The influence of tank size and impeller geometry on turbulent flocculation: I. Experimental

The effects of tank size and impeller geometry on the particle size distribution after 30 min of flocculation at constant characteristic average velocity gradient, Gm, were studied. Flocculation was performed in three square tanks 5-, 28-, and 560-L volume with Rushton turbines and A310 fluid foil impellers. The floc size was recorded in situ using a photographic technique, and the resulting photographs were analyzed with an image analysis system. The results show that the cumulative particle size distribution shifts to a smaller particle size range with increasing tank size, regardless of impeller type. A similar shift in the cumulative particle size distribution was observed when the impeller type was switched from the A310 fluid foil to the Rushton turbine, regardless of tank size. The volume mean floc size, floc standard deviation, and maximum stable floc size were found to he controlled by the turbulence intensity in the impeller discharge zone and not by Gm.