Flocculent Suspensions (Type II ) Settling

Flocculent particles such as those resulting from chemical precipitation or coagulation, or those found in biological treatment of wastewater will agglomerate while settling, with a resultant increase in particle size. The density of the composite particle may decrease due to the inclusion of water, but the overall result is generally an increase in settling velocity.
Settling analyses of such suspensions are performed in columns at least 300 mm in diameter and equal in depth to the proposed clarifier. Samples are withdrawn at regular intervals from multiple ports along the column and analyzed for percent removal. This percent removal is plotted as shown in figures below. From this plot percent removal is determined at selected time interval as shown in the following examples.
In order to obtain equivalent results in an actual tank the laboratory results should be modified to reduce SOR by a factor of 1.5 and increase the design time by a factor of 2.
Type II settling involves flocculating particles in dilute suspension.Flocculating suspensions cannot be generalized in the same manner as discrete particle suspensions. The Stokes equation cannot be used because flocculating particles are continually changing in size, shape, and, if a large aggregate of particles collect, specific gravity because of entrapment of water in the interstitial space. So many factors contribute to the flocculation process that it has been impossible to develop a general formula for determining settling velocities.