon or total organic carbon. Using Monte Carlo analyses, the lowest observed 5th percentile was 8.0 × 10 −6 min −1 and the highest observed 95th percentile was 1.6 × 10 −3. Total log 10 removals ranged from 23 to 200 m −1. These results have application for screening level colloid filtration modeling of riverbank filtration in these systems.

Simulation of the subsurface mobility of carbon nanoparticles at the field scale

The production and use of nanomaterials will inevitably lead to their disposal in the natural environment. To assess the risk that these materials pose to human and ecosystem health an understanding of their mobility and ultimate fate is essential. To date, however, relatively little research has been conducted on the fate of nanoparticles in subsurface systems. In this study the subsurface mobility of two carbon nanoparticles: nano-fullerenes (nC 60) and multi-walled carbon nanotubes (MWCNTs) is assessed. A two-dimensional finite element model was used to simulate the movement of these nanoparticles under a range of hydrologic and geological conditions, including a heterogeneous permeability field. The numerical model is based on colloid filtration theory (CFT) with a maximum retention capacity term. For the conditions evaluated the carbon nanotubes are much more mobile than nC 60 due to the smaller collector efficiency associated with carbon nanotubes. However, the mobility of nC 60 increased significantly when a maximum retention capacity term was included in the model. Model results also demonstrate that, for the systems examined, nanoparticles were predicted to be less mobile in heterogeneous systems compared to the homogeneous systems with the same average hydraulic properties.

GAC adsorption filters as barriers for viruses, bacteria and protozoan (oo)cysts in water treatment

Granular Activated Carbon (GAC) adsorption filtration is commonly used in drinking water treatment to remove NOM and micro-pollutants and on base of the process conditions a certain capacity to eliminate pathogenic micro-organisms was expected. The experiences with the mandatory quantitative microbial risk assessment of Dutch drinking water revealed a lack of knowledge on the elimination capacity of this process for pathogens. The objective of the current study was to determine the capacity of GAC filtration to remove MS2, Escherichia coli and spores of Clostridium bifermentans as process indicators for pathogens and more directly of (oo)cysts of Cryptosporidium parvum and Giardia lamblia. Challenge tests with fresh and loaded GAC were performed in pilot plant GAC filters supplied with pre-treated surface water at a contact time which was half of the contact time of the full-scale GAC filters. MS2 phages were not removed and the removal of E. coli and the anaerobic spores was limited ranging from ≤0.1–1.1 log. The (oo)cysts of C. parvum and G. lamblia, however, were removed significantly (1.3–2.7 log). On base of the results of the experiments and the filtration conditions the removal of the indicator bacteria and (oo)cysts was largely attributed to attachment. The model of the Colloid Filtration Theory was used to describe the removal of the dosed biocolloids in the GAC filters, but the results demonstrated that there is a lack of quantitative knowledge about the influence of collector characteristics on the two major CFT parameters, the single collector and the sticking efficiency.

Mobility of Multiwalled Carbon Nanotubes in Porous Media

Engineered multiwalled carbon nanotubes (MWCNTs) are the subject of intense research and are expected to gain widespread usage in a broad variety of commercial products. However, concerns have been raised regarding potential environmental and human health risks. The mobility of MWCNTs in porous media is examined in this study using one-dimensional flow-through column experiments under conditions representative of subsurface and drinking water treatment systems. Results demonstrate that pore water velocity strongly influenced MWCNT transport, with high MWCNT mobility at pore water velocities greater than 4.0 m/d. A numerical simulator, which incorporated a newly developed theoretical collector efficiency relationship for MWCNTs in spherical porous media, was developed to model observed column results. The model, which incorporated traditional colloid filtration theory in conjunction with a site-blocking term, yielded good agreement with observed results in quartz sand-packed column experiments. Experiments were also conducted in glass bead-packed columns with the same mean grain size as the quartz sand-packed columns. MWCNTs were more mobile in the glass bead-packed columns.

