[Chemistry Class Notes] on Sedimentation Pdf for Exam

We come across various incidents on a daily basis in which we have to separate one substance from the other to make it more useful. Different sedimentation methods are available by which we can separate substances that are mixed together. Sedimentation is the simplest separation method and an essential concept that is supposed to be understood. Its importance is unquestioned and plays a vital role in archeology. It’s a natural process that can be explained as building up layers of small particles like sand or mud. Weight and sedimentation are very related.

The Sedimentation definition is given by, it also includes deposits from glacial ice and such materials collected under the impetus of gravity alone, similar to talus deposits, or accumulations of rock debris at the base of cliffs. This term is commonly used as a synonym for sedimentology and sedimentary petrology.

Sedimentation Process By an Experiment

The sedimentation process can be observed by a small experiment. Take a glass jar and fill it with a garden variety of mud. Pour some water, shake it well and keep it untouched for a few minutes.

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In a while, we can notice that the gravel and rocks have settled below, sand on above, and so on. The garden variety of mud basically has formed layers of soil based on varied, which is seen below.

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If we look at the cliff, it can be observed several layers on the surface of the cliff. These resultant layers are formed by sedimentation – the grains of sand and mud build-up over a long period of time, forming the layers. Also, fossils happen to be found in these layers. Logically, the quicker the bones are buried, and the more survival chances are more because it can be protected from scavenging animals and limited damage by weather. The rivers, lakes, and sea are the best depositors of both sand and mud are some sedimentation examples. Dinosaur fossils were found near to the sea, lakes or rivers. A land-slide, where mud and rock-fall down a hill mountain, can also lead to a sedimentation type.

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Sedimentation can also be used to separate particles based on size by applying the centrifugal force to the required solution. In the Centrifugation process, a centrifugal force is applied to a heterogeneous mixture which separates the mixture based on its density. The high dense components stay away from the centrifugal axis, whereas the less dense components stay near to the centrifugal axis by therefore separating the constituents of the mixture.

Also, sedimentation helps to determine a person’s medical conditions. The sedimentation rate of RBC is one of the sedimentation examples. It is performed by measuring how long it takes the Red Blood cells (RBCs) to get settled in a test tube. As time passes by, the RBC’s start to separate from the other plasma contents, they settle down at the bottom and serum will be formed above. The ESR (erythrocyte sedimentation rate) or sedimentation rate is measured simply by recording how far the top of the Red Blood Cell layer has fallen (in millimeters) from the top of the serum layer in one hour.

The water treatment plant uses the sedimentation method to pull out unwanted particles from unclean water by filtering several layers of soil and sand, allowing specific sizes of particles to pass through.

Basic Principle of Sedimentation

Suspended water solids have a specific gravity that is greater than the water tends to get settled by gravity as soon as the turbulence is retarded by granting the storage. Inorganic suspended solids have a specific gravity of about 2.65, and Organic suspended solids have a specific gravity of about 1.04

The particles having a specific gravity of about 1.20 or so readily settle down at the bottom of the tank. But to cause the settlement of lighter particles, it’s difficult. 

This settling down of particles at the bottom of the sedimentation tank phenomenon is called hydraulic subsidence, and each particle has its own hydraulic settling value causing hydraulic subsidence on it.

Basin, where the flow is retarded, is known as a sedimentation basin. The average theoretical time for which the water is detained in the settling tank is known as the detention period.

Types of Sedimentation

Plain Sedimentation

It is the process of settling down of solids and impurities in the raw water to the bottom of the sedimentation basin by a natural gravity force alone, with no chemical added. This is a very cheaper sedimentation method and is mostly used in every water filtration and purification system. 

Sedimentation Using Clarifier and Contact

Here, the chemicals are mixed in water, and the same water is rotated with the help of pumps for a period of two hours per day, and suspended solids are settled down in the bottom of the reservoir or tank, and more. 

Chemically Assisted Sedimentation or Clarification

In this process, chemicals are added to water, and through mixing, the suspended solids, and other impurities are stuck together and form floc, settling at the bottom of the basin.

Generally, the most used process is chemically assisted as horizontal sedimentation based on some assumptions. Basically, water flows through a tank in an irregular flow, and thus the intention of sedimentation is to create conditions where the flow takes place uniformly for a long enough period, permitting the maximum practical amount of floc to get settled before the water reaches the end of the tank.