Biodegradation during contaminant transport in porous media: 4. Impact of microbial lag and bacterial cell growth

The reactive transport of trichloroethene is influenced by residence time and microbial numbers

The dechlorination rate in a flow-through porous matrix can be described by the species specific dechlorination rate observed in a liquid batch unless mass transport limitations prevail. This hypothesis was examined by comparing dechlorination rates in liquid batch with that in column experiments at various flow rates (3-9-12 cm day(-1)). Columns were loaded with an inoculated sand and eluted with a medium containing 1mM trichloroethene (TCE) for 247 days. Dechlorination in the column treatments increased with decreasing flow rate, illustrating the effect of the longer residence time. Zeroth order TCE or cis-DCE degradation rates were 4-7 folds larger in columns than in corresponding batch systems which could be explained by the higher measured Geobacter and Dehalococcoides numbers per unit pore volume in the columns. The microbial numbers also explained the variability in dechlorination rate among flow rate treatments marked by a large elution of the dechlorinating species' yield as flow increased. Stop flow events did not reveal mass transport limitations for dechlorination. We conclude that flow rate effects on reactive transport of TCE in this coarse sand are explained by residence time and by microbial transport and that mass transport limitations in this porous matrix are limited.

On the role of metabolic activity on the transport und deposition of Pseudomonas fluorescens in saturated porous media

A study was conducted to understand the role of cell concentration and metabolic state in the transport and deposition behaviour of Pseudomonas fluorescens with and without substrate addition. Column experiments using the short-pulse technique (pulse was equivalent to 0.028 pore volume) were performed in quartz sand operating under saturated conditions. For comparison, experiments with microspheres and inactive (killed) bacteria were also conducted. The effluent concentrations, the retained particle concentrations and the cell shape were determined by fluorescent microscopy. For the transport of metabolically-active P. fluorescens without substrate addition a bimodal breakthrough curve was observed, which could be explained by the different breakthrough behaviour of the rod-shaped and coccoidal cells of P. fluorescens. The 70:30 rod/coccoid ratio in the influent drastically changed during the transport and it was about 20:80 in the effluent and in the quartz sand packing. It was assumed that the active rod-shaped cells were subjected to shrinkage into coccoidal cells. The change from active rod-shaped cells to coccoidal cells could be explained by oxygen deficiency which occurs in column experiments under saturated conditions. Also the substrate addition led to two consecutive breakthrough peaks and to more bacteria being retained in the column. In general, the presence of substrate made the assumed stress effects more pronounced. In comparison to microspheres and inactive (killed) bacteria, the transport of metabolically-active bacteria with and without substrate addition is affected by differences in physiological state between rod-shaped and the formed stress-resistant coccoidal cells of P. fluorescens.

Transport and deposition of metabolically active and stationary phase Deinococcus radiodurans in unsaturated porous media

Bioremediation is a cost-efficient cleanup technique that involves the use of metabolically active bacteria to degrade recalcitrant pollutants. To further develop this technique it is important to understand the migration and deposition behavior of metabolically active bacteria in unsaturated soils. Unsaturated transport experiments were therefore performed using Deinococcus radiodurans cells that were harvested during the log phase and continuously supplied with nutrients during the experiments. Additional experiments were conducted using this bacterium in the stationary phase. Different water saturations were considered in these studies, namely 100 (only stationary phase), 80, and 40%. Results from this study clearly indicated thatthe physiological state of the bacteria influenced its transport and deposition in sands. Metabolically active bacteria were more hydrophobic and exhibited greater deposition than bacteria in the stationary phase, especially at a water saturation of 40%. The breakthrough curves for active bacteria also had low concentration tailing as a result of cell growth of retained bacteria that were released into the liquid phase. Collected breakthrough curves and deposition profiles were described using a model that simultaneously considers both chemical attachment and physical straining. New concepts and hypotheses were formulated in this model to include biological aspects associated with bacteria growth inside the porous media.

