A thermal profiling study was undertaken at four depths at each of nine sites, and at the inlets and outlets of a large waste stabilisation pond (WSP). Results were collected simultaneously using a network of 42 thermistors and dataloggers. Profiles at each site were categorised as either "stratified" or "unstratified", and persistence analysis was used to determine the frequency and persistence of stratification events at each of the nine sites. Stratification was found to persist most strongly at the site furthest upwind in the WSP, with respect to prevailing wind during the study, leading to the conclusion that stratification induced short-circuiting will be greatest in this region of the WSP. A computational fluid dynamics (CFD) model was constructed of the WSP, including an energy balance to predict the bulk stratification gradient in the pond. Environmental conditions and WSP inlet temperature during one day in June 2001 were used as boundary conditions. The pond thermal profiles measured during the profiling study, together with outlet temperature during the day, were used to validate the CFD model results. The model predicted mean pond temperature with a high degree of accuracy (r2 = 0.92). However it was evident that even modest winds (> or = 1.5 m/s) partially broke down stratification, leading to poor prediction of the gradient by the CFD model, which did not directly account for the impact of wind shear stress on mixing in the WSP.

Over the past fifty years, considerable research in waste stabilization pond operation has led to the development of a number of models used to describe the hydraulic regime and predict treatment efficiency. Models range in complexity from plug or completely mixed simplifications to computational fluid dynamics (CFD) models which are able to predict flow hydraulics at a local level. Information about the exit age of pond effluent can be used to estimate pollutant decay. However, a mechanistic approach to understanding pond operation highlights the importance of knowing both the time and spatial history of pond effluent. A CFD model of a large pond system was constructed to demonstrate various hydraulic scenarios under different boundary conditions. Two scenarios were compared to visually demonstrate the effects of differing hydraulic conditions. Typical mechanistic models were applied to each condition to quantify biological differences. This simple example indicates that integrating biological and localised flow models will lead to a more holistic understanding of pond operation and treatment efficiency.

The spatial and temporal variation of physical, chemical, and biological parameters was determined, in summer and winter, at nine sites in a large (112 ha) waste stabilisation pond (WSP) at the Bolivar Wastewater Treatment Plant. Each site was extensively sampled over the course of one day, with the nine sites sampled over successive days at exactly the same times of day, progressing in the direction of bulk flow through the pond. Analyses of covariance were used to test the independent impact of site and climate on the way in which the mean values and stratification gradient of the physical, chemical, and biological parameters varied diurnally at each site. In both winter and summer studies there was a very strong correlation at all sites between changes in temperature, pH and dissolved oxygen (DO). Mean pond temperatures were higher in summer than winter, and thermal stratification was more common in summer. In summer, during the day at each site, concentrations of chlorophyll-a, DO, suspended solids and pH increased with higher solar radiation levels. This relationship was less evident in winter. There was no systematic depth or temporal variation identified in either the summer or winter study for the broad range of chemical parameters measured. Mean values for these parameters, and to a lesser extent their stratification gradients, increased by varying extents throughout the day at the different sites in both summer and winter, irrespective of changes in climate when the different sites were sampled. Sites nearer the inlet to the WSP recorded lower NH4N and higher NO2N and NO3N concentrations than the rest of the WSP. This was indicative of nitrification. Somewhat surprisingly, high DO concentrations were also recorded at these sites near the inlets. Computational fluid dynamics (CFD) modelling, incorporating the predominant wind conditions, offers a rationale for these observations. Recirculation was evident, which may increase the residence time for the slow growing autotrophic nitrifying bacteria and recirculate oxygen rich water around these sites - conditions which would enhance nitrification. Understanding the effect of these variations, overlaid by the influence of hydraulic and temporal scenarios, assists in developing a mechanistic understanding of pond operation.

A cost-effective risk-based system was developed for assessing the performance and potential environmental impact of a large number of geographically dispersed pond systems, where cost and logistical issues prevent direct monitoring. In the process, a range of risk functions were calculated for each site to take into account pond performance, receiving environment, influent quality, surrounding land use and system size. Pond performance was estimated using traditional design equations, including Monte Carlo analysis to account for uncertainty in boundary conditions. The calculation of combined risk functions for all systems enabled the quantitative ranking of systems, which can be used to prioritise limited sampling resources.