Computational fluid dynamic modelling of a septic tank

A review of the literature has shown the septic tank component of an onsite wastewater treatment system to be a complex reactor having zones of differing biological and chemical composition. This variability is anticipated to translate into non-uniform partitioning of wastewater borne contaminants within the environment of the septic tank. As such the relative exposure of contaminants to the various partitioning regimes becomes of interest from a fate and contaminant transport perspective.; To facilitate a detailed understanding of the flow fields that exist within a septic tank the applicability of computational fluid dynamics (CFD) modelling to the situation was assessed. Models were developed in two and three dimensions for full-scale and lab-scale tanks respectively. It was found that when the inlet and outlet geometries were approximated as rectangular, grid and time independent solutions were obtainable. Removing these approximations introduced a more complex modelling scenario where grid independence was not achievable within available computational resources.; Tracer experiments were conducted in the laboratory to support model verification. With these experiments diffusion was found to play a limited role in the movement of dye through the tank. CFD simulations of these experiments produced mixed results. While there were a number of similarities in the bulk movement of dye through the laboratory and numerically simulated tanks, from a quantitative perspective the results were unconvincing. Time to peak and maximum tracer effluent concentrations were found to be similar between the two environments, however effluent concentrations as predicted by the numerical model were not consistent with those measured in the lab.; The effect of boundary configuration was made apparent during the course of this work. Tracer dye and discrete particles were found to behave differently depending on the geometrical configuration of the inlet and outlet. This indicated significant differences in the flow fields that had developed in relation to the configuration of boundary conditions.; Computational fluid dynamics has been found to exhibit the capacity to simulate the low velocity multiphase flows that would be associated with the normal operation of a septic tank. Techniques have been developed that may eventually be used to determine the relative exposure of a parcel of contaminant to the different partitioning regimes existing within a septic tank. As such CFD should be considered a useful tool for informing fate and contaminant transport modelling, which in turn may have an impact on guiding environmental waste management policy.