The influence of physical heterogeneity on microbial degradation and distribution in porous media

Abstract

Abstract. Intermediate-scale experiments (meter-long, two-dimensional flow cell) were performed with aerobic biodegradation of benzoate substrate in physically heterogeneous (bimodal inclusive) media. Clastic heterogeneities were represented in a quasi-twodimensional field, with low-conductivity inclusions embedded in a high-conductivity sandy matrix. The two media had similar pore-scale dispersivities but the conductivity ratio (•1:50) incurred macrodispersive spreading in the longitudinal direction. The highconductivity sand was uniformly inoculated with Pseudomonas cepacia sp., and a pulse input of substrate and chloride ion tracer were evaluated. Degradation and growth were oxygen-limited under nonlinear dual-Monod kinetics and controlled by spatial and temporal variations in nutrient flux. The low-conductivity inclusions created regions of slow transport that prolonged the dual availability of both oxygen and substrate, which in turn enhanced microbial growth in these regions. Bacterial detachment was significant, and the fivefold increase in biomass due to growth was entirely accounted for in the aqueous effluent which displayed a complicated nonlinear breakthrough curve. Highresolution deterministic modeling was applied to simulate the intermediate-scale experiment, with parameters of the relevant constitutive relations calibrated independently through batch and small-scale column experiments. Parameter fitting to match flow cell data was avoided. This approach was taken in order both to test the predictive modeling capability as it would necessarily be used in a field application and to avoid the a priori assumption that all relevant processes were adequately represented in the respective constitutive theories. Analyses of the fit between the independently calibrated model and the flow cell data were then used to isolate processes for further experimental study. This iterative experimental/modeling approach identified processes that contributed (surprisingly) to biodegradation in heterogeneous media and yet are not currently incorporated in most mathematical models: (1) buoyancy effects associated with very small solution density variations, amplified in heterogeneous media, and (2) dynamic biological processes associated with growth, namely, endogenous respiration, cell division partitioning to the aqueous phase, and active adhesion/detachment that are strongly coupled to the transport of dissolved nutrients or microorganisms.

Processes in Microbial Transport in the Natural Subsurface

“…1, [169]), similar to observations in field bioremediation efforts (US DOE, 1993). Column experiments have suggested that the response observed in [169] may be cell-division mediated transport, a mechanism long recognized in the microbiology literature [60,99,161,179]. Cell-division mediated transport has also been referred to as mother-daughter or shedding cells and is when the ''mother'' cell, attached perpendicular to the mineral surface, grows, divides, and the ''daughter'' cell is released into the aqueous phase [163].…”

“…Therefore, in unstructured models the biomass is a fully penetrable volumeless component which assumes that a linear relation exists between mass of substrate consumed and mass of biomass produced and that no diffusion limitations affect the transfer of substrate mass from solution into the biomass. This approach has been taken in model construction [25,144,159,162], in column studies that focus on bacterial transport [69,120] and in intermediate-scale flow cell studies that focus on active degradation and growth and coupled transport [130,169]. For instance, Macquarrie et al [162] used this approach in treating biomass involved in aerobic degradation as a volumeless species undergoing transport, with equilibrium partitioning of biomass between aqueous and attached phases.…”

“…In the so-called unstructured biophase approach, no structural presumption is imposed on the biophase [22,23,162,180,191] and the biomass is treated as a suspended, but kinetically sorbing/desorbing species [169,191]. Therefore, in unstructured models the biomass is a fully penetrable volumeless component which assumes that a linear relation exists between mass of substrate consumed and mass of biomass produced and that no diffusion limitations affect the transfer of substrate mass from solution into the biomass.…”

“…This model may be viewed as a modification of the colloid filtration theory (CFT) approach noted above, wherein a linear detachment process is included (no such detachment occurs in original CFT), and wherein dependence of the rate coefficient for attachment on porewater velocity is ignored. Murphy et al [169] extend the linear reversible model to include non-linear dependence of the rate coefficients on ionic strength of solution, manifest in intermediate-scale experiments. Tan et al [120], Lindqvuist et al [69], and Saiers and Hornberger [105] all introduce the classical site-saturation limiting factor on the attachment rate coefficient, in order to account for potential depletion of available surface sites as attached microbe densities increase, which may occur when aqueous microbes cannot attach to attached microbes.…”

“…Attachment of bacteria in the natural subsurface via filtration is often (but not always; e.g., [169]) treated using colloid filtration theory (CFT) [94], which posits the kinetic rate of attachment as kinetic attachment rate ¼ 3 2…”