Environmental Engg. 3(2+1)

Lesson-12 Water treatment - sedimentation and coagulation

Sedimentation

            Having examined the quality of water, a line of treatment is to be recommended for impure water to make it potable or fit for drinking purposes.  The first stage of treatment is the prefiltration of water and it includes provision of sedimentation tanks or settling tanks or clarifiers.  These tanks remove inorganic impurities and make water fit for the next process of filtration.

            The sedimentation tanks are designed to give complete rest to the flowing water or water is allowed to flow at a very low velocity. The heavier inorganic impurities settle at the bottom of tanks and the lighter inorganic impurities float on the surface of liquid level. The former impurities are removed from the bottom while the latter impurities are removed from the top.

            The actual amount of matter removed by sedimentation tanks depends on several factors such as design of tank, detention period, size of suspended particles, velocity of flow, etc. It is estimated that plain sedimentation tank can remove about 60% of suspended matter and about 75% of bacterial load from water.

            In order to make the sedimentation tanks effective, coagulants are added to water before it is bought to the sedimentation tanks. The topic of coagulation is discussed at length in the next chapter.

            The sedimentation tanks are located near filter units and in case of variations in demand, they may even be called upto to work as storage reservoirs. The height of location of sedimentation tank should be decided by keeping in view the natural configuration of locality and the pressure head required.

            Where filtration is to be adopted, it is essential to provide sedimentation tanks.  The treated water from sedimentation tanks enters filter units for further purification.

Theory of sedimentation

            The particles which are heavier than water are naturally likely to settle down due to force of gravity. In water, there are mainly two types of impurities.

(1)   Inorganic suspended solids having specific gravity of about 2.65; and

(2)   Organic suspended solids having specific gravity of about 1.04

The particles having specific gravity of about 1.20 or so readily settle down at the bottom of tank. But it is difficult to cause the settlement of lighter particles.  This phenomenon of settling down of particles at the bottom of sedimentation tank is known as hydraulic subsidence and every particle has its own hydraulic settling value which will cause its hydraulic subsidence.

The process of settlement of a particle is obstructed or opposed by the following three forces:

(1) Velocity of flow: The particle is moved in the horizontal direction by the velocity of flow

(2) Size and shape of particle : The force of gravity depends on the weight of particle and tends to move the particle in vertical direction. Consequently, small particles will settle down very slowly.

(3) Viscosity of water: This force offers frictional resistance to the movement of water and it mainly depends on the temperature of water

In 1815, G.G. Stokes developed and expression known as Stokes’s law and it is applicable to the discrete particles. The particles which do not change in size, shape or mass during settling are known as discrete particles.  The expression is as follows:

\[v=418{d^2}\left( {s-{s_1}}\right)\frac{{3\cdot T+70}}{{100}}\]

v = Velocity of settlement in mm per second

s= Specific gravity of the particle

s₁= Specific gravity of water

d = Diameter of particle in mm

T= Temperature in ^oC

            It was found by Hazen the Stoke’s law was applicable for particles having diameter small than 0.10mm or so. For particles of greater diameter than 0.0mm, he found that velocity of settlement was proportional to the first power of diameter and not to the second power of diameter as expressed in Stoke’s law.  Hence, Stokes’s law for bigger particles would be :

            Now, out of three forces which oppose the tendency of settlement of particle, attempts are made to control the first and second forces in purification process of water. The third force, namely, viscosity of water, is unpracticable to control as it is dependent on temperature. The control of temperature of a huge quantity of water becomes unreasonable and uneconomical.

            The velocity of flow can be decreased by increasing the length of travel and thus a particle is allowed to stay for a longer period in the sedimentation tank. The particle is thus given maximum opportunity to come down and settle at the bottom of tank.

            The size and shape of the particle are altered by the addition of certain chemicals in water. These chemicals are known as coagulants.

Coagulation

            The source of water supply for the most of public water supply project is surface water.  This water is turbid and contains many suspended impurities. It also possesses colour which may be due to colloidal matter and dissolved organic material in water.  The turbidity is mainly due to the presence of very fine particles of clay, silt and organic matter.

            All these impurities are in a finely divided state and it is not possible to detain them in plain sedimentation tanks unless such tanks are designed for longer detention periods.  The other alternative to remove such particles is to increase their size so that they become settleable.  The purpose of coagulation is thus to make particles of bigger size by adding certain chemicals know as coagulants to water.  The coagulants react with the impurities in water and convert them in settleable size.

            The coagulation is to be adopted when turbidity of water exceeds about 40 p.p.m. It should, however, be remembered that it is not complete process by itself.  It simply assists plain sedimentation and it is to be followed by the process of filtration. Thus, coagulation is merely a process by which impure water is prepared for successful purification by rapid sand filtration.

Principle of coagulation

            The principal of coagulation can be explained from the following two considerations.

1. Floc formation

When coagulants are dissolved in water and thoroughly mixed with it, they produce a thick gelatinous precipitate. This precipitate is known as floc and this floc has got the property of arresting the suspended impurities in water during its downward travel towards the bottom of tank.

           2.Electric charges

The ions of the floc are found to possess positive electric charge.  Hence, they will attract the negatively charged colloidal particles of clay and thus they cause the removal of such particles from water.

Flocculation

            The floc produced by the action of coagulants with water is heavy and hence, it starts to settle down at the bottom of tank.  As it descends, it absorbs and catches more and more suspended impurities present in water. It thus slowly goes on increasing in size.  During this process, some amount of bacterial removal also takes place. The surface of floc is sufficiently wide to arrest colloidal and organic matter present in water. The term flocculation is used to denote the process of floc formation and thus flocculation follows the addition of coagulant and its efficiency depends on the following factors.

1.      Dosage of coagulant

The dosage or quantity of coagulant should be carefully determined so as to cause visible floc.  The quantity of coagulants should be such that turbidity of water is brought down to the limit of 10 to 25 p.p.m.

2.      Feeding

The feeding of coagulants may be in powder form or in solution form, the latter being more popular

3.      Mixing

The coagulants should be properly mixed with water so as to cause a uniform mass. In the beginning, the mixing may be quick for a period of about 30 to 60 seconds or so.

4.      pH value

Depending upon the quality of water and coagulant adopted, suitable pH value should be determined. The pH value should be actually tested in the laboratory at regular intervals. To remove acidity, lime is added to water and to remove alkalinity, sulphuric acid is added to water.

5.      Velocity

The floc  should be allowed to move gently after initial quick mixing, The gentle movement of floc results in collision of particles and ultimately, the floc grows in size.  The detention period of coagulated sedimentation tanks is about 3 to 4 hours.

The processes of coagulation and flocculation are greatly influenced by the physical characteristics of water, its dissolved constituents and the temperature. The  failures in coagulation plant are due to incorrect does of the coagulant, inadequate mixing arrangements, improper tank design, etc. Hence, the characteristics of water to be submitted to the coagulation plant should be properly studied before deciding the details of the plant.

Usual coagulants

Following six are the usual coagulants which are adopted for coagulation

1. Aluminium sulphate

2. Chlorinated chopperas

3. Ferrous sulphate and lime

4. Magnesium carbonate

5. Polyelectrolytes

6. Sodium aluminate