Abstract Summary (Max 250 words)
Secondary settling tanks (SSTs) serve as the final stage of wastewater treatment. These units rely heavily on chemically induced aggregation to remove fine impurities, but frequently suffer from off-spec effluent quality. Although extensive laboratory testing is conducted to evaluate chemical performance and understand aggregation mechanisms (coagulation and flocculation), laboratory results often show poor correlation to full-scale behavior. This disconnect stems from a lack of physically meaningful scale-up/down parameters, with industry typically selecting average shear rates (G-value) to mimic full-scale conditions in laboratory tests – with limited predictive success. The purpose of this study is to evaluate the hydrodynamic scaling parameters governing flocculation in three bench-scale mixing vessels: two geometrically similar baffled cylindrical tanks (T = 0.139 m and 0.243 m) and one square tank (T = 0.139 m), equipped with a T/3 Rushton turbine. A high molecular weight polyacrylamide flocculant was used to induce bridging flocculation in dilute slurries (Cv < 1%) of 2 μm particles. Experiments were performed for average shear rates of 30–100 s⁻¹ (0.001–0.01 W/kg). Maximum local energy dissipation was estimated for each condition using M-Star CFD simulations. In the experiments, floc size evolution was measured in-line using a SOPAT probe (9–1200μm). Trends in Sauter mean diameter (D32) show that the frequency of floc passages through the impeller discharge region strongly impacts floc evolution and steady-state size. The findings demonstrate the importance of replicating local energy dissipation from key mixing zones across scales, providing a framework for designing physically meaningful lab-tests for full-scale SSTs.
