What occurs as the original slurry is made more and more concentrated? That question was answered in the work done by Scott. The short of it is that the more dilute the original suspension, the faster the settling occurs. The only thing is, why?
As seen above, not only does the settling take more time as the slurry becomes more concentrated, but the critical sedimentation point occurs at progressively higher interface heights.
What occurs is that the type of settling changes from free settling to hindered settling as you get more and more concentrated. And the graph above shows just how much of an affect hindered settling has on compression times.
It should also be mentioned that there are three modes of settling that occur, that roughly correspond with concentration of the suspension.
In the figure below, it can be seen that in type 1, which is zone settling, as time moves on, there is a sharp drop in the level of separated flocs, or groups of flocculated particles, while a porous medium begins to appear below this zone, gradually increasing the interface height until finally there are no more flocs, at which point the level of the porous medium begins to very slowly decrease. In type 2, or channelling, the porous medium drops off until a certain amount of compression is reached, and then the rate at which the interface height drops slows dramatically. And finally, for compression, which is seen as type 3, the interface is an almost linear drop, very slow, but almost a straight line, until it reaches the final compression limit.
