Sedimentation

Introduction

The phenomenon of settling of particles in motionless water is of great importance, which can be observed in naturally occurring process like sea, river, canals etc. and particularly in water treatment for the purification of water.

Several processes are there for the removal of suspended solids from the water like Cyclones, Centrifugal, Thickeners, and Sedimentation tanks. All these processes are important but the later one is the most commonly used process in the industry due to multiple reasons i-e capital cost, operational cost are relatively low as compare to other processes.

For this very reason, study of settling of particles in a fluid is an important practice regarding purification and separation.

Sedimentation

When water has little or no movement, suspended solids sink to the bottom under the force of gravity and form sediment. This process is called sedimentation. We have discussed a similar process, while separating solids from the water.

The theory of sedimentation would seem to be quite simple. Earlier we had a widening river flowing more slowly, so if we make the settling tank large enough and the flow slow enough, this will enhance the rate of fall of the sediment towards the bottom of the tank.

When a particle moves through fluid, three forces act upon it which elects the behavior particle i-e whether it will sediment or not.

Principle of Sedimentation

Basic principle behind the settling of particles is given by Stokes law:

                                                                                 G   =       gravitational acceleration

                                                                                 D_p  =       particle diameter

                                                                                 µ    =       fluid viscosity

Depending upon the concentration of the particles, when it is low the effects of mutual interference of the particle is less and vice versa.

Zones in Sedimentation Tank

During the settling of particles, different compositions within the sedimentation tank could be observed, having different concentration of particles are termed as zones. Different zones are shown in the fig.

Clear Liquid	Zone free of particles is called clear zone.
Suspension Interface	Point separating the constant composition zone and clear liquid.
Constant Composition Zone	In constant composition zone, the suspension moves at the uniform rate. Free settling occurs in this zone.
Variable Composition Zone	Hindered Settling occurs in this zone. Concentration of the particles does not remain same in different areas.
Sludge Interface	Point separating the sediment and liquid.
Sediment	Solid particles which are obtained at the end of sedimentation process.

As the interface of the particles moves down, sludge builds up and the point where two levels becomes equal is known as Critical Point.

If particles size is equivalent to the 6:1 setting happens with defined zones, discussed above, according to the Stokes law. However, the range of particles is greater than 6:1 there would be no sharp boundary among different zones. In this case, sediment is difficult to observe.

Sedimentation Processes

Basically, two processes are generally used to carry out sedimentation at industrial scale. Both the processes are employed, depending upon the characteristics of the material to be sediment.

These two processes are:

·         Gravity settling processes

·         Centrifugal settling processes

Gravity Settling Processes

Particles heavies than the suspending fluid may be removed from a gas or a liquid in a large settling tank, in which the fluid velocity is low and the particles have ample time to settle out.

Different kinds of gravity settlers used for separation are discussed below:

·         Clarifier: That virtually removes all the particles from a liquid.

·         Classifier: That separates the solid into two fractions.

Gravity Classifier

Separate particles on the basis of size, however, the density of the fine particles are the same as that of the larger ones.

Gravity Clarifier

Clarifiers are used for gravity separation under hindered settling conditions to convert dilute slurry of fine particles into a clarified liquid and a concentrated suspension.

 

Centrifugal Settling Processes

In order to increase the settling rate, the force of gravity acting on the particle is replaced by a much stronger force called centrifugal force. Particles are removed more efficiently, in a short period from fluids as compared to gravity process.

Centrifugal settlers used are mentioned below:

·         Cyclones: Used for the separation of solids from gases.

·         Hydro Cyclones: Used for separating solids from liquids.

 

Cyclones

Cyclones operate to collect relatively large size particulate matter from a gaseous stream through the use of centrifugal forces. Dust laden gas is made to rotate in a decreasing diameter pathway forcing solids to the outer edge of the gas stream for deposition into the bottom of the cyclone. Efficiencies are 90% in case of particle size of 10 microns or greater are possible.

