In this article we will discuss about:- 1. Meaning of Plain Sedimentation 2. Process of Treating Water by Plain Sedimentation 3. Principle.

Meaning of Plain Sedimentation:

If the water contains suspended impurities of large size, it is very economical to remove them by preliminary sedimentation. The suspended impurities make the water-turbid; therefore, when they will be removed more uniform water will be available for the further treatment processes.

Plain sedimentation is the process of removing suspended matters from the water by keeping it quiescent in tanks, so that suspended matter may settle down in the bottom due to force of gravity.

Plain sedimentation has the following advantages:


(i) It lightens the load on the subsequent process.

(ii) The operation of subsequent purification process can be controlled in a better way, because plain sedimentation delivers less variable quality of water.

(iii) The cost of cleaning the chemical coagulation basins is reduced.

(iv) No chemical is lost with sludge discharged from the plain settling basin.


(v) Less quantity of chemicals are required in the subsequent treatment processes.

Process of Treating Water by Plain Sedimentation:

In the process of treating water by plain sedimentation the water is retained in a basin so that the suspended particles may settle down due to force of gravity only. After the settlement of suspended particles has taken place, the water is taken out from the basin without causing any disturbance to the suspended impurities.

Plain sedimentation is suitable for relatively pure water which contains undesirable amounts of suspended matters. Practical experience has shown that the water containing large amount of suspended matter can be easily clarified by sedimentation than a water containing less suspended matter.

In the ancient days plain sedimentation was done by ‘Draw and Fill’ method. In this method first, the water was filled in the tank, then it was allowed to remain quiescent so that suspended impurities may settle, after this it was taken out. But now-a-days only uniform rate of flow (i.e., continuous flow type) sedimentation tanks are in practice. All the suspended impurities settle in the bottom and the clear water is drawn from the top.

Principle of Plain Sedimentation:


Any particle which does not alter its size, shape and weight while rising or settling in any fluid is called discrete particle. All the particles having more specific gravity than the liquid, will move vertically downward due to gravitational force.

When any discrete particle is falling through a quiescent fluid, it will accelerate until the frictional resistance or drag force becomes equal to the gravitational forces acting upon the particle. At such stage the particle will settle at uniform velocity. This uniform velocity is called ‘Settling Velocity’ and is a very important factor.

The impelling force at uniform settling velocity is equal to the effective weight of the particle in fluid.

If all the other values are known anything can be determined.


In metric units the values will be written as:

VS = settling velocity in cm/sec.

g = 981 cm/sec.


pS = specific gravity of the particle

d = diameter of settling particle in cm.

µ = viscosity of water in centi-strokes.

Moreover, since viscosity is dependent upon temperature, it is possible to convert this relationship (Eqn. 11.6) by changing viscosity term into temperature term, so as to avoid the trouble of reading the value of viscosity for different temperatures.

The equation, expressing settling velocity in terms of temperature is given as:

For small sized particles, i.e., d < 0.1 mm where vS is in mm/sec., d is in mm, and T is in °C.

If the diameter of the particles is more than 0.1 mm, the (eqn. 11.7) gets modified, and Hazen has given the following relation:

The value of differs with temperature.

Its values at various temperatures are as follows:

Hydraulic Settling Values of Various Particles in Still Water