Validation of a Relative Permeability Model for Bioclogging in Unsaturated Soils

Biological clogging of unsaturated soils is an important process that can lead to the development of a biomat and failure of biofilters used to treat various wastewater streams. Several conceptual models have been developed to simulate clogging in saturated soils; however, efforts to develop similar models for unsaturated soils have been limited. Recently, Mostafa and Van Geel proposed three conceptual models to simulate bioclogging in unsaturated systems. These models included the impact of biomass growth on the relative permeability term for unsaturated flow. The models were incorporated into a one-dimensional unsaturated flow and transport numerical model that simulates biological clogging in which microbial growth was simulated using Monod kinetics. The conceptual models have not been validated with experimental data. In this study, column experiments were conducted to study the clogging process in three different sand soils; filter media sand, concrete sand, and septic bed sand. Monod kinetic parameters were also evaluated in a separate experiment for the same feed solution used in the column study. The aim of this study was to validate the conceptual models proposed by Mostafa and Van Geel. Significant improvements to the flow and transport numerical model were required to accommodate the laboratory conditions. Improvements included implementing a minimum relative permeability of the biomat layer and a more detailed description of the biomass structure, which includes active biomass, extracellular polymeric substances (EPS), and an inert fraction. Comparisons between experimental data and numerical simulations indicated that the improved conceptual model for relative permeability appears to be appropriate for modeling bioclogging in the experiments considered in this study.