Effect of hydrodynamic breakage on floc evolution and turbidity reduction in flocculation and sedimentation processes

Abstract

Coagulation and sedimentation process is one of the most popular processes in drinking water treatment. Hydrodynamic breakage has a significant impact on the evolution of floc characteristic and the efficiency of turbidity removal. In this work, the effects of hydrodynamic breakage on floc size, fractal dimension, and floc morphology were investigated with an in-situ recognition system. The experiments were conducted in a continuous flocculation and sedimentation reactor equipped with perforated plates to provide different hydrodynamic breakage conditions. The experimental results indicated that the hydrodynamic conditions significantly influenced the floc destabilization and restructuring processes. A low hydrodynamic shear forces provided by P1 led to the increase of both bigger sized flocs but accompanied with small particles (0–10 μm). Excessive velocity gradient provided by P3 produced smaller and looser flocs. An appropriate velocity gradient (i.e., the flow velocity through the perforated plate P2 at 18.9 × 10−3m s−1) was conducive for the formation of larger and more compact structures, with higher average floc size and fractal dimension. This flocculation condition in turn resulted in effective improvements in the turbidity removal efficiency. Floc evolution models were described based on the mechanism of the breakage and restructuring process.