Riverbank Filtration: Comparison of Pilot Scale Transport with Theory

Pilot-scale column experiments were conducted in this study using natural soil and river water from Ohio river to assess the removal of microbes of size ranging over 2 orders of magnitude, i.e., viruses (0.025-0.065 microm), bacteria (1-2 microm), and Cryptosporidium parvum oocysts (4-7 microm) under conditions representing normal operation and flood scour events. Among these different organisms, the bacterial indicators were transported over the longest distances and highest concentrations; whereas much greater retention was observed for smaller (i.e., viral indicators) and larger (i.e., Cryptosporidium parvum oocysts) microbes. These results are in qualitative agreement with colloid filtration theory (CFT) which predicts the least removal for micrometer size colloids, suggesting that the respective sizes of the organisms was a dominant control on their transport despite expected differences in their surface characteristics. Increased fluid velocity coupled with decreased ionic strength (representative of major flood events) decreased colloid retention, also in qualitative agreement with CFT. The retention of organisms occurred disproportionately near the source relative to the log-linear expectations of CFT, and this was true both in the presence and absence of a colmation zone, suggesting that microbial removal by the RBF system is not necessarily vulnerable to flood scour of the colmation zone.

On colloid retention in saturated porous media in the presence of energy barriers: The failure of α, and opportunities to predict η

This contribution reviews recent findings that illuminate the processes governing colloid retention in porous media under environmentally relevant conditions. In the environment, colloids act as conveyors of contaminants, or even as contaminants themselves; however, despite decades of research, we are unable to accurately predict the retention of colloids in granular aquifer media under environmental conditions, where repulsion exists between colloids and surfaces. This failure cannot be blamed solely on the complexities of the subsurface, since colloid filtration theory (CFT) works well in the absence of colloid‐collector repulsion despite its idealization of porous media as consisting of spherical grains completely surrounded by fluid envelopes. Rather, the failure of CFT stems from failure to incorporate the correct mechanisms of retention when repulsion exists. Recent observations implicate wedging in grain‐to‐grain contacts and retention in secondary energy minima as dominant mechanisms of colloid retention in the presence of an energy barrier. Mechanistic simulations in unit cells containing grain‐to‐grain contacts corroborate these mechanisms of colloid retention. The resulting concept for colloid retention in the presence of an energy barrier involves translation of colloids across the collector surfaces until they become wedged within grain‐to‐grain contacts, or are retained via secondary energy minima (without attachment) in zones where the balance of fluid drag, diffusion, gravitational, and colloid‐collector interaction forces allow retention. The above findings highlight the pore domain geometry as a dominant governor of colloid retention in so far as the geometry gives rise to grain‐to‐grain contacts and zones of relatively low fluid drag.

A new method to evaluate polydisperse kaolinite clay particle removal in roughing filtration using colloid filtration theory

Previous application of colloid filtration theory to roughing filtration has not considered a reliable method for determining a representative attachment factor for a polydisperse suspension (of constant particle density). Establishment of such a method would broaden the application of trajectory modelling in roughing filtration, and progress the development of a comprehensive database of attachment factors and surface charge potentials for various particle and fluid types. This study establishes a methodology for the application of colloid filtration theory to roughing filtration and incorporates recent advancements in theoretical single-collector efficiency. A polydisperse kaolinite clay suspension was passed through a series of four gravel upflow roughing filters and removal efficiencies were calculated. Both the classical and Tufenkji and Elimelech's more recent correlation equations were used to calculate theoretical single-collector efficiencies and associated attachment factors for three different filter media sizes, flow rates, and suspended solids concentrations (0.137±0.023). The use of Tufenkji and Elimelech's modified correlation equation resulted in reduced variability in the estimation of theoretical single-collector efficiencies.

