Impact of Single Fracture Roughness on the Flow, Transport and Development of Biofilms

This study examined the impact of systematically increasing roughness in a single fracture and the effects on the hydraulic properties, solute transport and biofilm development in those fractures. Biofilms were modeled using a newly developed two-dimensional Lattice-Boltzmann Method (LBM) fluid flow model with the additional capability to simulate substrate transport using a discrete Random Walk (RW) algorithm. The discrete modeling methods, including LBM, RW and Cellular Automata (CA) for biofilm modeling, were able to capture small scale effects that emerged into large scale behaviour of biofilm growth and structure development. The two-dimensional fluid flow model using LBM was developed and validated against analytical solutions for simple cases of parallel plate flow and a backward facing step. Fracture flow results showed a pronounced deviation from predicted cubic law flow rates as expected and previously reported in the literature. Two-dimensional fracture cross-sections were synthetically produced to control the fracture roughness and results from the LBM model extended the three-zone non-linear model of hydraulic behaviour from porous media to include fractured media. Simulations with the LBM model showed that secondary flows, or flows not contributing to bulk flow, could occur at Reynolds numbers lower than previously reported in the literature.;A numerical solute transport method was added to the LBM flow simulations to model solute transport in fractures of increasing roughness. Here the delay of breakthrough curves, including initial, peak and tail were associated with the development and growth of eddies.;A biofilm growth model, implemented with a discrete CA method, was used to capture the local small scale effects of fracture roughness, hydraulics and substrate transport on biofilm development in terms of biomass and bio-structure. In two dimensions, biofilm growth was controlled by clogging, which occurs earlier at lower biomass levels in rougher fractures. Sensitivity analysis of biofilm development assuming a variation in bacteria shear strength was completed. Lower biofilm shear strength does not allow for any biofilm development within the fracture, however, when the shear strength is increased above a threshold, dependent on Reynolds numbers and fracture roughness, biofilms begin to develop. Above the shear strength threshold the same general trends of biofilm development hold compared to the results when no sloughing due to shear is considered.