Envlron. Scl. Technol. 1992, 26, 1053-1058
Application of Clean-Bed Filtration Theory to Bacterial Deposition in Porous Media Robert E. Martin,+#*Edward J. Bouwer,* & and Linda M. Hannafnll Department of Environmental Health Sciences and Department of Geography and Environmental Engineering, The Johns Hopkins University, 34th and Charles Streets, Baltimore Maryland 21218
The objective of this work was to test the use of filtration theory to describe the deposition of bacterial cells in porous media. Suspensions of Pseudomonas aeruginosa were applied to a column packed with glass beads, and effluent cell concentrations were monitored. Experiments and 10" M were conducted with cells suspended in NaC1, conditions which alter the collision efficiency (probability of cell attachment) of bacteria with the surM NaC1, breakthrough of bacteria was comface. At plete and similar to that of a fluorescein tracer. In contrast, the effluent concentration attained only about 78% of the influent value at M NaCl. The greater removal observed at the higher salt concentration was consistent with the hypothesis that interaction of similarly charged diffuse layers can impede the deposition of bacteria. The removal of cells was modeled by combining theoretical calculation of particle transport and empirical estimates of collision efficiencies obtained independently in rotating-disk experiments. The extent of agreement obtained between observed and predicted values suggests that mechanistic models of particle transport can be successfully applied to bacteria. Applications of this work include the movement of bacterial contaminants in groundwater and in situ biological remediation of contaminated aquifers. Introduction Bacterial interactions with surfaces in porous media are of widespread significance in water supply, wastewater treatment, and groundwater contamination. Pollution of aquifers by pathogenic bacteria, for example, has long been a concern associated with septic tanks, wastewater injection, and land application of sewage sludge (1). Whether bacteria appear in potable water supplies depends as much on processes of attachment, growth, and detachment occurring at soil surfaces as it does on survival of the microorganisms. Biofilms that develop from deposited cells can have an important effect on the chemical quality of water. Thus, porous media biofilm reactors have been used to remove organic material, ammonia, nitrate, iron, and manganese from surface water, groundwater, and wastewater (2, 3). In the natural environment, the fate of many synthetic organic contaminants in groundwater can be influenced by the activity of bacteria attached to soil particles (4). This has led to the current interest in promoting the activity of indigenous bacteria and/or introducing bacteria capable of degrading contaminants to actively treat pollution of soil and groundwater (5). Optimization of remediation techniques, however, will require accurate models of the transport of bacteria and biofilm development in porous media. 'Department of Environmental Health Sciences. Present address: Lyonnaise des Eaux-Dumez, 72 Avenue de la Libert6, 92000 Nanterre, France. 5 Department of Geography and Environmental Engineering. I' Present address: BCM Engineers, One Plymouth Meeting, Plymouth Meeting, PA 19462.
*
0013-936X/92/0926-1053$03.00/0
The work presented here was part of an effort to describe the transient phase of initial biofilm development in which bacteria attach to bare surfaces and then grow to produce a biologically active film. This paper will focus on experimental work designed to test the use of filtration theory to describe initial cell deposition in clean porous media. The modeling approach relied on empirical estimates of collision efficiencies, or cell attachment probabilities, obtained in a rotating-disk system (6). Calculation of cell transport to surfaces was based on the particle trajectory model of Rajagopalan and Tien (7). Deposited cells can grow and accumulate in the presence of suitable substrates; the coupling of filtration and biofilm growth models will be treated in a subsequent paper. Model Development The removal of suspended particles by a one-dimensional clean-bed filter can be described by the following equation (8): Ne/Ni = e-[1.5(1-t)a~(L/d)l (1) in which Ni is influent particle concentration; Ne is effluent particle concentration; L is bed length; d is media diameter; e is porosity; I] is single collector efficiency; and a is collision efficiency or (no. of attached)/ (total collisions). This expression gives the fraction of particles remaining in suspension after passage through a filter of length L. Its value depends explicitly on geometric characteristics of the porous medium, including media size and porosity, as well as the length under consideration. It also depends, however, on the