Image Analysis of Floc Blanket Dynamics: Investigation of Floc Blanket Thickening, Growth, and Steady State

Abstract

Floc blankets are fluidized, concentrated beds of particles utilized in some upflow sedimentation tank reactor configurations in water treatment. The presence of a floc blanket can significantly enhance removal of turbidity when coupled with lamellar sedimentation. Blanket dynamics have not been extensively studied, but are important for understanding blanket formation and operational control in full-scale water treatment plants. This research employed image analysis to reveal the temporal dynamics of floc blanket suspended solids concentration and floc-water interface height. Turbidity measurements were used to corroborate image analysis as a valid method for measuring solids concentration in the floc blanket and in the floc blanket supernatant. Observations from image analysis reveal relatively distinct stages in the creation of a floc blanket: thickening (increasing suspended solids concentration) in the absence of a floc-water interface, thickening with an interface, and steady state. Preliminary performance results suggest that turbidity removal is more significantly impacted by blanket concentration than blanket height. Two important factors were found to impact blanket dynamics during the interval when the height of the floc-water interface was increasing: (1) thickening, and (2) mass transfer between the supernatant above the floc-water interface and the floc blanket. Floc blanket thickening during formation has been confirmed to occur at both variable and constant interface upflow velocity. This observation suggests that floc properties (size and density) change during floc blanket formation.