Revisiting Hydraulic Flocculator Design for Use in Water Treatment Systems with Fluidized Floc Beds

Abstract

In water treatment, flocculation creates aggregated particles or flocs large enough to be removed by downstream processes of sedimentation and filtration. Fluidized beds of flocs (floc blankets) are sometimes included in upflow sedimentation processes because they can significantly improve sedimentation tank particle removal when plate or tube settlers are used. Overall performance of a sedimentation tank containing a floc blanket is influenced by the characteristics of incoming flocs from the flocculator. Floc blankets provide an additional opportunity for aggregation of colloidal particles that enables a reduction in the size of the flocculators, while improving particle removal efficiency. In this study, laminar flow flocculators varying in velocity gradient (G) and residence time (θ) were tested in a laboratory scale water treatment system with a floc blanket. Results indicate that increasing G (range of 74–251 s−1), while decreasing residence time from 269 to 80 s, and maintaining a constant Gθ (∼20,000) improved particle removal. These results indicate that velocity gradients greater than existing design recommendations may be used to reduce flocculator residence times (and associated construction costs) when upflow sedimentation with a floc blanket and overlying lamellar plate or tube settlers are employed subsequent to flocculation. Further reduction of the residence time to 24 s with a G of 251 s−1 resulted in a settled water turbidity of 14 nephelometric turbidity unit (NTU). When varying θ at a constant G of 72 s−1, an apparent minimum in settled effluent turbidity at 0.15 NTU was observed at the middle residence time tested (θ = 211 s), suggesting that long hydraulic flocculator residence times may be suboptimal.