 

Hydro cyclone

A hydro-cyclone is a device to classify, separate, or sort particles in a liquid suspension based on the densities of the particles. A hydro cyclone may be used to separate solids from liquids or to separate liquids of different density.

A hydro cyclone will normally have a cylindrical section at the top where liquid is being fed tangentially, and a conical base. The angle, and hence length of the conical section, plays a role in determining operating characteristics.

Factors Affecting Rate of Sedimentation

Sedimentation is not an independent process. It is governed by several factors. More important factor is to know the density and the size of the particles to calculate their rate of fall and there should be no turbulence in the tank as it will tend to reduce settlement. There must be an even flow through the tank to prevent a narrow stream flowing through quickly from one end to the other.

Moreover, other effects like concentration, height, wall resistance etc. affecting. the rate of settling are discussed below:

Effect of Concentration

Rate of sedimentation has an inverse relation with the concentration of particles. Greater the concentration, smaller would be the rate and vice versa.

Particle Size

Greater the size of the particles, which is to be sediment, greater the gravitational force will act upon it and vice versa. Force of gravity on the particle is given by the following mathematical equation:

W = mg

Temperature

Temperature has an inverse relation with the sedimentation rate. High temperature means low sedimentation rate. In order to increase the sedimentation rate temperature should be minimized.

 

Height Effect

In general the height of the initial suspension does not affect the rate of sedimentation. However, in case of larger height, greater amount of sediment has to build up before the critical point.

 

Wall Effect

Provided the particle size to the tube diameter is not greater than 1:100, no wall effect s should be apparent.

 

Density of The Particle

Density of the particles greatly affects the rate of the sedimentation. Greater the density, in compare to medium through which sedimentation takes place, greater would be rate of sedimentation and vice versa.

Sedimentation Tanks

Because of these factors we shall now look at Sedimentation tanks. Generally, sedimentation tanks are divided into following zones.

 

Inlet zone

The inlet or influent zone should provide a smooth transition from the flocculation zone and should distribute the flow uniformly across the inlet to the tank. The normal design includes baffles that gently spread the flow across the total inlet of the tank and prevent short circuiting in the tank. The baffle could include a wall across the inlet, perforated with holes across the width of the tank.

Settling Zone

The settling zone is the largest portion of the sedimentation basin. This zone provides the calm area necessary for the suspended particles to settle.

Sludge Zone

The sludge zone, located at the bottom of the tank, provides a storage area for the sludge before it is removed for additional treatment or disposal.

Outlet Zone

The basin outlet zone or launder should provide a smooth transition from the sedimentation zone to the outlet from the tank. This area of the tank also controls the depth of water in the basin. Weirs set at the end of the tank control the overflow rate and prevent the solids from rising to the weirs and leaving the tank before they settle out. The tank needs enough weir length to control the overflow rate, which should not exceed 20,000 gallons per day per foot of weir.

Types Of Sedimentation Tanks

Various kinds of tank are used depending upon the particles which are to be sediment. Commonly three types of tanks are used on industrial scale are discussed below:

Horizontal Flow–Rectangular Tank

Long rectangular basins are hydraulically more stable, and flow control for large volumes is easier with this configuration.

A typical, long rectangular tank has length ranging from 2 to 4 times their width. The bottom is slightly sloped to facilitate sludge scraping. A slow moving mechanical sludge scraper continuously pulls the settled material into a sludge hopper from where it is pumped out periodically.

Radial Flow–Circular Tank

Circular settling basins have the same functional zones as the long rectangular basin, but the flow regime is different. When the flow enters at the center and is baffled to flow radially towards the perimeter, the horizontal velocity of the water is continuously decreasing as the distance from the center increases.

Thus, the particle path in a circular basin is a parabola as opposed to the straight line path in the long rectangular tank. Sludge removal mechanisms in circular tanks are simpler and require less maintenance.

Upward Flow–Hopper Bottom Tank

Water enters through the centrally placed inlet pipe and deflected downward by the action of deflector box. Water travels vertically downwards, sludge settles at the bottom of the tank. Where it is removed by a sludge pipe connected to a sludge pump.

