Simple Model for Coupled Hydraulic Flocculation-Sedimentation Performance

Because coagulation and flocculation do not remove suspended particles but condition particles for removal downstream, performance of these processes is assessed by relative removal of particles and organic matter in downstream processes. Predictive models of coagulation and flocculation performance do not include information about downstream processes, but they could be more generally applied if they did. In this paper, the AguaClara hydraulic flocculation model is modified so that a fitting parameter, k, becomes a function of sedimentation capture velocity. Experimentally, tap water with 90 NTU of kaolinite clay was treated with six PACl doses of 0-2.79 mg/L as Al, a hydraulic flocculation residence time of 6.5 min with a dimensionless flocculation product of 62,200, and six tube settler detention times of 8.4-1.4 min for capture velocities of 0.1 to 0.6 mm/s, achieving settled turbidities of 0.21-82.5 NTU. The analysis revealed an exponential relationship between k and the sedimentation capture velocity, which appears related to the shape of typical particle size distributions. An addition to the model that incorporates capture velocity lowered the normalized root mean square error of the data from 0.414 to 0.113 logarithmic performance ratio units. The updated model is more readily applied to water treatment plant design and analysis and allows for design trade-offs to be made when considering hydraulic flocculation and sedimentation process parameters.