This study investigated the impact of turbulent heterogeneity in a flocculation reactor on particle aggregation and breakup. In particular, the influence of average characteristic velocity gradient (G), particle concentration, and coagulation mechanism (sweep floe vs charge neutralization) on the floe growth, steady-state size, and variance was analyzed. Experiments were performed in a bench-scale reactor with a low-shear axial-flow impeller using a photometric dispersion analyzer (PDA). Results indicated that as G increased, floe growth increased while the mean size and variance in the floe size distribution decreased. In addition, floe growth, mean size, and variance increased with increasing primary particle concentration and when the coagulation mechanism was switched from charge neutralization to sweep floe. Lastly, floe growth, mean size, and variance were found to vary spatially in the reactor at low G values with larger floe size and growth rate in the bulk region and a larger variance in the impeller discharge region. (C) 2003 Elsevier Inc. All rights reserved.