Sedimentation Apparatus

The apparatus we have deployed for the study of sedimentation is shown in fig.

·            Stopper– It is used to close the mouth of the sediment tube to avoid any accident in case of mishandling.

·            Clamps – Fixed with sediment apparatus are used to hold the glass tubes in a vertical position.

·            Measuring Scales –Graduated scale (cm) on each tube is used to observe the correct position of the solid particles.

·            Sample Tube – It is the main part of the apparatus in which whole practical is carried out. Made up of transparent plastic for quick observation.

·            Tube Lights – The only purpose of the tube lights is to make the clear view of the small particles in the sample tube.

·            Stopwatch – It is of secondary importance, used for taking the readings with the settling of particles.

Experiment Performed

Objectives

1.      Effect of initial concentration on the sedimentation characteristics

a.       Plot initial mass settling rate against concentration

b.      Plot the settling velocity against the limiting concentration

2.      Construction of the rate of settling curve against concentration from a single batch test

3.      Effect of initial height on sedimentation characteristics

 

Procedure

1.      Select a suitable, well mixed powder such a chalk. Weigh out five separate quantities to make up five equal volume of chalk in water called slurry of 21/2%, 5%, 71/2%, 10% and 121/2% concentration by weight,

2.      Slurry should make up in a separate beaker and volume in each should be identical.

3.      Stir slurry well in the preparation vessels and then fill each sedimentation tube in turn, starting with the most concentrated.

4.      The tubes should than remove from the retaining clips; cork should use to close off the open ends. To tube should be well shaken to give consistent suspension.

5.      The tubes should be replaced in the clips on the supporting frame, at the same interval of time between mixing and testing. Reading of the interface should be noted.

6.      In addition to noting the fall of the interface in each sedimentation tube at convenient time intervals, the rise of the sludge interface at the bottom of the tube should be recorded.

 

Graphical Analysis

Results obtained from the above analysis are elaborated with the help of following graphs.

Height Against Time Graph

Following graph simply depicts the height variation of interface in the sedimentation tube. As the time passes, height of the interface decreases. Solid particles, in quiescent water, settle under the force of gravity.

More particles will settle sediment interface rise up. With the passage of time sediment layer become thicker.

Different sedimentation curves display the behavior of solid particles of different concentration.

Log of Height Graph

As the sedimentation proceeds, the rate becomes non-linear due to hindered settling effects. A plot of time against log (H-H_o) Produces In general a straight line for the zone in which final compaction occurs and a typical plot is shown in the following diagram.

Concentration Rate Graph

Industrial Applications

Sedimentation is one of the methods used in industry to separate liquid-liquid or solid-liquid mixtures. By definition, sedimentation is the separation of a dilute slurry or suspension by gravity settling into a clear fluid and slurry of higher solids content. The resulting liquid is essentially particle free. In industry, either the particle free liquid or the particles itself is the desired product.

Sedimentation is applied to accomplish the following process:

Separation

For the separation of liquid, solid mixtures, when the solid is in form of colloid or suspended. For example separation of solid particles from inorganic compound likes oil, ester, carbon tetra chloride etc.

Purification

On industrial scale, most important application of sedimentation is the purification of water. Removal of physical impurities like stones, straws, leafs etc. Separation of impurities is done on the basis of density of the particle.

Definitions

Free Settling                      When the particle is at sufficient distance from the boundaries of the container or from other particles so that, its fall is not affected by them.

Hindered Settling              If the motion of the particles is impeded by other particles which will happen when the particles are near each other, though they may not be colliding, the process is called hindered settling.

Flocculants                        Flocculants, or flocculating agents, are chemicals that promote flocculation by causing colloids and other suspended particles in liquids to aggregate, forming a floc.

Short circuiting                  It is the term used for a situation in which part of the influent water exits the tank too quickly, sometimes by flowing across the top or along the bottom of the tank.