Sediment Diffusion in Primary Shallow Rectangular Clarifiers

Abstract

The steady, diffusive transport of discretely settling, single‐sized solids through a primary, shallow, rectangular clarifier is modeled in this study. The analysis proceeds in the assumed absence of fluid stratification, coagulation, unsteadiness, resuspension, wind, and short circuiting, although qualitative attention is given to some of these real‐world phenomena. The inlet zone is identified as the dominant source of tank turbulence and an independent estimate of the resulting turbulent diffusivity is put forth, based on crude turbulent transport theory and data. A simple, analytical model of sediment transport through the settling zone is derived, and the resulting removal efficiencies are well‐established functions of conventional design parameters. The predictions are successfully tested against laboratory data without calibration. Finally, a resuspension criterion yields a useful design constraint which is cast into a sizing algorithm for uniform sediment loads, predicted on the derived conclusion that shallow tanks are the most economical and efficient.