The Basics

Settling

Settling or sedimentation is a physical water treatment process used to settle out suspended solids in water under the influence of gravity. In a flocculation plant it is used to seperate the flocs which by now contain the particles and colloids (the colour and turbidity) from the water. Water leaving a settling tank or "clarifier" is often called "clarified water".

In settling process theory, a particle will settle only if:

1. In a vertical upflow arrangement, the up-flow water velocity is lower than the particle settling velocity.

2. In a horizontal flow arrangement, the ratio of the length of the tank to the height of the tank is higher than the ratio of the water velocity to the particle settling velocity.

Any phycisist will at this point start talking about Newtons Law and Stokes Law and will tell you that it is easy to calculate the terminal particle settling velocity of a given particle. And they are correct in that it is easy to calculate the settling velocity of a single ballbearing (a perfectly round object, with a very high density) in, say, glycerine (which will slow the ball bearing down so that there is very little turbulence). On a water treatment plant however there are the following issues that make the whole thing much more complicated:

1. Flocs, especially flocs from upland water, with almost no turbidity, have a density that is not much higher than the density of water. This means that settling is slow (hours instead of seconds in the case of the ballbearing) and also that the slightest turbulence in the water will create a "cloud" of flocs in the clarified water at the top of the settling tank.

2. Flocs are not homegenous and contain much liquid, a lot of it bound to the floc, in and between the (sub) units. This means that water has trouble flowing smoothly around the floc.

3. When flocs are settling, they interact with each other. This phenomenon is called hindered settling. Hindered settling is where the concentration of particles is so high that each particle affects the flow of liquid around its neighbour.