Effect of the micro-flocculation stage on the flocculation/sedimentation process: The role of shear rate

Dynamic analysis on the variation of particle size distribution (PSD) and the fractal characteristics of PSD (D_f) were investigated to better understand the continuous procedure of the floc growth and optimize the control of flocculation process. It was found that the flocculation process could be divided into three stages, i.e., the micro-flocculation stage, the growth stage and the steady (or breakage) stage. As the stage which is crucial to the morphology of micro-flocs (the building blocks of large flocs), the micro-flocculation stage plays an important role on flocculation/sedimentation process. The results showed that an increase in shear rate (11s^-1<G<30s^-1) during the micro-flocculation stage contributed to micro-flocs with larger size and more compact structure. As shear rate further increased (30s^-1<G<55s^-1), the micro-floc average size gently decreased from 13.61μm to 10.91μm, whereas two-dimension fractal dimension of micro-flocs gradually increased from 1.85 to 1.89. This indicated that further increase of shear rate during the micro-flocculation was incline to the formation of smaller micro-flocs with more compact structure. According to the results of final floc properties, the moderate shear rate (G=30s^-1) benefited to the micro-floc formation to form final flocs with desired properties, further improved the treatment efficiency in the whole process. Based on the kinetics in the micro-flocculation stage, a conceptual model was proposed to describe the micro-floc growth under different shear rates, further revealed the reason for the different properties of final flocs under various shear rate during the micro-flocculation stage. Combining the results with model, it was concluded that shear rate during the micro-flocculation stage mainly affected final flocs by the domination of micro-floc structure. This research gives indications both for theoretical and actual works to improve the efficiency in the solid/liquid process.