The influence of system complexity on bacterial transport in saturated porous media

A series of miscible-displacement column experiments were conducted under saturated flow conditions to systematically investigate the influence of physical and biological complexity on bacterial activity and fate in the presence and absence of a non-sorbing growth substrate, salicylate. Bacterial elution was monitored for three different systems; System I--a sterilized, inoculated, well-sorted sand, System II--a sterilized, inoculated, heterogeneous loamy sand (Hayhook), and System III--two different unsterilized loamy sands (Hayhook and Vinton) each with their associated indigenous microbial community. Results show that System I behaved ideally with respect to both cell and substrate transport, wherein: (1) growth occurred in response to substrate addition, (2) cell elution increased in response to the substrate pulse, and (3) breakthrough curves were reproducible for both substrate and cell elution. In contrast, System II showed ideal behavior with respect to substrate transport but showed variable behavior for cell transport. Further, there was no measurable growth in response to substrate addition and no increase in cell elution during the salicylate pulse. System III exhibited non-ideal behavior for both substrate and cell transport. Of particular interest is the fact that the indigenous communities of the two soils behaved differently. Specifically, for the Hayhook soil, an increased elution response was observed for the heterotrophic population while the salicylate-degrading community was preferentially retained in the column. In contrast for the Vinton soil, the substrate pulse did not elicit an elution response from either the heterotrophic or salicylate-degrading community from the culturable, indigenous Vinton microorganisms. For Systems II and III, the observed variability appears to be associated with the biological component of the system, since sterile controls were reproducible. This type of systematic study is critical for understanding cell and substrate transport behavior in complex, heterogeneous systems, and illustrates the potential uncertainty associated with measurements in such systems.

Spatial variability of in situ microbial activity: biotracer tests

Biotracer tests have been proposed as a means by which to characterize the in situ biodegradation potential for field-scale systems. In this study, field experiments were conducted at two sites to evaluate the utility of the biotracer method for characterizing the spatial variability of microbial activity. The first site is a mixed waste-contaminated surficial aquifer in Utah, and the second site is a chlorinated solvent-contaminated regional aquifer in Tucson, Arizona. Mass recovery of the biotracer decreased approximately linearly with increasing residence time for the Tucson site. Similar behavior was observed at the Utah site, except in the region adjacent to the injection zone, where percent recoveries were much lower than those predicted using a correlation determined using data collected downgradient of the injection zone. First-order biodegradation rate coefficients obtained from model calibration of the tracer data varied between 0.2 and 0.5/day for the Tucson site. For the Utah site, the values varied between 0.1 and 0.6/day downgradient of the injection wells, and between 0.7 and 2.6/day near the injection wells. Considering the large range over which biodegradation rate coefficients can vary, the rate coefficient exhibited relatively minimal spatial variability (factor of 2.5) for the Tucson site. Conversely, the spatial variability of the rate coefficient was an order of magnitude greater for the Utah site. These differences in variability are consistent with conditions associated with the respective sites. For example, the greater microbial activity observed in the vicinity of the injection wells for the Utah site is consistent with the biomass distribution determined from analysis of core samples, which shows larger bacterial cell densities for the region near the injection wells. These results illustrate the utility of biotracer tests for in situ characterization of microbial activity (e.g., biodegradation potential), including evaluation of potential spatial variability.

Temporal change in culturable phenanthrene degraders in response to long-term exposure to phenanthrene in a soil column system

Widespread environmental contamination by polycyclic aromatic hydrocarbons (PAH) has led to increased interest in the use of natural attenuation as a clean-up strategy. However, few bioremediation studies have investigated the behaviour of the indigenous PAH-degrading community after long-term exposure to a PAH. In this study, a column packed with sandy loam soil was exposed to a solution saturated with phenanthrene ( approximately 1.2 mg l-1) for a 6-month period to examine the temporal response of the indigenous phenanthrene-degrading community. Initial soil, effluent, and final soil samples were collected and analysed for phenanthrene concentration and culturable phenanthrene degraders. Phenanthrene-degrading isolates were grouped by colony morphology. For each unique group, 16S rDNA polymerase chain reaction was performed, and then sequencing analysis was used to identify the isolate at the genus level. Twenty-five phenanthrene-degrading isolates, potentially representing 19 genera, were obtained from this analysis. Of these, eight genera have not been reported previously to degrade phenanthrene, including Afipia, Janthinobacterium, Leptothrix, Massilia, Methylobacterium, Rhizobium, Sinorhizobium and Thiobacillus. Results indicate that the dominant phenanthrene-degrading population changed over the course of this 6-month experiment. Specifically, the isolates obtained initially from the soil were not subsequently found in either effluent samples or the soil at the end of the experiment. Furthermore, several isolates that were found in the soil at the end of the experiment were not observed in the soil initially or in the effluent samples. This study confirms earlier findings indicating that a diverse community participates in phenanthrene degradation in the environment, and also suggests that the composition of this community is temporally variable.

