Fluid shear influences on the performance of hydraulic flocculation systems

Gravity driven hydraulic flocculators that operate in the absence of reliable electric power are better suited to meet the water treatment needs of green communities, resource-poor communities, and developing countries than conventional mechanical flocculators. However, current understanding regarding the proper design and operation of hydraulic flocculation systems is insufficient. Of particular interest is the optimal fluid shear level needed to produce low turbidity water. A hydraulic tube flocculator was used to study how fluid shear levels affect the settling properties of a flocculated alum-kaolin suspension. A Flocculation Residual Turbidity Analyzer (FReTA) was used to quantitatively compare the sedimentation velocity distributions and the post-sedimentation residual turbidities of the flocculated suspensions to see how they were affected by varying fluid shear, G, and hydraulic residence time, θ, while holding collision potential, Gθ, constant. Results show that floc breakup occurred at all velocity gradients evaluated. High floc settling velocities were correlated with low residual turbidities, both of which were optimized at low fluid shear levels and long fluid residence times. This study shows that, for hydraulic flocculation systems under the conditions described in this paper, low turbidity water is produced when fluid shear is kept at a minimum. Use of the product Gθ for design of laminar flow tube flocculators is insufficient if residual turbidity is used as the metric for performance. At any Gθ within the range tested in this study, best performance is obtained when G is small and θ is long.

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► The effect of fluid shear levels on flocculated alum-kaolin suspensions was studied.
► Results show that floc breakup occurred at all velocity gradients evaluated.
► Use of the product Gθ for design of laminar flow tube flocculators is insufficient.
► At any given Gθ, best performance is obtained when G is small and θ is long.