Revisiting the Cape Cod Bacteria Injection Experiment Using a Stochastic Modeling Approach

Bromide and resting-cell bacteria tracer tests conducted in a sandy aquifer at the U.S. Geological Survey Cape Cod site in 1987 were reinterpreted using a three-dimensional stochastic approach. Bacteria transport was coupled to colloid filtration theory through functional dependence of local-scale colloid transport parameters upon hydraulic conductivity and seepage velocity in a stochastic advection-dispersion/attachment-detachment model. Geostatistical information on the hydraulic conductivity (K) field that was unavailable at the time of the original test was utilized as input. Using geostatistical parameters, a groundwater flow and particle-tracking model of conservative solute transport was calibrated to the bromide-tracer breakthrough data. An optimization routine was employed over 100 realizations to adjust the mean and variance of the natural-logarithm of hydraulic conductivity (InK) field to achieve best fit of a simulated, average bromide breakthrough curve. A stochastic particle-tracking model for the bacteria was run without adjustments to the local-scale colloid transport parameters. Good predictions of mean bacteria breakthrough were achieved using several approaches for modeling components of the system. Simulations incorporating the recent Tufenkji and Elimelech (Environ. Sci. Technol. 2004, 38, 529-536) correlation equation for estimating single collector efficiency were compared to those using the older Rajagopalan and Tien (AIChEJ. 1976, 22, 523-533) model. Both appeared to work equally well at predicting mean bacteria breakthrough using a constant mean bacteria diameter for this set of field conditions. Simulations using a distribution of bacterial cell diameters available from original field notes yielded a slight improvement in the model and data agreement compared to simulations using an average bacterial diameter. The stochastic approach based on estimates of local-scale parameters for the bacteria-transport process reasonably captured the mean bacteria transport behavior and calculated an envelope of uncertainty that bracketed the observations in most simulation cases.

Influence of solution ionic strength on the collision efficiency distribution and predicted transport distance of a Sphingomonas sp. flowing through porous media

The effects of solution ionic strength on the collision efficiency ( α) distribution of a Sphingomonas sp. were investigated using multiple sand columns of varying lengths and analyzing the bacteria clean-bed breakthrough concentrations using a distributed colloid filtration theory (D-CFT). Five different probability density functions (PDFs) were investigated and all accurately replicated the lab-scale experimental data, whereas a single α value could not. The α distribution shifted toward smaller values with decreasing ionic strength and the PDF parameters were strongly correlated to the Debye length, indicating that electrostatic interactions had a direct impact on the α distribution. The results indicate that while ionic strength has a large impact on bacterial transport distances for a concentration reduction of a few orders of magnitude, as occurs at the laboratory scale, due to the distributed nature of the collision efficiency, it has a minor effect on predicted transport distances required to achieve concentration reductions on the order of 10 6, which occurs at the field scale. Because of this, bacterial inactivation (e.g., death), rather than physically removing the bacteria from solution via filtration, is likely the key process impacting the transport of viable bacteria at the field scale. Overall, for systems with a distributed α, the results indicate that ionic strength has a strong influence on the transport of bacteria at the lab-scale (centimeters to one meter), both ionic strength and bacterial inactivation are important at the meso-scale (tens of meters), and inactivation becomes the dominant mechanism for reducing the transport of viable bacteria at the field scale (hundreds of meters).

Transport of Escherichia coli in saturated porous media: Dual mode deposition and intra-population heterogeneity

Because of heterogeneity among members of a bacteria population, deposition rates of bacteria may decrease upon the distance bacteria are transported in an aquifer. Such deposition rate decreases may result in retained bacteria concentrations, which decrease hyper-exponentially as a function of transport distance, and may therefore significantly affect the transport of colloids in aquifers. We investigated the occurrence of hyper-exponential deposition of Escherichia coli, an important indicator for fecal contamination, and the causes for such behavior. In a series of column experiments with glass beads of various sizes, we found that attachment of E. coli decreased hyper-exponentially, or, on logarithmic scale in a bimodal way, as a function of the transported distance from the column inlet. From data fitting of the retained bacteria concentration profiles, the sticking efficiency of 40% of the E. coli population was high ( α=1), while the sticking efficiency of 60% was low ( α=0.01). From the E. coli total population, an E. coli subpopulation consisting of slow attachers could be isolated by means of column passage. In subsequent column experiments this subpopulation attached less than the E. coli total population, consisting of both slow and fast attachers. We concluded that the main driver for the observed dual mode deposition was heterogeneity among members of the bacteria population. Intra-population may result in some microbes traveling surprisingly high distances in the subsurface. Extending the colloid filtration theory with intra-population variability may provide a valuable framework for assessing the transport of bacteria in aquifers.

