Flocculation differential settling

The increasing sedimentation velocities for samples taken at the lower ports on Figures 11-14 are good evidence that flocculation is occurring. This result is expected since differential settling velocities in the columns probably increased particle numbers in the lower region of the column, and since the flocculation rate increases with increasing particle number. 

Particle concentration and size distribution in raw water have extensive and complex effects on the performance of individual treatment units (flocculator, sedimentation tank, and filter) and on the overall performance of water treatment plants. Mathematical models of each treatment unit were developed to evaluate the effects of various raw water characteristics and design parameters on plant performance. The flocculation and sedimentation models allow wide particle size distributions to be considered. The filtration model is restricted to homogeneous suspensions but does permit evaluation of filter ripening. The flocculation model is formulated to include simultaneous flocculation by Brownian diffusion and fluid shear, and the sedimentation model is constructed to consider simultaneous contacts by Brownian diffusion and differential settling. The predictions of the model are consistent with results in water treatment practice. 

Friedlander (11) has examined the effects of flocculation by Brownian diffusion and removal by sedimentation on the shape of the particle size distribution function as expressed by Equation 9. The examination is conceptual the predictions are consistent with some observations of atmospheric aerosols. For small particles, where flocculation by Brownian diffusion is predominant, p is predicted to be 2.5. For larger particles, where removal by settling occurs, p is predicted to be 4.75. Hunt (JO) has extended this analysis to include flocculation by fluid shear (velocity gradients) and by differential settling. For these processes, p is predicted to be 4 for flocculation by fluid shear and 4.5 when flocculation by differential settling predominates. These theoretical predictions are consistent with the range of values for p observed in aquatic systems. 

Flocculation by Brownian diffusion is characterized by the collision frequency function presented in Equation 14. For flocculation by differential settling, the corresponding function is ... 

Plant performance in the absence of flocculation by fluid shear (G == 0 sec ) is examined in Figure 18. Some particle growth occurs by Brownian diffusion in the flocculator and by differential settling and Brownian diffusion in the settling tank. Removal efficiency by sedimentation is smaller than in the standard case (74% compared with 85% ), and filter runs are reduced from 28 to 16 hr. Filtrate quality is good throughout the run. 

With the exception of sedimentation where differential settling velocities of floes create relative motion, hence orthokinetic flocculation, particle flocculators rely on the liquid drag past the particles to create the velocity gradients. 

The equations cannot be treated exactly like other orthokinetic equations because of the limitation i j. If i = j there would be no differential settlement and, therefore, no collisions and aggregation. Similarly, it cannot be applied to an initially monodisperse suspension because aU particles would settle at the same rate. If the suspension were subject initially to significant perikinetic (Brownian diffusion) flocculation then it would become hetero-disperse and flocculation by differential settling would follow. 

If a sample of suspension to be settled is available it is not difficult to determine whether flocculation by differential settling will occur. An appa-rams similar in principle to the Andreasen pipette method (chapter 2) is used, but with sampling at more than one depth, such as the settling mbe with multiple sampling ports shown in Figure 4.22. Samples are taken at the various depths h, h2,h etc., below the surface level of the suspension, at... 

Another type of flocculation results from particle—particle collisions caused by differential settlement. This effect is quite pronounced in full-size plants where large rapidly falling particles capture small particles that settle more slowly. 

Flocculation is accelerated and higher overflow rates are achieved by external or internal recirculation of settled soflds into the feed which leads to the collection of fine particles by interception. Addition of conditioned fine sand to the feed induces separation by differential sedimentation, and sometimes increases overflow rates to 6—8 m/h. 

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