SEDIMENTATION

The water contains variety of particles, mineral as well as organic. Some particles may be box, colloidal, very fine or coarse. Also, the specific gravity of the particles may range from 1.01 to 2.65. This indicates that different particles will require different time for their settlement.

Sedimentation is a process of removal of suspended particles of impurities by gravitational settling. It occurs when particles are heavier than water. This process takes place in a basin or tank usually known as sedimentation tank. Sometimes the tank is also called as settling tank, settling basin or clarifier.

Principle of Sedimentation

It is noticed that large amount of suspended impurities have a specific gravity greater than that of water (i.e. 1). These impurities, however, remain in suspension due to turbulence in water. If this turbulence is brought down to zero (i.e. still water) or reduced to a considerable extent by reducing flow velocity, these impurities will settle down.

Hence, in short, reducing the velocity of water to cause the settlement of suspended solid: by gravity is the principle of sedimentation.

Principle of sedimentation is applied for the removal of

(i) Grit in grit chamber, (ii) destabilized floc in clariflocculators, (iii) particulate matter in primary and secondary settling basins.

Types of Sedimentation

1. Plain sedimentation.
2. Sedimentation with coagulation or chemically assisted sedimentation.

1. Plain Sedimentation

When suspended matter in water is separated by the action of gravitation and natural aggregation, the process is called plain sedimentation. Suspended particles of size 0.01 mm and more and having specific gravity greater than 1 are removed by plain sedimentation. When the turbidity of raw water is greater than 400 mg/lit, plain sedimentation process is recommended.

Discrete Particles

When a unigranular particle of spherical shape do not undergo any change in its size, shape and density during settling or rising in water, it is called as Discrete Particle'. These particles have little or no tendency to flocculate. They behave as individual particles and hence settle freely. Particle size is greater t an 1$0^{-4}$ cm.

Theory of Sedimentation

The settling of particles in water depend upon the following factors

• Velocity of flow of water.

• Viscosity of water.

• Size, shape and specific gravity of suspended particle.

• Detention time It is assumed that discrete settling takes place in plain sedimentation as majority of particle are assumed to be discrete particles. When discrete particle is placed in still fluid, it will accelerate until the frictional resistance (drag force) of water equals the driving force acting on the particle. At this instant the discrete particle attains a uniform or terminal velocity and settles down with this constant velocity, often called as settling velocity.