Flocculation-Sedimentation Performance Model for Laminar-Flow Hydraulic Flocculation with Polyaluminum Chloride and Aluminum Sulfate Coagulants

摘要

Mechanistically based scalable algorithms for design and operation of hydraulic flocculators were developed in this research based on observations of residual turbidity for a range of influent turbidities (5-500 NTU) and coagulant doses (0.01-0.15 mM Al), for two hydraulic residence times (800 s and 1,200 s) and for two coagulant types (polyaluminum chloride and aluminum sulfate). Kaolin clay was used as a model colloid to create synthetic raw water turbidities. Data were obtained over a range of sedimentation capture velocities using a bench-scale laminar-flow tube flocculator and quiescent settling column. Tap water with a pH of approximately 7.6 was used for all experiments. Seemingly disparate results were unified through creation of a composite dimensionless parameter that considers collision potential in the flocculator and coagulant surface coverage of colloids. One adjustable model parameter was used to fit data (R~2 ≈ 0.9) from over 136 experiments to create a model for each of the two coagulants. The model was found to be applicable over a range of sedimentation tank capture velocities and accurately reflected the effects of coagulant dose, raw water turbidity, flocculator residence time, and coagulant type. The model was validated by successfully predicting results from independent data sets. The predictive model is expected to be a useful tool for evaluating design trade-offs between coagulant cost to increase surface coverage relative to capital cost to increase residence time and energy cost used to increase the velocity gradient.