Experimental and numerical investigations of changes in flow and solute transport processes in porous media affected by bioclogging

Tracer studies in laboratory column and sandbox experiments were performed to investigate the processes of bioclogging. Model simulations of the experiments investigated the coupling between the bioclogging process, microbial migration, and attachment/detachment of biomass, and examined the effect of these processes on the flow pattern and solute transport in porous media. Tracer tests conducted in three column and three sandbox experiments revealed that the flow pattern, and hereby solute transport, changed due to bioclogging from being uniform to non-uniform exemplified through increasing hydrodynamic dispersion, diffusive mass transfer between the mobile phase and the immobile biomass, and development of preferential flow paths. The bulk hydraulic conductivity decreased by up to three orders of magnitude in the 5 cm long columns, and by two orders of magnitude in the 44 cm long sandbox. The most significant reduction in the hydraulic conductivity occurred quickly (within the first 20 days of the experiments). The changes in tracer transport revealed a reduction in the porosity down to 70%-80% of the initial value, i.e., only 20-30% of the pore space was filled with biomass. These observations indicate that bioclogging is controlled by biomass in micro-colonies plugging the pore throats, as biofilm models have revealed that a significantly larger amount of biomass as a biofilm (>90% of the porosity) is needed to produce the observed decrease in the hydraulic conductivity. The sandbox experiments revealed that microorganisms migrated upgradient toward the substrate source with a velocity of about 0.5 cm/day. Model simulations using a numerical bioclogging model, which was extended to include migration of microorganisms by random motility and chemotaxis, were able to reproduce the observed microbial migration and the subsequently upgradient bioclogging. In the model it was assumed that microorganisms migrated in pores with stagnant water or close to the sand grain surface where the pore water velocity is reduced. In the model simulations it was necessary to assume that the reduced pore water velocity affecting the microorganisms was 1% of the average pore water velocity. The migration of microorganisms is sensitive to the detachment rate of biomass from the solid to the aqueous phase, which depends on the substrate concentration. Sandbox experiments with different substrate concentrations revealed a faster microbial migration with a higher substrate concentration.