Transport of Escherichia coli bacteria through laboratory columns of glacial-outwash sediments: Estimating model parameter values based on sediment characteristics

Bacterial transport through cores of intact, glacial-outwash aquifer sediment was investigated with the overall goal of better understanding bacterial transport and developing a predictive capability based on the sediment characteristics. Variability was great among the cores. Normalized maximum bacterial-effluent concentrations ranged from 5.4 × 10 − 7 to 0.36 and effluent recovery ranged from 2.9 × 10 − 4 to 59%. Bacterial breakthrough was generally rapid with a sharp peak occurring nearly twice as early as the bromide peak. Bacterial breakthrough exhibited a long tail of relatively constant concentration averaging three orders of magnitude less than the peak concentration for up to 32 pore volumes. The tails were consistent with non-equilibrium detachment, corroborated by the results of flow interruption experiments. Bacterial breakthrough was accurately simulated with a transport model incorporating advection, dispersion and first-order non-equilibrium attachment/detachment. Relationships among bacterial transport and sediment characteristics were explored with multiple regression analyses. These analyses indicated that for these cores and experimental conditions, easily-measurable sediment characteristics – median grain size, degree of sorting, organic-matter content and hydraulic conductivity – accounted for 66%, 61% and 89% of the core-to-core variability in the bacterial effective porosity, dispersivity and attachment-rate coefficient, respectively. In addition, the bacterial effective porosity, median grain size and organic-matter content accounted for 76% of the inter-core variability in the detachment-rate coefficient. The resulting regression equations allow prediction of bacterial transport based on sediment characteristics and are a possible alternative to using colloid–filtration theory. Colloid–filtration theory, used without the benefit of running bacterial transport experiments, did not as accurately replicate the observed variability in the attachment-rate coefficient.

Application of a dual deposition mode model to evaluate transport of Escherichia coli D21 in porous media

Controlled laboratory‐scale column deposition experiments were conducted using a well‐characterized mutant of the Escherichia coli (E. coli) K12 strain to obtain insight into the mechanisms that give rise to the observed deviation from classical colloid filtration theory (CFT). Both the suspended effluent bacteria concentration and the spatial distribution of retained bacteria were systematically measured over a wide range of solution conditions using columns packed with spherical glass beads. Calculations of Derjaguin–Landau–Verwey–Overbeek (DLVO) interaction energies based on measured cell zeta potentials indicated that the bacteria should experience considerable repulsive interaction forces when approaching the glass bead surface. In spite of these predictions, bacterial adhesion was observed even at the lowest solution ionic strength investigated (3 mM) and increased with solution salt concentration. Comparison of these results with measurements obtained using model colloidal particles (polystyrene latex microspheres) and a different microbe (Cryptosporidium parvum) suggested that another non‐DLVO‐type interaction may be contributing to the observed deposition behavior. Furthermore, predictions based on a discrete dual deposition mode (DDM) model disagreed with measured fractions of released cells. Taken together, the experimental and modeling results suggest that the deposition behavior of bacteria in saturated porous media is influenced by additional interaction mechanism(s) or factors not considered in classical DLVO theory, such as local charge heterogeneities of the cell membrane and surface biomolecule‐specific interactions.

