Sedimentation - Theory

Where will I ever need settling and sedimentation?

Settling and sedimentation can be found in industry quite frequently, once you realize what it is. It is valuable for the removal of solids from liquid sewage wastes, getting crystals from what they precipitated from, settling solid food particles from a liquid food, and settling the slurry from a soybean leaching process.

Sedimentation or settling can be used to separate any particle from any fluid, whether it is a liquid from a gas, or a solid from a liquid, but for the rest of this report, I am going to refer to the solid-liquid version, as I feel that it is perhaps a bit more tangible, and is definitely easier to explain.

There are actually two different styles of settling, known as free settling and hindered settling. The process of separating a suspension by gravity settling into a clear fluid and a slurry of solids is called sedimentation.

Separation of Solids and Liquids

In order to separate a solid-liquid mixture, there is often a two-step procedure that needs to occur. First is what is known as thickening, in which the particles in the mixture settle into a thickened, denser, underflow, with a clear overflow. This step is sometimes marked by the addition of a coagulant or other thickening agent. The second stage involves taking the underflow and further reducing it, attempting to convert it into a compact solid with relatively small amounts of liquid. The second stage is more or less defined by the first stage, and, as such, most attention is focused on the original separation and thickening of the solid-liquid mixture.

Gravity Settling

Here's an experiment. Look at one of the "snow globes" you can buy at almost any gift shop. Take it, and shake it up, and see all the particles of white "snow" flying around in the water. Then, if you set it down and wait a few minutes, the "snow" will begin to start hitting the ground. In a few minutes, you will reach what is sometimes called a critical point or critical sedimentation point. At that point, everything has reached the ground state. It is all "sediment" so to speak. In a few more minutes, compression continues, and eventually, everything settles back to normal; the water on top, and the compacted white "snow" particles on the ground. However, this is just a very simplified version of settling and sedimentation.

Graphically, it can be seen that as time goes on, the liquid-solid interface changes from being non-existant, almost a true solution, to having very distinct edges.

Looking at the above figure, also notice that while a clear overflow appears fairly early on, the interface height is quite high, and only when the settling has taken a bit of time does the interface height begin to reach its final, much lower value.

There is the rub. Sedimentation, or more precisely, thickening, takes time. The less time it takes, the more often you can run through the batch, and the more you run through the batch, the more money the company you might be working for can make. And so that is one of your first important variables, determining the time required for settling, or, more appropriately, the settling velocity.