nature of the flow, the physical properties of the suspended particles, and the forces that arise between the surfaces of particles and media grains; these factors are contained in the single-collector (I]) and collision (a) efficiencies. The single-collector efficiency indicates the fraction of particles flowing toward a grain of media, or collector, that actually collides with the collector. In a packed bed, the suspended particles may be much smaller than the pore sizes and thereby traverse the porous medium without contacting any surfaces. However, several mechanisms exist which can result in particles being removed from suspension by colliding with media surfaces and attaching. Interception, a major mechanism in liquids, is due to suspended particles of finite size which simply intercept or contact media surfaces as they move with the fluid. However, there also exist mechanisms that may cause particle trajectories to deviate from the fluid streamlines resulting in collisions with the media. Through Brownian motion, particles may diffuse to the surface. In contrast, larger particles may experience sufficient gravitational force to settle onto the media before traversing the length of the filter. Inertial impaction, deviation of particles from streamlines due to particle momentum, is of little consequence in liquid. In this work, the expression presented by Rajagopalan and Tien (7) for application to deep-bed filtration was used to calculate the single-collector efficiency. Their analysis
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 5, 1992 1053
was based on the trajectory of a spherical particle in the vicinity of a spherical collector. The trajectory equation was developed by formulating a force balance for a suspended particle which included effects due to gravity, fluid drag, van der Waals interactions, and increased viscous resistance to particle motion near the collector surface. Flow in the porous medium was represented by Happel's sphere-in-cell model in which the packed bed is assumed to consist of spherical grains, each of which is encapsulated by a spherical liquid envelope. The diameter of the envelope is chosen such that the porosity of the unit formed by the grain and the liquid envelope is equal to the overall porosity of the bed. The contribution of Brownian diffusion to collection was based on an analytical solution of the convective diffusion equation for the Happel flow model (7). Combining this with the trajectory analysis yielded the following expression for the single-collector efficiency in terms of dimensionless parameters, N , representing mechanisms of particle-media interactions:
Here, NR is the ratio of suspended particle size to collector size and indicates the importance of interception, NG accounts for gravity effects; NLoreflects van der Waals interactions, and NPeexpresses the role of diffusion. These quantities are given by
+
where A, = 2 ( 1 - p 5 ) / w ,w = 2 - 3p 3p5 - 2p6,p = ( 1 E ) ~ /=~ a s / b , and D B M (Brownian diffusion coefficient) = kt/[6npap].In these definitions, up and a, are the radii of the suspended particles and the media grains, respectively, I* and p are the absolute viscosity and density of water, U is the approach velocity, E is the medium porosity, b is the radius of the liquid envelope surrounding a grain, g is the gravitational acceleration, p p is the density of the suspended particle, and H is the Hamaker constant. The term A,, which depends only on the porosity, enters the equation because the collector is surrounded by other grains in the packed bed. The model of clean-bed filtration (eq 1 and 2 ) was originally developed to describe the performance of manmade filters used in water treatment. It is therefore restricted to one-dimensional vertical flow in a homogeneous medium with the particle-settling velocity less than the flow velocity. Furthermore, it is best suited to describing deposition occurring on relatively clean surfaces where previously deposited particles do not affect further deposition. In principle, however, the basic approach represented by this model is applicable to particle deposition in saturated soil systems. While the single-collector efficiency describes the transport in terms of collision mechanisms, it gives no indication of the likelihood that a collision will result in attachment and, hence, deposition. The collision efficiency is a measure of the likelihood of attachment and is defined as that fraction of particles colliding with a collector surface that result in attachment. It is a general concept, applicable to any system, whether a porous medium, a pipe, or a rotating disk. 1054
Environ. Sci. Technol., Vol. 26, No. 5, 1992
