Analysis of flocculation in a jet clarifier. Part 1 – Global and local hydrodynamic analysis

Abstract

Jet clarifier combines jet hydrodynamics, flocculation and settling in a unit operation. Generally, Camp and Stein G t parameter is recommended to evaluate clarifier efficiency, where G stands for a global velocity gradient and t a characteristic time scale (contact time). In this work, a quasi-two-dimensional jet clarifier is developed to make easier the hydrodynamic analysis of the flocculation zone of a jet clarifier. Measurements of instantaneous velocity field are performed by means of particle image velocimetry (PIV). PIV data are processed to visualise the strong circulation induced by the jet in the flocculation zone. Characteristic time scales related to macromixing are then extracted. Based on PIV data processing, local and instantaneous shear rate are estimated. The analysis of space averaged velocity gradient G is presented. The range of G is 2–15 s−1 whereas the residence time decreases from 4 to 1 h. Based on the hydrodynamic analysis, the parameter Gt is shown to be constant around 30,000 for different jet flow rates. Efficiency of such jet clarifier can thus be foreseen.