THE INFLUENCE OF PHYSICAL HETEROGENEITY ON MICROBIAL-DEGRADATION AND DISTRIBUTION IN POROUS-MEDIA

Intermediate-scale experiments (meter-long, two-dimensional flow cell) were performed with aerobic biodegradation of benzoate substrate in p hysically heterogeneous (bimodal inclusive) media. Clastic heterogenei ties were represented in a quasi-two-dimensional field, with low-condu ctivity inclusions embedded in a high-conductivity sandy matrix. The t wo media had similar pore-scale dispersivities but the conductivity ra tio (similar to 1:50) incurred macrodispersive spreading in the longit udinal direction. The high-conductivity sand was uniformly inoculated with Pseudomonas cepacia sp., and a pulse input of substrate and chlor ide ion tracer were evaluated. Degradation and growth were oxygen-limi ted 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 fivef old increase in biomass due to growth was entirely accounted for in th e aqueous effluent which displayed a complicated nonlinear breakthroug h curve. High-resolution deterministic modeling was applied to simulat e the intermediate-scale experiment, with parameters of the relevant c onstitutive relations calibrated independently through batch and small -scale column experiments. Parameter fitting to match flow cell data w as avoided. This approach was taken in order both to test the predicti ve modeling capability as it would necessarily be used in a field appl ication and to avoid the a priori assumption that all relevant process es were adequately represented in the respective constitutive theories . Analyses of the fit between the independently calibrated model and t he flow cell data were then used to isolate processes for further expe rimental study. This iterative experimental/modeling approach identifi ed processes that contributed (surprisingly) to biodegradation in hete rogeneous media and yet are not currently incorporated in most mathema tical 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 respi ration, cell division partitioning to the aqueous phase, and active ad hesion/detachment that are strongly coupled to the transport of dissol ved nutrients or microorganisms.