The magnitude of the collision efficiency depends on the forces between the particle, the fluid, and the collector as the separation distance between the surfaces decreases. Efforts to quantitatively predict collision efficiency on the basis of such forces have not been effective, even for relatively ideal, well-defined systems (9). In light of this limited success and considering the complexity of bacterial surfaces, no attempt was made here to quantify interfacial forces which may result in attachment. Instead, collision efficiencies in two solutions of differing ionic strength were determined empirically in independent deposition experiments using a rotating-disk system (6). These studies demonstrated a dependence of the collision efficiency on the ionic strength of the solution. In the rotating-disk experiments, collision efficiencies for Pseudomonas aeruginosa depositing on glass in the presence of solutions containing and M NaCl were determined to be 0.0021 and 0.41, respectively. Since both bacteria and glass surfaces would bear a negative net surface charge under the solution conditions of the experiment, the dependence of the collision efficiency on NaCl concentration was explained by the effect of ionic strength on repulsive double-layer interactions. To test the efficacy of the model to predict bacterial filtration, an experimental filtration system was developed using a packed column of glass beads. The filtration studies were performed using two solutions of different ionic strength. The collision efficiencies under these same two ionic strengths measured with the rotating disk (6) were used in eq 1 to predict the removal of bacteria in the experimental filtration system. The predictions were compared to observations of bacterial filtration in the packed column. Approach The basic experimental approach to testing the use of this model to describe bacterial deposition in terms of single-collector and collision efficiencies was to apply suspensions of P . aeruginosa to a column packed with clean glass beads and monitor the concentration of cells in the effluent. In the column-deposition experiments, three replicate trials were conducted for two salt concentrations consistent with the rotating-disk experiments ( and M NaCl). Since both surface properties and ionic strengths in the glass bead column were similar to those of the rotating-disk experiments, the different collision efficiencies yielded different removals of bacteria in the glass bead column. The fraction of cells remaining in the column effluent was compared to the predicted value based on eqs 1 and 2 using the empirical collision efficiencies evaluated in the rotating-disk system (6). The comparison between the predicted and empirical filtration was used in this study to determine the relative importance of collision efficiency in the initial development of a biofilm. Methods All water used for glassware washing and solution preparation was obtained from a five-stage Milli-Q Plus system (Millipore, Bedford, MA). System Design. The experimental column consisted of a 50 mm i.d. x 300 mm Spectrum glass liquid chromatography column with an adjustable bed support (Figure 1) (Fisher Scientific, Pittsburgh, PA). It was packed with 3-mm borosilicate glass beads (Fisher Scientific, Pittsburgh, PA), which were cleaned by soaking in hot 10 g/L Alconox detergent solution (Fisher Scientific, Pittsburgh, PA) for at least 2 h, rinsing three times, soaking in 5 N nitric acid for at least 12 h, and then finishing with
n
n I
Measurement Flush Solution
a
Glass Bead
column
Bacterial Feed Suspension
Figure 1.
Schematic diagram of glass bead column reactor.
Table I. Parameters Used for Filtration Model Calculations
parameter column specifications length, m diameter, m media size, m porosity particle characteristics diameter. m specific gravity fluid characteristics temperature, "C absolute viscosity, (N-s)/m2 density, kg/m3 volumetric flow rate, m3/s physical constants gravitational acceleration, m/sz Boltzmann constant, J / K Hamaker constant, J collision efficiency
value 0.27 0.05
0.003 0.39 7.0 x 10-7, 1.2 x 10'6, 1.6 X lo* 1.04, 1-10,1.13 25
8.94 x 10-4 997.1 1.67 x 10-7 9.806 1.381 X 1.00 x 10-20 0.0020, 0.10, 0.41, 1.0
a thorough rinse with sterile water. Column specifications are given in Table I. Sterile solutions and bacterial suspensions were fed to the column by a peristaltic pump (Ismatec variable speed minicartridge pump, Cole-Parmer Instrument Co., Chicago, IL). The effluent line included a three-way valve for sample collection and a bypass to a vertically mounted pipet for checking flow rates. The mean residence time for a volumetric flow rate of 10 mL/min was obtained by applying a step input of a 100 ppb fluorescein solution to the column. The effluent dye concentration was continuously monitored with a Model 10 Turner fluorometer (Turner Designs, Inc., Mt. View, CA) fitted with a 2-mL continuous-flow cell. The mean residence time and variance, calculated according