Operational Limitations of Arctic Waste Stabilization Ponds: Insights from Modeling Oxygen Dynamics and Carbon Removal

Abstract

Presented here is a mechanistic model of the biological dynamics of the photic zone of a single-cell arctic waste stabilization pond (WSP) for the prediction of oxygen concentration and the removal of oxygen-demanding substances. The model is an exploratory model to assess the limiting environmental factors affecting treatment performance in arctic WSPs. A sensitivity analysis was used to provide a quantification of the relative uncertainties of parameters that exist within the described modeling framework. The model was able to qualitatively reproduce mesocosm experiment trends in phytoplankton growth, dissolved oxygen concentration, and the reduction of carbonaceous biochemical oxygen demand on Day 5 (CBOD5). These results demonstrated that CBOD5 reduction and oxygen state are very sensitive to organic loading regimes at low temperatures (5-15°C). The sensitivity analysis identified that it was the difference in phytoplankton growth rates, and the associated change in photosynthetic oxygen production, that mainly contribute to creating differences in CBOD5 removal rates and the development of aerobic conditions. The model was also sensitive to atmospheric aeration rates at low temperature, providing further evidence that low oxygen availability limits the treatment of CBOD5 in cold-climate WSPs. During the development process, it was discovered that common formulations of depth-integrated phytoplankton growth performed poorly for the arctic system to be modeled, which was a quiescent eutrophic environment. This paper presents a new phytoplankton growth formula within the paradigm of a poorly mixed eutrophic system that may find utilization in other eutrophic, colored, or turbid systems. The novel aspect of the approach is that the depth-integrated phytoplankton growth function was formulated upon the premise that the phytoplankton would be capable of orienting themselves to optimize their growth under poorly mixed conditions, and the average growth rate of the phytoplankton population must decrease as crowding puts pressure on shared resources. The general agreement of the model with the experiments, combined with the simplicity of the depth-integrated box model, suggests there is potential for further development of the model as a tool for assessing proposed arctic WSP designs. The sensitivity analysis highlighted the uncertainty and importance of the parameterization of bacterial and phytoplankton physiology and metabolism in WSP models.