Quantifying the sources of uncertainty in calculating the limiting flux in secondary settling tanks using iCFD

Solids-flux theory and state point analysis (SPA) are used for the design, operation and control of secondary settling tanks (SSTs). The objectives of this study are to assess uncertainties, propagating from flow and solids loading boundary conditions as well as compression settling behaviour to the calculation of the limiting flux (J_L) and the limiting solid concentration (X_L). The interpreted computational fluid dynamics (iCFD) simulation model is used to predict 1-D local concentrations and limiting solids fluxes as a function of loading and design boundary conditions. A two-level fractional factorial design of experiments is used to infer the relative significance of factors unaccounted for in conventional SPA. To move away from using semi-arbitrary safety factors, a practical approach is proposed to calculate the maximum SST capacity by employing a factor of 23% and a regression meta-model to correct values of J_L and X_L, respectively – critical to abate hydraulic effects under wet-weather flow conditions.