UNDERFLOW GEOMETRY IN SECONDARY SEDIMENTATION

Previous work by the authors suggested that activated sludge sedimentation might be highly sensitive to underflow geometry. Field, physical model, and stratified flow numerical model tests were conducted to evaluate this hypothesis. Field work included solids settling velocity, dye dispersion, and flow pattern/solids distribution tests. Physical tests included operation of a pie-shaped 1:12.5 scale model of a circular, center-feed tank with sludge withdrawal simulating counter-current, uniform, and cocurrent geometries. A stratified flow numerical model that simulated the effects of density currents was calibrated to the results of the full-scale testing. The results failed to confirm the experimental hypothesis. Model performance was relatively insensitive to underflow geometry in a circular tank when density effects were simulated.