| The coagulation and flocculation process is used in many water and wastewater treatment systems to change particulate characteristics to improve removal in the subsequent separation processes. The performance of the coagulation and flocculation process is dependent on complex interactions between many chemical and physical phenomena which include colloidal interactions, surface chemistry effects, and mixing and fluid processes.;To investigate mixing in flocculation, detailed hydrodynamic measurements using a 2-D laser doppler anemometer were completed on two types of flocculators, with radial flow impellers: (1) a standard jar test apparatus; and, (2) the reactor zone of an upflow solids-contacting clarifier. Rather than rely on vessel-average parameters, localized velocities, energy dissipation rates, turbulent length scales and shear forces were determined for various zones of the flocculator. For these hydrodynamic factors it was also determined how they change with scale and impeller speed.;It was found that flow in impeller-agitated vessels is highly non-homogeneous, with areas of high energy dissipation and velocities occurring in the region of the impeller. It was also found that flow in the impeller discharge zone could be represented by a swirling radial jet which provided the basis for the development of many scaling relations. Based on results gained from the analysis of turbulent flow in these flocculators, forces on the floc were determined for the various zones within the flocculator. It was found that local forces within the vessel can be much greater than those calculated based on vessel average values. Of importance, it was found that high forces on floc particles occur due to high turbulence or energy dissipation rates in the discharge stream of the impeller, mean shear stresses due to fluid flowing over the impeller blades and mean shear stresses caused by the impeller discharge jet impinging on vessel walls. The latter may be the most significant and explain many of the difficulties found in scaling up mixing for flocculation. Results also indicate that vessel average parameters such as average energy dissipation per unit mass, $bar{varepsilon}$ and $bar{G}$ are not appropriate for the design, assessment and operation of flocculation facilities as local forces on the floc are not represented by them. A better approach for the scaling of the mixing process may be the matching of local floc forces for different scales. As results indicate that the dominant forces are those associated with the impinging jet, it appears that scaling of mixing based on matching these forces between flocculation systems is an improvement over the traditional use of $bar{G}$. (Abstract shortened by UMI.). |
