Sedimentation: Process, Lab Testing, and Best Practices

The sedimentation of solids from a suspension in a liquid by gravity settling is called sedimentation. In this process, a dilute material (slurry) is separated into a clear liquid and slurry of higher solids content. The Dorr thickness is a common piece of equipment used for sedimentation.

Sedimentation Process

Sedimentation is one of the most widely used processes in the treatment of water. The simplest method of removing suspended impurities is plain sedimentation. The water is allowed to stand quiescent or move very slowly through the basin until the suspended impurities settle to the bottom and relatively clear water is drawn off from the top. The degree of removal of suspended impurities depends upon the length of the retention period, the size of the suspended contaminants, and the temperature.

{tocify} $title={Table of Contents}

Pree settling refers to the process wherein the fall of the particle in a gravitational field through a stationary fluid is not affected by the walls of a container and other particles. The necessity is that the particles be at a sufficient distance from the wall of the container and also from each other. Practically, the free settling conditions exist if the concentration of the particles in suspension is less than 1 percent by weight by solids. 

In such a case, as the particle falls its velocity increases and will continue to increase until the resisting force and accelerating force. When this point is reached, the particles will settle at a definite constant velocity during the remainder of their fall. This ultimate constant velocity is called terminal settling falling velocity.

Hindered Settling

When the concentration of particles in suspension is large, particles will be so close to each other that the surrounding particles will interfere with the motion of other individual particles. If the fall of particles through a stationary fluid is impeded by the other particles the process is called hindered settling. 

For hindered flow, the settling velocity is considerably less than the terminal falling velocity under free settling conditions. Hindered Settling is encountered in sedimentation. In this case, the particle is settling through the suspension of particles in fluid rather than through the fluid itself.

Laboratory Batch Sedimentation Test.

The mechanism of settling may be best described by batch settling test in a glass cylinder, which gives in images a series of observations cylinder of batch settling test.

Shown in the image is a cylinder containing a newly prepared slurry of a uniform concentration of uniform solid particles throughout. As soon as the process starts, all the particles begin to settle and are believed to approach rapidly terminal settling velocities under hindered settling conditions. Various zones of concentration then are established as shown in the picture.

The heavier faster-settling particles settled at the bottom of a glass cylinder are indicated by area D. Above area D forms another layer, called area C, a region of variable size distribution and non-uniform concentration. The boundary between C and D is usually obscure and is marked by a vertical channel through which fluid rises from the lower area D as it compresses. Above area, C is area B which is a zone of uniform concentration of approximately the same concentration as that of the original pulp. Above area, B is A, which is an area of clear liquid. If the original slurry is closely sized to the smallest particles the boundary between A and B is sharp.

As sedimentation continues, the heights of each zone vary as shown in pictures b,c, and d. The heights of zones D and A increase at the expense of that of area B while that of C remains constant. After further settling areas B and C disappear, and all the solids appear in area D. Still, area D may shrink further settling areas B and C disappear, all the solids appear in area D, but area D may shrink further because of compression. During compression, the liquid associated with the solids in area D is expelled in a clear area.

In a batch sedimentation operation, as discussed, depths of the various areas vary with time. The same area will be present in the continuous thickener, but in a continuous sedimentation process, once the steady state is set up, the heights of each area will be constant. The picture shows how the area may be arranged in a piece of continuous operating equipment such as a thickener.

In batch settling tests carried out in the laboratory, the height of the liquid-solids interface is measured as a function of time. When the experimental data of height of interface v/s time are plotted, we get the curve as shown in the picture. The slope of this curve at any point in time represents the settling velocity of the suspension at that instant. 

During the settling process's early stage, the settling velocity rate decreases and steadily drops until the ultimate height is reached. The batch settling test will give a different curve for every sludge and a somewhat different one for different concentrations. Such batch tests are the basis for the design of continuous thickeners. 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane