ANALYTICAL SOLUTION FOR DENSITY CURRENTS IN SETTLING BASINS

Settling basins are extensively used to remove suspended solids from effluents. Experimental information on the magnitude of density currents in settling basins shows that the velocities of sediment-induced density currents near the upstream region of the settling basin are one order of magnitude larger than the flow-through velocities in the basins. Therefore, performance of settling basins is controlled, primarily, by density currents and not by flow-through velocity. An analytical model is described to predict density currents induced by settling solids in rectangular sedimentation basins. The conservation equations for fluid and sediment mass, fluid momentum, and fluid energy, on integration over the flow depth using similarity profiles for concentration and velocity, are reduced to a system of ordinary differential equations solved analytically. Analytical expressions for longitudinal variations of sediment concentration and velocity along the settling basin are presented. The density-induced velocity decreases linearly along the length of the basin for both laminar and turbulent regimes. The decay of sediment concentration is quadratic for laminar flows and cubic for turbulent flows. The analytical results compare reasonably well with the available experimental data.