Contaminant transport processes in onsite waste disposal systems
Doctor of Philosophy
School of Civil, Mining and Environmental Engineering
Bunsri, Thidarat, Contaminant transport processes in onsite waste disposal systems, Doctor of Philosophy thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, 2006. http://ro.uow.edu.au/theses/642
Groundwater contamination is an important environmental problem to the present day. The groundwater pollution caused by pathogens from human excreta, particularly in developing countries is directly associated with the lack of safe drinking water and proper sanitation. A septic system is the simplest and the most economic onsite sanitation unit. It includes a tank and a drainage field. A septic tank is designed to receive domestic wastewater, especially black and grey, or a combination of both. Due to the worldwide spread of septic tank usage, it was estimated that they disposed of the largest volumes of wastewater into the ground through their drainage fields. Poorly discharged effluent often contains high concentrations of organic carbon, nutrients and pathogens. Therefore, there is a need to develop a model for estimating the migration of contaminants in a drainage field. The aim of this study was to develop a model for the transport and fate of pollutants discharged by a septic tank below the ground surface.
Firstly, a conceptual model was developed to describe removal mechanisms and the fate of contaminants such as chemical oxygen demand (COD), nitrate, phosphate and Escherichia Coli (E.Coli) in unsaturated soil conditions. The governing equations were formulated to support the established conceptual model. The migration of contaminants depends on the advection-dispersion transport and retardation processes that can be evaluated using a modified form of the Richards equation. The retardations are mainly due to adsorption and biodegradation processes that are traditionally described using the multiplicative Monod’s equations and isotherm equation, respectively. Secondly, a mathematical model was developed by modifying these governing equations. The mathematical model is in the form of hyperbolic/parabolic partial differential equations with strong nonlinearity due to pressure head dependencies in the specific moisture capacity, hydraulic conductivity terms and complex retardation processes. In order to solve the mathematical model, a numerical approach was employed. The developed model was solved numerically using the Galerkin finite element method. The numerical model was coded with MATLAB software. Finally, the developed model was calibrated using the writer’s laboratory data and other data obtained from case studies.
The movement of water and wastewater and the retardation of contaminants were investigated experimentally and the results of these experiments were compared with the simulations of the developed model. Two types of porous media were used, sand and topsoil. Sand is a uniform and non-reactive porous medium which provides an effective permeability for infiltration. Topsoil is a non-uniform and reactive porous medium which provides various rates of retardation. The infiltration experiments were conducted using laboratory and pilot scale columns of 20 and 120 cm effective heights, respectively. Laboratory scale sand and soil columns were used to determine the hydraulic properties of sand and soil, the movement of water with various boundary conditions such as gravitational, static equilibrium of capillary and infiltrationredistribution flows. A pilot scale soil column was used to examine the transport of contaminants in field conditions.
The computational code used in the advective-dispersive transport (Richards’ equation) was applied to the data obtained from the laboratory scale soil columns. The simulation results for the hydraulic pressure head and moisture content in both sand and soil columns matched the observed data well. The contaminant transport model could satisfactorily estimate the contaminant concentration and retardation zone. The simulation results indicate that all contaminants except E.Coli were reduced significantly across a 15 cm depth (elevation of 105 cm) whereas the E.Coli reduction zone was observed within 10 cm depth (elevation of 110 cm) of the soil column.
Eight case studies were used in the model verification processes. The developed model could effectively predict the profiles of pressure head and moisture content observed in infiltration and infiltration-redistribution systems. Furthermore, the developed model could predict the profiles of non-reactive and reactive contaminant concentrations presented in all the case studies. This indicates that the developed model is an effective alternative tool for predicting the migration of contaminants in the ground underneath a septic effluent drainage field.
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