Biological clogging in unsaturated soils is an important concern in the design of biofilters that are used to treat wastewater in rural areas. Several conceptual models have been developed to simulate biological clogging in saturated flow systems but limited research has been performed to develop similar conceptual models in unsaturated soils. This study developed three conceptual models for biological clogging in unsaturated soils. The model formulations varied from microscale to macroscale and from analytically derived to empirical equations. They were all formulated based on the approaches proposed by Burdine and Mualem to estimate the relative permeability based on the effective water saturation and the soil moisture curve. A one-dimensional unsaturated flow and transport code was developed, which incorporates Monod kinetics to simulate the biodegradation of an organic substrate. The three conceptual models that were developed relate the relative permeability to the microbial growth term in the unsaturated flow equation. The models were implemented in a numerical model to illustrate the impact of microbial growth on the biological clogging of unsaturated soils. Also the effect of continuous loading versus pulse loading was simulated to illustrate the difference between the loading scenarios on the clogging process within biofilters.

No abstract

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.

No abstract

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. Septic beds and peat filters used to treat septic tank effluent are important applications. Several conceptual models have been developed to simulate clogging in saturated soils; however, limited effort has been conducted to develop similar models for unsaturated soils. Different conceptual models have been proposed to simulate biological clogging in unsaturated systems. These models include the impacts of biomass growth on the relative permeability term for unsaturated flow, but limited experimental data have been used to validate these models. In this study, column experiments were conducted to study the clogging process in loose and dense peat, filter media sand, and septic bed sand. Experimental data indicated that the pore structure of the peat, in comparison to two commonly used sands for septic drainage fields, allowed the biomass to distribute itself over a greater depth within the peat biofilter and delayed the formation of a biomat at the surface and eventual clogging of the filter medium.