Exploring Positive Feedback Between Biofilm Growth and Mixing in porous media: Insights from Darcy-Scale Flow-Through Column Experiment

Biofilms in porous media host microbial communities that play a central role in the degradation of nutrients and Contaminants of Emerging Concern, significantly enhancing water quality through contaminant removal. Current research focuses on strategies to prevent bio-clogging in natural and engineered systems while promoting controlled and widespread biofilm growth. This dual approach aims to maintain permeability while leveraging biofilm activity for bioremediation purposes. Biofilm growth dynamics and spatial distribution are shaped by factors such as the mixing of nutrients (electron donors and acceptors), flow and transport processes, and the inherent heterogeneity of the porous medium, although these processes are not yet fully understood. In this context, we seek to understand how mixing affects biofilm growth and vice versa, as well as whether it is possible to maximize the bio-reactive zone while minimizing clogging.

To achieve this, we conducted a flow-through experiment in a 60 cm homogeneous sand-packed column. By injecting multiple sequences of electron donor and electron acceptor solutions, we created periodic reactive mixing zones with complementary reactants to stimulate microbial activity. This setup mimics a fluctuating geochemical environment, resulting in multiple mixing areas that evolve in both space and time. The interplay between limited nutrient supply and biofilm development turned biofilm growth into a process driven by mixing and transport dynamics. The monitoring system enabled us to indirectly assess integrated microbial activity alongside overall flow and transport behavior. We recorded the temporal evolution of (i) downstream redox potential, (ii) differential pressure, and (iii) tracer breakthrough curves. Data suggest an early onset of microbial activity, inferred from a rapid decrease in redox potential, as well as a gradual decline in hydraulic conductivity and an increase in BTC tailing, indicating enhanced immobile porosity due to biofilm growth. Moreover, the spatial and temporal evolution of microbial activity inside the column—directly linked to the evolution of mixing dynamics—was characterized through semi-continuous measurements of pH and CO₂ at ten non-invasive sensor spots. Results indicate an expansion of the bio-reactive zone (i.e., the mixing zone) over time, likely driven by increased dispersion, and a spatial shift of the area with the highest activity over time.