FLOC RESTRUCTURING IN VARIED TURBULENT MIXING

Floc structure was examined after the following mixing program: (i) 1 min of intense initial mixing of latex spheres with an aluminum coagulant (characteristic velocity gradient, Gi = 1500 s-1), (ii) slow flocculation for 30 min at Gf = 35 s-1, (iii) breakup for 1 min in the range Gb = 150-1800 s-1, (iv) reflocculation for 30 min at Gr = 35 s-1, and (v) quiescent settling (60 min). Depending upon the level of energy input during the short breakup step, significant alterations were found in the final floc characteristics. The density and fractal dimension of all floc subjected to the above mixing steps were greater than those for the standard mixing conditions, which did not include the breakup and reflocculation steps. However, floc properties did not vary monotonically with increasing breakup mixing intensity. For example, floc density passed through several minima and maxima over the range of Gb examined. These and other measurements of the floc size distribution following breakup are shown to support the multilevel floc model. © 1991.