E. coli fate and transport in the Happel sphere-in-cell model

Rates of mass and gene transfer reactions involving biotic phases are often expressed as proportional to local number densities of bacteria. When the reactions involve attached bacteria, reaction rates depend on local densities of bacteria attached to surfaces. Such may be the case with microbially-facilitated redox reactions involving mineral electron donors and mineral electron receivers (e.g., Sani RK, Peyton BM, Amonette JE, Dohnalkova A. Reoxidation of uranium in the presence of iron(III)-(hydr)oxides under sulfate reducing conditions. Environ Sci Technol 2005;39:2059–66), biofilm formation induced by quorum sensing (Purevdorj B, Costerton JW, Stoodley P. Influence of hydrodynamics and cell signaling on the structure and behavior of Pseudomonas aeruginosa Biofilms. Appl Environ Microbiol 2002;68(9):4457–64) and horizontal gene transfer among attached phase bacteria (Beaudoin DL, Bryers JD, Cunningham AB, Peretti SW. Mobilization of broad host range plasmid from Pseudomonas putida to established biofilm of Bacillus azotoformans. I. Experiments. Biotech Bioeng 1998a;57(3):272–79; Beaudoin DL, Bryers JD, Cunningham AB, Peretti SW. Mobilization of broad host range plasmid from Pseudomonas putida to established biofilm of Bacillus azotoformans. II. Modeling Biotech Bioeng 1998b;57(3):280–86). Here we use the conceptual Happel sphere-in-cell model to determine the microscopic distribution of attached bacteria on idealized spherical grains of porous media, assuming azimuthal symmetry. We extend a Lagrangian model of colloid filtration to investigate the effects of motility of Escherichia coli on attachment rate and on the attachment distribution as a function of location on grain surface. The hydrodynamics of the Happel model is implicitly 3D and represented in 2D polar coordinates under the assumption of axisymmetric flow, while the motility of the E. coli cells is explicitly 3D. The model incorporates the fate and transport processes of colloid filtration theory in the particle tracking approach of Nelson and Ginn (Nelson KE, Ginn TR. Theoretical investigation of bacterial chemotaxis in porous media. Langmuir 2001;17:5636–45; Nelson KE, Ginn TR. Colloid filtration theory and the Happel sphere-in-cell model revisited with direct numerical simulation. Langmuir 2005;21:2173–84) and includes statistics reported in Biondi et al. (Biondi SA, Quinn JA, Goldfine H. Random motility of swimming bacteria in restricted geometries. AIChE J 1998;44(8):1923–29) to describe the “run & tumble” behavior of individual E. coli cells. Incorporation of the motility allowed for comparative modeling of diffusion-controlled vs. motility-controlled transport and attachment in the Happel sphere. It was found that, whereas the Lévy distribution is often associated with the distribution of run lengths for this species, the data of Biondi is better fit by a Gamma distribution due to the low frequency of short runs, and that the turn distribution is non-uniform. Further, the motility mean free path is much larger than that corresponding to Brownian diffusion and the attachment frequency is higher in the case of motile cells as a result. However the distribution of attached cells is similar in both motile and diffusive cases.

Adaptable method for estimation of parameters describing bacteria transport through porous media from column effluent data: Optimization based on data quality and quantity

Bacterial transport through porous media is often modeled using the colloid filtration theory with a Langmuirian blocking function (CFT-LB), with experimentally derived transport parameters being the collision efficiency, the blocking parameter, and the clean-bed breakthrough concentration. Research trends are moving towards comparing small variations in these parameters as a function of experimental conditions, and the objective of this study was to determine impact of experimental design, including number of data points and experimental duration, on the suitability of the CFT-LB model to provide sufficient resolution to permit these comparisons. The results indicate that while the CFT-LB model captures the general trends in bacterial transport data, there are cases where the CFT-LB model cannot replicate the details in the column effluent curves, and this results in the estimated transport parameters varying as a function of the experimental duration. Impacts of these variations on interpretation of the transport parameters are discussed, and a recommended procedure is provided when applying the CFT-LB model to column effluent data. Ultimately, when the CFT-LB model is used to estimate bacterial transport parameters, the process should be completely transparent, allowing the general reader to know exactly how the transport parameters were obtained from the experimental data.