to Levenspiel (IO), were 21 min and 20 min2, respectively. Cell Suspensions. The P. aeruginosa culture used in this study was an ATCC strain originally obtained from The Johns Hopkins Hospital Department of Clinical Microbiology (Baltimore, MD). Cells were maintained on nutrient agar slants and transferred to fresh media every 4-6 weeks. Cultures for experimental use were grown for about 20 h in a water bath shaker at 37 "C in 250-mL Erlenmeyer flasks containing 100-mL of tryptone yeast extract (TYE) broth (10 g/L tryptone, 8 g/L NaC1, and 1g/L yeast extract). These conditions yielded a bacterial suspension containing about 5 X lo9 cfu/mL. Motility was always observed, and aggregates were never seen in wet mounts of 20-h cultures observed under phase-contrast microscopy. Negative staining with India
ink as described by Norris and Swain (11) failed to reveal capsules on cells from 20-h cultures. Examination of cells grown under these conditions by transmission electron microscopy revealed that the cells did have single-polar flagella as is characteristic of the species P. aeruginosa, but did not possess fimbriae. Singlet cells ranged from 1.1 to 1.9 p m in length and from 0.66 to 0.75 Km in diameter. Suspensions were prepared by diluting the culture 100-fold in 0.1% peptone and then adding 3 mL of the diluted culture to 1500 mL of a solution containing M NaHC03 for pH control and either 10" or low2M NaCl. This yielded a cell concentration of about lo5 cfu/mL. A fresh culture was employed for each trial. The average pH was 7.94 for the trials conducted with M NaCl and 8.14 for those with 10" M NaC1. The average pH during the rotating-disk experiments was 5.52 (6). The main reason for raising the pH in the filtration experiments was concern about the recovery of cells stressed by a low-pH solution. In the rotating-disk experiments, enumeration was accomplished by direct-counting techniques and therefore was independent of cell viability. System Operation. Prior to each experimental run, the entire system was disinfected by filling with a 100-200 mg/L chlorine solution and flushing thoroughly with the appropriate sterile NaCl solution after at least 6-h contact time. Before cell suspensions were introduced, the column effluent was checked for residual free chlorine by the DPD method (12). Experiments were carried out at room temperature (21.0-22.5 "C), with downward vertical flow to maintain consistency with the constraints of the filtration model. After the flow rate was adjusted to 10 mL/min (approach velocity of 7.33 m/day), step inputs of bacterial suspensions were applied to the column by switching the intake from the sterile reservoir of flushing medium to the cell suspension reservoir. A total of eight samples of 5-mL volume were taken at 10-min intervals from 0 to 70 min after switching the feed. In order to flush the sample line, about 5 mL was wasted before collecting samples in sterile test tubes. Samples of the suspension in the feed reservoir were collected at 20-min intervals starting at time zero. Cell Enumeration. Suspended P. aeruginosa were enumerated by the spread-plate technique (12). Appropriate sample dilutions were obtained by serial dilution in 0.1% peptone. The plating medium was tryptone yeast extract (TYE) agar, containing 10 g of tryptone, 8 g of sodium chloride, 1 g of yeast extract, and 15 g of agar (Bacto-Agar, Difco Laboratories, Detroit, MI) per liter. Plates were incubated at 37 "C for 12-24 h before counting. Examination of the variability associated with this technique showed standard deviations to be about 10% of the mean. Results Samples of the feed suspensions were taken at regular intervals over the course of all filtration runs in order to detect changes in viability. The results for all six trials with P. aeurginosa in and M NaCl solutions M NaHC03 at 20.0-22.5 "C are shown in containing Figure 2. Values were normalized with respect to that obtained at the start of a run (No).Samples were plated on tryptone yeast extract agar and incubated at 37 "C for at least 12 h. The solid lines in this figure are drawn through the average of values for each time. It was generally observed that cell concentrations declined by 10-20% over a 60-min period. P. aeruginosa breakthrough curves for all trials are shown by the data points in Figure 3. To account for changes in viability over time, the ratio of effluent to inEnviron. Sci. Technol., Vol. 26, No. 5, 1992 1055
1.2
7 Table 11. Predicted Values for N J N i Using Equations 1
0
:1 t
09 08
0' 7
O
3
0
z z
O0 5E 1
+
log [NaCl] = - 4 log [NaCl]
"1
A
= -2
U
I 0 A
and 2 with Input Parameters Listed in Table I
particle diameter, fim 0.70 1.2
1.6
02 I
-----00 20
0
60
40
Time, min
Figure 2. Variation over time in feed suspension viable counts relative to the initial value, N o (approximately io5 cfu/mL in all cases).
