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.