Temperature Stratification in an Operational Waste-Stabilization Pond

Abstract

Waste stabilization ponds (WSPs) rely on natural processes to improve water quality. WSPs require less capital or operational resources than traditional treatment and are commonly used in rural communities or for secondary disinfection. Typically, WSPs designs use empirical equations without three-dimensional (3D) circulation, thermal stratification, or varying water levels. In the present study, a 3D numerical model (Delft3D version 4.04) was applied combined with observations of an operational WSP for nine months (May–November 2017). After calibrating light attenuation and eddy viscosity, model results were in agreement with temperature profiles at five locations, with an average root mean square errors of 1.3°C. The results indicate minimal effects from inflows temperatures and vertical temperature differences of up to 8.0°C between the surface and bed, which inhibited vertical mixing. The buoyancy frequency, a measure of density stratification, was inversely proportional to wind speed and was used to determine that the threshold wind speed required for mixing varied seasonally between 1 and 3 m/s. The results from a simulated tracer indicate that empirical methods for estimating hydraulic retention times—the average length of time water remains in a WSP—are inaccurate and varied by an average of 22% from the simulated tracer.