Validation and Application of a Simple Model for Circular Secondary Settling Tanks

A simple model is presented for the dynamic calculation of the sludge blanket height and concentrations of suspended solids (diluted, sludge blanket and return sludge) in circular secondary settling tanks. The model combines a mass balance equation with three empirical equations, which account for the main processes. These equations form a non-linear, dynamic system, which cannot be solved explicitly; thus, a trial-and-error method is adopted. Field data were obtained from a full-scale treatment plant in transient and steady state conditions and were used in the calibration, the verification and the application of the model. Model calibration involved the determination of only two coefficients, which were found to be relatively constant in various operating conditions. Model verification and application were successful showing a satisfactory agreement with field data. A discrepancy of the model to over-predict sludge blanket height during sudden hydraulic overloads was noted and explained.