0
10
20
30
40
50
60
70
Time, rnin Flgure 3. Breakthrough of P. aeruginosa and fluorescein dye in the experimental porous media column expressed as the ratio of the effluent and influent concentrations.
fluent cell concentrations, N e / N i ,was based on effluent and influent values for the same point in time. Since influent samples were taken at 20-min intervals and effluent samples every 10 min, Ni for intermediate times was obtained by averaging consecutive values. In Figure 3, breakthrough curves for P.aeruginosa and a fluorescein tracer are shown. The dashed line represents the breakthrough curve obtained with a fluorescein tracer for a flow rate of 10 mL/min (approach velocity 7.33 m/day) and indicates complete breakthrough in about 30 min. The breakthrough curves for bacteria are given as a continuous line through the average values for replicate trials. At M NaC1, the breakthrough of bacteria was similar to that of the dye tracer in that the effluent concentration rose to that of the influent within 30-40 min M NaC1, the effluent for all three trials. In contrast, at concentration attained only about 78% of the influent value during the experimental period. It thus appeared that little or no removal occurred for the low-salt concentration, while at the higher concentration a detectable amount of deposition did occur throughout the time course of the experiment. This was consistent with expectations based on previous experience with a rotating glass disk (6) and provided further evidence that bacterial deposition can be limited by repulsive double-layer interactions. Comparison of Observed and Predicted Results. Of primary interest in this investigation was whether filtration theory could be used to predict the initial rates of bacterial deposition suggested by these results. In order to make 1056
Environ. Sci. Technol., Vol. 26, No. 5, 1992
specific gravity
0.0020
collision efficiency 0.10
0.41
1.0
1.04 1.10 1.13 1.04 1.10 1.13 1.04 1.10 1.13
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98
0.92 0.91 0.91 0.94 0.91 0.94 0.91
0.81 0.80 0.79 0.85 0.82 0.80 0.86 0.80
0.97
0.90
0.77
0.92
the comparison, the ratio of effluent to influent cell concentration was calculated according to the filtration model given by eqs 1and 2. The parameters used in the calculations are listed in Table I. Of the parameters listed in Table I, collision efficiency, cell size, and cell specific gravity involved a degree of uncertainty with respect to their values. To assess the impact of this uncertainty on model predictions, a sensitivity analysis was performed by using a range of plausible values. On the basis of the rotating-disk experiments (6), values of 0.41 and 0.002 were used for the collision efficiency for P.aeruginosa depositing on glass in the presence of M NaCl and M NaC1, respectively. Values of 0.1 and 1.0 were also used. In the filtration model, particles are assumed to be spherical although bacteria are rod shaped (0.7 pm in diameter by 1.6 pm in length). To accommodate the uncertainty of size and shape effects, predictions of filtration were made for spheres of diameter 0.7, 1.2, and 1.6 pm. Finally, since the specific gravity of the cells was not determined experimentally, filtration predictions were made using values of 1.04, 1.1, and 1.13, which were based on values reported in the literature obtained by density gradient centrifugation (13). Variation of the Hamaker constant from to J was also examined, but a negligible effect on model predictions was observed. A value of J was therefore adopted as a reasonable estimate (14). The filt-ation model calculations are summarized in Table 11. For collision efficiencies of 0.002 and 0.1, variation in the particle size and specific gravity had little effect, Ne/Nibeing 1.00 and 0.97-0.98, respectively, for all cases considered. Using the collision efficiency