Temporal and spatial dynamics of blocking and ripening effects on bacterial transport through a porous system: A possible explanation for CFT deviation

The studies on transport of particles across porous systems are based on the Colloid Filtration Theory (CFT). According to CFT, the collision efficiency is constant along the system length [J.N. Ryan, M. Elimelech, Colloids Surf. A: Physicochem. Eng. Aspects 107 (1996) 1–56]. Decreasing values of collision efficiency have been reported, a phenomenon that has been interpreted as a deviation from the CFT [X. Li, T.D. Scheibe, W.P. Johnson, Environ. Sci. Technol. 38 (2004) 5616–5625; N. Tufenkji, J.A. Redman, M. Elimelech, Environ. Sci. Technol. 37 (2003) 616–623; N. Tufenkji, M. Elimelech, Langmuir 20 (2004) 10818–10828; N. Tufenkji, M. Elimelech, Langmuir 21 (2005) 841–852]. This paper presents data on transport of Bacillus megaterium spores through quartz sand columns. The occurrence of consecutive phases of increase and decrease of the values of C/ C 0, the effluent spore concentration expressed as a fraction of the influent spore concentration, is reported. These patterns of change in C/ C 0 were interpreted as the result of the concomitant occurrence of blocking and ripening, the prevalence of these phenomena in different moments of the experiment, and the spatial distribution of the prevalence of blocking and ripening effects along the porous system. It is argued that this spatial distribution in the predominance of blocking and ripening, what leads to the intensification of ripening at the entrance of the porous system, might be a possible explanation for the reported deviation from the CFT for experimental conditions where ripening and blocking take place.

Noninvasive Quantitative Measurement of Colloid Transport in Mesoscale Porous Media Using Time Lapse Fluorescence Imaging

We demonstrate noninvasive quantitative imaging of colloid and solute transport at millimeter to decimeter (meso-) scale. Ultraviolet (UV) excited fluorescent solute and colloid tracers were independently measured simultaneously during co-advection through saturated quartz sand. Pulse-input experiments were conducted at constant flow rates and ionic strengths 10(-3), 10(-2) and 10(-1) M NaCl. Tracers were 1.9 microm carboxylate latex microspheres and disodium fluorescein. Spatial moments analysis was used to quantify relative changes in mass distribution of the colloid and solute tracers over time. The solute advected through the sand at a constant velocity proportional to flow rate and was described well by a conservative transport model (CXTFIT). In unfavorable deposition conditions increasing ionic strength produced significant reduction in colloid center of mass transport velocity over time. Velocity trends correlated with the increasing fraction of colloid mass retained along the flowpath. Attachment efficiencies (defined by colloid filtration theory) calculated from nondestructive retained mass data were 0.013 +/- 0.03, 0.09 +/- 0.02, and 0.22 +/- 0.05 at 10(-3), 10(-2), and 10(-1) M ionic strength, respectively, which compared well with previously published data from breakthrough curves and destructive sampling. Mesoscale imaging of colloid mass dynamics can quantify key deposition and transport parameters based on noninvasive, nondestructive, spatially high-resolution data.

Collision efficiency distribution of a bacterial suspension flowing through porous media and implications for field-scale transport

The collision efficiency ( α) distribution of a bacterial population was determined using multiple packed-bed columns of varying lengths and analyzing the bacteria clean-bed breakthrough concentrations using a distributed colloid filtration theory. This technique allows the α distribution to be determined independently from other effects that can cause non-exponential deposition, including detachment and blocking. It was found that multiple probability density functions (PDF's) could accurately replicate the experimental data. Regardless of which PDF was used, a distributed α resulted in significantly greater predicted field-scale transport than when using a single α. However, there were wide variations in the predicted field-scale transport between the different distributions, suggesting that lab-scale experiments may not be readily utilized to determine the specific PDF that best represents α at the field scale. Finally, blocking was observed in the column effluent curves, underscoring the fact that if non-clean-bed processes occur then an approach such as that utilized in the current study may be used to separate the non-clean-bed and clean-bed processes when determining the collision efficiency distribution.