A pore scale study of substrate mixing and biological degradation in porous media

Groundwater remediation projects are increasingly using in situ treatment methods such as bioremediation. Often, degradation occurs exclusively within a narrow region along the contaminant plume fringe where both contaminant and sufficient nutrients are present. This work develops and validates a biomass growth model under mixing conditions. The model system is a 2D porous medium with a specified average flow rate. Two substrates enter the porous medium parallel to each other and completely unmixed, and microbes degrade them after they mix. The lattice Boltzmann method is used to simulate Stokes flow through the porous medium. Substrate transport and reaction is modeled with the advection-diffusion-reaction equation, using the finite volume method. Finally, a cellular automaton algorithm models biomass growth and spreading in the porous medium. The most significant parameters that enable reasonable agreement with experimentally observed biomass are the biomass shear strength and the maximum substrate utilization rate.; The effect of biomass on substrate degradation rate is explored by comparing results from the developed biomass to two simple biomass morphologies, a uniformly distributed biomass and a biofilm around all solids. For slow reaction rates and fast flow rates, degradation rates become linearly dependent on the total amount of biomass in the reaction zone. Because flow is reduced in pores containing biomass, the limiting substrate is depleted at decreased distances from the inlet so that the reaction zone moves further from the system centerline in the transverse direction, which may result is decreased degradation rates a long distances.; Finally, degradation rates obtained from a simple continuum model with uniform flow field are compared to results from the full pore scale model. Substrate degradation rates are consistently underpredicted when transverse dispersion coefficients obtained from nonreactive transport simulations are used for reactive transport. The continuum model with values of transverse dispersion fitted to the reactive case is able to match downstream degradation rates from the pore scale model, but not rates near the inlet. Adjusting the reaction rate constant in the continuum model does not achieve satisfactory agreement with pore scale results.