of 0.41 obtained from the rotating glass disk experiments for M NaC1, values ranged from 0.90 to 0.94 for the various combinations. With a collision efficiency of 1.0, the range was 0.77-0.86. The utility of these results is that they give an indication of the potential error associated with estimating the physical properties of the bacteria for different values of the collision efficiency. A more narrow range of parameters can be justified for comparison with the data, however. First, on the basis of the literature, the best estimate for the specific gravity was 1.1. Second, the rotating-disk results for M NaCl strongly suggested a collision efficiency higher than 0.1. Thus, a reasonably conservative range of predicted values for the higher salt concentration is 0.80-0.91. To obtain experimental estimates for comparison, the values of N e / N ifor 40, 50, 60, and 70 min were averaged (12 data points for each ionic strength). The average and one standard deviation for the low- and high-salt concentrations are given in Table 111. Also, a summary of the filtration model values based on the range of collision efficiencies (a)obtained from the rotating disk (6) is given in Table 111. The experimental values for M NaCl
Table 111. Summary of Predicted and Observed Values for NeINi
source experiment M NaCl M NaCl model M NaCl (01 = 0.0005-0.0036) M NaCl (01 = 0.19-0.86)
Ne/Ni
for 600 mL filtered total cfu % removed“ coverage“
1.03 f 0.11
0
0.78 f 0.09
1.7 X lo7 2.5 X
1.00
4.0
0.83-0.96
1.3 X lo7 1.9 X 3.2 x 106 4.7 x 10-4
0
x 104 5.7 x i o 4
” Estimates are based on the average influent cell concentration for all six experimental runs, Ni = 1.33 X lo5 cfu/mL, the physical specifications for the column given in Table I and an assumed m2. Total cfu removed = Ni(l - N,/Ni) single cell area of 1 X X (volume filtered). % coverage = total cfu removed X (single cell area/media surface area) X 100. showed good agreement with the values predicted for the corresponding collision efficiency range of 0.0005-0.0036, the imperceptible change in the bacterial concentration over the length of the column suggested by the model being consistent with the observations. In addition, the observed 95% confidence interval for Ne.Ni at M NaCl (0.60-0.96) overlapped with the predicted values corresponding to the collision efficiency range between 0.19 and 0.86. Since bacterial concentrations were determined by plate counts, the experimental results reflect the removal of aggregates as well as single cells, both of which qualify as “colony forming units” (cfu). By increasing the mean particle size in the influent suspension, the presence of aggregates could be expected to result in a lower effluent concentration of colony forming units and hence lead to lower estimates of Ne/Ni than would be expected for a homogeneous suspension of single cells. To evaluate the significance of the range in values for Ne/Nidue to uncertainty in the collision efficiency, estimates of the total number of cfu deposited in the column for each case were made as shown in Table 111. The concentration of P. aeruginosa in the influent was taken to be 1.33 X lo6 cfu/mL, which was the average value of Ni for the six trials. For the range of predicted values Ne/Ni = 0.83496, the number of cfu deposited varied by a factor of 2.5. For the purpose of modeling initial biofilm growth, whether it is sufficient to be able to predict removal rates within such a range depends on the sensitivity of the overall process to the initial level of attached bacteria. Biofilm growth experiments to be reported in a subsequent paper as well as computer simulations suggested that order of magnitude differences in deposition would be required to demonstrate a difference in the time required to achieve a given level of biofilm activity, such as steady-state removal of an organic substrate.
