This thesis was part of the Oxley Creek Treatment Wetland (OCTW) Project. The primary research aim of the OCTW Project was to produce a coherent design and performance model for Horizontal Subsurface Flow Treatment Wetlands (HSSFTWs). The first stage of experimental work at the wetland, and the focus of this thesis, was to create a solid baseline of data about wastewater flow patterns on which to build the remaining experimental work and the model. Therefore, the primary objectives of this thesis were:
• To measure the distribution of wastewater residence times and wastewater velocities in the wetland beds both before and after the growth of the plants.
• To determine the effects of the plants and the gravel (bed media) sizes on the wastewater residence times and on the wastewater velocities within the beds.
• To model the wastewater residence time distributions.
This thesis represents the first time that such a comprehensive set of wastewater residence time distribution studies, with supporting hydraulic conductivity studies and wetland characterisation, have been undertaken for a HSSFTW. Initial characterisation of the wetland showed that the plant rhizomes and roots increased the gravel surface levels in the planted sections of the beds by about 2% of the bed depth, and would have contributed to a decrease in the overall porosity of the beds. However, the porosity of the bottom halves of the beds was only found to be about 1 % higher than the porosity of the top halves of the beds. This finding was supported by consideration of the vertical distribution of plant rhizomes and roots and the settling of the unplanted sections of the beds. Evapotranspiration rates from the planted beds were found to be significant and were therefore considered in the water balances undertaken during the wastewater residence time distribution studies.
The residence time distribution studies showed that there was some wastewater dispersion with flow through the gravel sections of the beds prior to planting, and that this dispersion increased after planting, particularly in the beds containing the larger gravel sizes. However, prior to planting there was little correlation between the wastewater velocity and the level in the beds, and there was a random variation of velocities at each of the three sample point levels. After the growth of the plants the average wastewater velocities in the bottom halves of the planted beds containing the larger gravels were found to be 2 to 3 times higher than the velocities in the top halves of these beds. The porosity differences between the top and bottom halves of the beds did not fully explain these variations in wastewater velocities. However, there was a correlation between the velocity distributions and the hydraulic conductivities of the bottom and top halves of the beds. This suggests that the plants may have increased the resistance to wastewater flow through the top halves of the beds containing the larger gravels. The most probable explanation for this, is that because the plant roots and rhizomes have a higher surface area per unit volume, than the larger gravels, they caused an increase in the viscous drag between the wetland beds and the wastewater flowing through them. The overall results of these studies show that the size of the gravel (bed media) and the presence or absence of plants are both important factors in determining the wastewater velocities within the beds and the wastewater residence time distributions.
Initial modelling of the wastewater residence time distributions showed that a single parameter model, such as the Tanks in Series model, did not adequately describe the tracer response curves. Therefore a multiparameter model, which incorporated main flow channels and zones of diminished mixing was used to characterise the wastewater residence time distributions. This is the first reported use of this type of model for HSSFTWs, and it was shown to adequately simulate the wastewater residence time distributions for the unplanted bed and the planted beds containing each of the gravel sizes. The average overlap between the experimental and the model tracer response curves was 93 %.
Finally, the OCTW, was planted with one species, Phragmites australis, and contained three different sizes of gravels (with nominal diameters of 5, 20 and 40 mm). However, it is probable that the trends observed in this work are more widely applicable.