Investigating the relationship between shear and floe fate using CFD

The size, density and strength of floes play a major role in the removal of contaminants from water in physico-chemical treatment processes. The efficiency of the main removal processes is a function of floe size, strength and density. Changes in these parameters affect floe removal and hence the removal of adsorbed organic matter. Coagulation and flocculation efficiency and floe strength are often assessed using ajar tester. Here, computational fluid dynamics (CFD) was used to model the flow field within standard jar test apparatus and, using a Lagrangian particle trajectory model, to study the effects of turbulence on individual floes. The hydrodynamic environments were also investigated experimentally using laser Doppler anemometry (LDA) and particle image velocimetry (PIV) measurement techniques. Combining numerical and experimental data, velocity gradient values at which floe breakage occurs were postulated for three different floe suspensions. Although the threshold values are determined using jar test and CFD data in combination, they are based on the floes’ resistance to induced velocity gradients. This is a significant result, as previous breakage thresholds have been expressed only in terms of mixing speed and cannot be applied at full scale. With this in mind, work was subsequently undertaken to use CFD to model numerically the hydrodynamic conditions within two full scale flocculation vessels; one mechanically mixed, the other hydraulically mixed. This section of work had two principal aims; firstly, to investigate the perceived benefits of using CFD to model the hydraulic performance of the flocculation process at two large surface water treatment works, and secondly, to investigate the practicality and effectiveness of using CFD and jar test results in combination to consider floe fate in the flocculation vessels (in terms of growth, breakage and residence time). This work drew upon the results and conclusions of the previous laboratory scale work and facilitated a greater insight into flocculation processes. Improved understanding of flocculator hydrodynamics can only serve to improve design procedures and standards for future installations.