Discussion The extent of agreement obtained between observed removal in the experimental porous media column and prediction based on filtration theory suggests that mechanistic models of particle transport can be successfully applied to bacteria in relatively simple systems. The independent evaluation of the collision efficiency (a)using the rotating-disk system and the use of a filtration model that includes the influence of neighboring collectors (Happel flow) and hydrodynamic retardation contributed to the success of this work relative to other models that apply filtration theory to quantify bacterial transport in
the subsurface (15-1 7). While the results are encouraging with regard to the basic approach, they are not conclusive. One requirement for testing the application of filtration theory to bacterial suspensions is a reliable, independent measure of the collision efficiency. The estimates used here carried some uncertainty due to limitations of the rotating-disk system (6). Further work is needed to develop reliable measures of the collision efficiency. Additional refined studies using the approach described here should be carried out to validate the suggested conclusions of this work. In practical applications, such as bacterial transport in groundwater systems, the usually complex, heterogeneous structure of the subsurface can lead to deviations between the models and observations. Under such conditions, the simplifying assumptions that allow calculation of particle trajectories become increasingly untenable. Despite the limitations of the model, certain relationships could be expected to remain at least qualitatively valid and, therefore, useful as a rational basis for anticipating the influence of system properties on the movement of bacteria. For example, from eq 1 it can be seen that the distance required to achieve a given reduction in the concentration of suspended bacteria is inversely proportional to the collector efficiency (7)and hence the collision efficiency (a).The low flow velocities characteristic of groundwater would tend to favor the deposition of cells, i.e., reduce their travel distance, by increasing the collector efficiency. Other investigators have calculated travel distances for bacteria under typical groundwater conditions and concluded that, even for very low collision efficiencies, cell concentrations should be reduced to undetectable levels within 20 m (9, 15, 18). The fact that bacteria have been observed to travel hundreds of meters, however, has led others to question the utility of applying particle transport models to bacteria in groundwater (19). Gerba (20) reviewed observations of bacterial movement in natural soil systems and cited distances up to 920 m. High initial cell concentrations and flow-through macropores or channels could contribute to long-range transport. In Gerba’s work, the flow velocities associated with extreme distances range from 150 to 350 m/day (2). Groundwater velocities are usually less than 1m/day. Order of magnitude differences in velocity can result in comparable differences in the collector efficiency and, hence, in the distance necessary to achieve a given reduction in cell concentration. This observation is also relevant to schemes to seed chemically contaminated aquifers with bacteria capable of degrading specific compounds. From an operational standpoint, a key issue for in situ biological remediation is delivery of organisms to the desired points in the aquifer. Maximizing flow velocities for cell injection might significantly increase the range of delivery and hence reduce the number of injection wells needed. The transport of bacteria in aquifers also depends on whether cells stick to the surfaces with which they collide. The collision efficiency, expressing the probability of attachment, has the same degree of influence as the collector efficiency on the relationship between distance and removal of suspended cells. However, manipulation of collision efficiencies is not as straightforward as changing flow velocities. The collision efficiency is determined by the particle-surface interactions, which is also influenced by the chemistry of the solution. Because collision efficiency was shown to effectively alter filtration in this study, it is a viable method to consider since halving the collision efficiency could be expected to Environ. Sci. Technoi., Vol. 26,
No. 5, 1992 1057
double the range of bacterial transport. However, in seeding an aquifer for biological remediation, the range of solution conditions may be constrained by the necessity to maintain cell viability as well as by the ambient chemistry and water quality considerations. The potential for controlling the collision efficiency of cells by altering their surface properties is not clear at this time. Strategies to control this parameter could be very valuable to development of remediation techniques. Approaches for quantifying bacterial movement and fate in porous media are largely derived from studies of nonbiological particles. More research is needed to determine the extent to which these concepts apply to biological particles. Bacteria have diverse morphological features in comparison to nonbiological particles and differ markedly in their extent of transport because of differences in cell properties, such as electrophoretic mobility, hydrophobicity, cell size, and presence of capsules and appendages (e.g., flagella, fimbriae, and pili) (21-24). Their metabolic activity also can alter the chemical character of the cell surface as well as the surroundingmedium. Our knowledge of the relationship of such physical and chemical factors to the modeling of bacterial transport and attachment is poor and merits further study. In summary, we have presented the application of a filtration model to the initial deposition of bacteria in clean porous media. Suspensions of P. aeruginosa were applied to a column packed with glass beads, and the effluent cell concentrations were monitored. The collision efficiency was varied by using solutions of and M NaC1. The removal of P. aeruginosa in the experimental column was modeled by combining theoretical calculation of particle transport with independent empirical estimation of the collision efficiency under similar ionic strength conditions in a rotating-disk system. The extent of agreement obtained between observed and predicted removals suggests that mechanistic models of particle transport can be successfully applied to bacteria. The filtration model used here permits identification of factors influencing the movement of bacteria in porous media of which the collision efficiency is clearly a parameter that merits attention. The beneficial use of such a model is the development of rational approaches to practical problems such as the movement of bacterial contaminants in the subsurface and in situ biological remediation of chemically contaminated aquifers.
