Three-dimensional Investigation of Retention Time Distribution of Waste Stabilisation Ponds

Waste stabilisation ponds (WSPs) use natural microbiological, photosynthetic, biochemical, physico-chemical and hydrodynamic processes to treat wastewater. They require little technical attention during operation and are less demanding in terms of construction cost and energy consumption than other engineered wastewater treatment systems. Practical engineering experience and research over the past few decades have established that hydrodynamics is crucial in determining the treatment efficiency of WSPs. After reviewing a large number of pond systems operated in Australia, Wood et al. (1995) stated that many systems were found to operate below an optimal level due to a reduced retention time of wastewater. It is therefore required that the hydrodynamic retention time distribution be precisely understood so that the hydrodynamic behaviour and the overall treatment efficiency of WSPs can be accurately evaluated. A substantial number of models have been developed to look into various hydrodynamic aspects of WSPs. However, most of the work has been limited to one or two dimensions due to computational capabilities. The present study aims to establish a generic model fully describing the three-dimensional hydrodynamic behaviour of WSPs. MIKE by DHI is chosen as the modelling tool considering the following favourable features: (1) applying a non-hydrostatic engine to simulate unsteady three-dimensional flows; (2) taking into account density variation, bathymetry and external forces such as meteorology, tidal events and currents; (3) incorporating several modules such as hydrodynamic, transport and ecological, which allows integrated modelling of hydrodynamic and biological processes for a more complete analysis and prediction of wastewater treatment. A typical pond model with the dimensions of 50 m (length) by 20 m (width) by 1.5 m (depth) was adopted. The model was validated against an empirical formula for the wind-driven circulation in a tank. An optimised meshing scheme of 1 m by 1 m horizontally and 7 layers vertically was determined to achieve the best computational performance-time ratio. Subsequently, the validated model was employed to formulate WSP retention time analysis. A parametric study was conducted in terms of varying length to width ratio (L/W), inlet/outlet positioning, inlet direction changes, varying wind speed and direction. It was found that: • Pond L/W ratio has a significant influence on pond retention time. It was noted that a larger L/W ratio is associated with longer retention time. • Winds are a predominant factor in WSP performance, especially wind direction. The retention time of wastewater was found to be longer due to the circulation in the transverse direction generated in the pond when wind direction was perpendicular to the inflow direction. • The inlet/outlet position change and the inflow direction variation present a rather mild influence on pond retention time in comparison to pond L/W ratio and winds. This is partially due to the local boundary effect in the modelling. Another possible reason can be associated with the pond configuration specified in this study. This study performs a systematic investigation of WSP retention time based on three dimensional models. It is expected to establish a modelling framework for extended WSP studies. The parameter analysis presented in this work will be further developed to address the interrelated effects of multiple parameters on WSP hydrodynamics. Ultimately, this study will lead to the development of a 3D model which will incorporate the fate of pathogens linked to hydrodynamic parameters. This model will allow overall efficiency evaluation of the design, operation, retrofit and maintenance of WSPs.