Temporal and spatial variation of physical, biological, and chemical parameters in a large waste stabilisation pond, and the implications for WSP modelling.

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.