(3) Asano, T. Artificial Recharge of Groundwater; Butterworth Publishers: Boston, 1985; pp 1-747. (4) Bouwer, E. J.; McCarty, P. L. Ground Water 1984, 22, 433-440. (5) Thomas, J. M.; Ward, C. H. Environ. Sci. Technol. 1989, 23, 760-766. (6) Martin, R. E.; Hanna, L. M.; Bouwer, E. J. Environ. Sci. Technol. 1991,25, 2075-2082. (7) Rajagopalan, R.; Tien, C. AZChE J . 1976, 22, 523-533. (8) Yao, K.-M.; Habibian, M. T.; O’Melia, C. R. Environ. Sci. Technol. 1971,5, 1105-1112. (9) Tobiason, J. E.; O’Melia, C. R. J. Am. Water Works Assoc. 1988,80, 54-64. (10) Levenspiel, 0. Chemical Reaction Engineering, 2nd ed.; John Wiley & Sons: New York, 1972. (11) Norris, J. R.; Swain, H. In Methods in Microbiology; Norris, J. R., Ribbons, D. W., Eds.; Academic Press: New York, 1971; Vol. 5A, pp 105-134. (12) American Public Health Association. Standard Methods for the Examination of Water and Wastewater, 16th ed.; American Public Health Association: Washington, DC, 1985. (13) Martin, R. E. Quantitative description of bacterial deposition and initial biofilm development in porous media. Ph.D. Dissertation, The Johns Hopkins University, Baltimore, MD, 1990. (14) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1985. (15) Corapciologlu, M. Y.; Haridas, A. Adv. Water Resour. 1985, 8, 188-200. (16) Taylor, S. W.; Jaffe, P. R. Water Resour. Res. 1990, 26, 2181-2194. (17) Harvey, R. W.; Garabedian, S. P. Environ. Sci. Technol. 1991,25, 178-185. (18) McDowell-Boyer,L. M.; Hunt, J. R.; Sitar, N. Water Resour. Res. 1986, 22, 1901-1921. (19) Germann, P. F.; Douglas, L. A. Water Resour. Res. 1987, 23, 1697-1698. (20) Gerba, C. P. In Groundwater Pollution Microbiology; Bitton, G., Gerba, C. P., Eds.; John Wiley & Sons: New York, 1984; pp 225-234. (21) van Loosdrecht, M. C. M.; Lyklema, J.; Norde, W.; Schraa, G.; Zehnder, A. J. B. Appl. Environ. Microbiol. 1987,53, 1893-1897. (22) van Loosdrecht, M. C. M.; Lyklema, J.; Norde, W.; Schraa, G.; Zehnder, A. J. B. Appl. Environ. Microbiol. 1987,53, 1898-1901. (23) van Loosdrecht, M. C. M.; Lyklema, J.; Norde, W.; Zehnder, A. J. B. Microbiol. Rev. 1990,54, 75-87. (24) Gannon, J. T.; Manilal, V. B.; Alexander, M. Appl. Environ. Microbiol. 1991, 57, 190-193.
Registry No. NaC1, 7647-14-5.
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Received for review August 15,1991. Revised manuscript received January 10, 1991. Accepted January 14,1992. This research was supported by the U.S. Geological Survey (Project 14-080001-G1284) and the National Science Foundation (Grant ECE-8451060 Presidential Young Investigator Award). R.E.M. also gratefully acknowledges the support of the Washington Chapter of the ARCS Foundation.