The influence of substrate and electron acceptor availability on bioactive zone dynamics in porous media

Two approaches were used to investigate the influence of dissolved oxygen (DO) and substrate availability on the formation and dynamics of "bioactive zones" in a water-saturated porous medium. A bioactive zone is defined as a region where a microbial community is sufficiently active to metabolize bioavailable substrates. In the first approach, microbial activity was characterized by monitoring the spatial and temporal variability of DO and aqueous substrate (salicylate and naphthalene) concentrations during miscible-displacement experiments. In the second approach, microbial activity was monitored using multiple fiber optics emplaced in the porous medium to detect luminescence produced by Pseudomonas putida RB1353, a bioluminescent reporter organism that produces light when salicylate (an intermediate of naphthalene degradation) is present. The results of both approaches show that the location and size of the bioactive zones were influenced by in situ DO and substrate availability. When DO was not a limiting factor (i.e., lower substrate input concentrations), the bioactive zone encompassed the entire column, with the majority of the microbial activity occurring between the inlet and midpoint. However, as the availability of DO became limiting for the higher substrate input experiments, the size of the bioactive zone shrank and was ultimately limited to the proximity of the column inlet.

Biodegradation during contaminant transport in porous media: V. The influence of growth and cell elution on microbial distribution

This study investigated the interaction between microbial growth and cell elution, and their influence on resultant microbial distribution between the aqueous and solid phases during solute transport in a sandy, low-organic-carbon-content porous medium. Miscible displacement experiments were conducted with salicylate as the model compound, and with different initial conditions (e.g., substrate concentrations and cell densities) to attain various degrees of microbial growth. For each experiment, salicylate and dissolved oxygen concentrations as well as cell densities were monitored in the column effluent. Cell densities were also measured in the porous medium at the beginning and end of each experiment. Total microbial growth was determined in two ways, one based on a cell mass balance for the system and the other based on total amount of salicylate degraded. For conditions yielding a considerable amount of microbial growth, the majority of the biomass was associated with the aqueous phase (68-90%). Conversely, under minimal-growth conditions, most cells (approximately 60-70%) were attached to particle surfaces. Significant cell elution was observed for most conditions, the rate of which increased in the presence of the substrate. The results suggest that the increase in aqueous-phase cells observed for the experiments exhibiting the greatest growth is associated with the production of new cells, and that under appropriate conditions aqueous-phase biomass can contribute significantly to contaminant biodegradation.

Biodegradation during contaminant transport in porous media: 3. Apparent condition-dependency of growth-related coefficients

The biodegradation of organic contaminants in the subsurface has become a major focus of attention, in part, due to the tremendous interest in applying in situ biodegradation and natural attenuation approaches for site remediation. The biodegradation and transport of contaminants is influenced by a combination of microbial and physicochemical properties and processes. The purpose of this paper is to investigate the impact of hydrodynamic residence time, substrate concentration, and growth-related factors on the simulation of contaminant biodegradation and transport, with a specific focus on potentially condition-dependent growth coefficients. Two sets of data from miscible-displacement experiments, performed with different residence times and initial solute concentrations, were simulated using a transport model that includes biodegradation described by the Monod nonlinear equations and which incorporates microbial growth and oxygen limitation. Two variations of the model were used, one wherein metabolic lag and cell transport are explicitly accounted for, and one wherein they are not. The magnitude of the maximum specific growth rates obtained from calibration of the column-experiment results using the simpler model exhibits dependency on pore-water velocity and initial substrate concentration (C0) for most cases. Specifically, the magnitude of micron generally increases with increasing pore-water velocity for a specific C0, and increases with decreasing C0 for a specific pore-water velocity. Conversely, use of the model wherein observed lag and cell elution are explicitly accounted for produces growth coefficients that are similar, both to each other and to the batch-measured value. These results illustrate the potential condition-dependency of calibrated coefficients obtained from the use of models that do not account explicitly for all pertinent processes influencing transport of reactive solutes.