Coagulation Process

8/10/2019 Coagulation Process

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CEG 507 Lecture Series No. 5: Tr eatment Process- Coagulati on. Apr il 14, 2014/Prof /EOL

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COAGULATION PROCESS

Remember:

Sedimentation process is used for the separation and removal of discreteparticles from water. What is

a discrete particle?

Also remember thatsettling velocityof particle is dependent on the followingfactors:

i. Density of the particle

ii. Density of the suspending liquid

iii. Particle size

iv. Shape of the particles

v. Fluid or liquid viscosity.

The above factors are only true for settling under quiescent conditions. However, for laminar

settling:-

i. Settling velocity of a particle is proportional to the square of its diameter.

-Colloidal-sized m1 particles: Particles are separated from water by forces that couldinhibitBrownian movement.

ii. Examples include: clay, (clay gives turbidity)

iii. Large molecules of complex organic acids (give coloration)

iv. Proteins, carbohydrates.

v. Oxides of iron, manganese and silica

Colloidal particles are removed by coagulation and flocculation.

Definitions

Coagulation: This is a process whereby colloidal suspensions are destabilized for particles

agglomeration. Destabilization is the physicochemical change which accompanies the addition of

chemicals and this allows particles to adhere to one another.

Flocculation: This is a process of particles agglomeration through stirring or agitation. Smaller

particles are brought together to form larger particles of adequate size, which settles under a velocity

accepted for separation. Particle collision opportunities occur as a result of relative particle transport.

Increasing the likelihood of collision is accomplished in a number of ways including the use of

paddles, baffled basins, hydraulic jets and turbulence in pipes and channels.

Coagulation describes the overall process of particle aggregation including both particle

destabilization and particle transportation. Flocculation on the other hand describes only the transport

process.

Colloids

Discrete particles and can remain in suspension in a dispersion medium due to Brownian

movement

Sizes range between 1 and 10m, note 1m equals 10 -6mm.

Examples include: - aerosols, fog, emulsions, smoke, solid sols.


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Properties of Colloids

i. Range of particle size, from 2 x 10-4to 10-5 m for dissolved materials.

ii. Cannot be removed from suspension by filtration. Removal is achieved by ultra filtration

or dialysis.

iii. Particles cannot settle under gravity due to Brownian motion.

iv.

Occur as aggravates of molecules or as single large molecules i.e.starches and proteins.

v. Have tendency to concentrate substances at their surface a process known as adsorption

vi. They have electrokinetic properties i.e. they are electrically charged.

vii. Colloids exhibit hydrophilic and hydrophobic properties

There are two types of solid colloidal dispersions in a liquid, hydrophilic (solvent-loving) and

hydrophobic (solvent-hating). Colloids in water and wastewater are not purely hydrophobic. Metallic

oxide and non-metallic oxide are hydrophobic colloids. Soap, detergents, soluble starch, and

soluble proteins are examples of hydrophilic colloids

Table 1: Examples of Colloids in Water

S/No Colloids Effects On Water1 Colloidal clay turbidity

2 Proteins turbidity

3 Carbohydrates turbidity

4 Fats Colour, taste

5 Large or organic molecules Colour

6 Oxides of Fe, Mn and silica Colour, taste.

The stability of colloids depends upon the hydration and the electric charge on their surface. This is

explained by their excessively large surface-to-volume ratio resulting from their very small size. The

electric charge influences the stability of hydrophobic colloids while adsorption helps in increasing

the charge on the colloids, depending on the valence and the number of ions adsorbed.

Table 2: Settling velocities of various particles* (after Peavy et al., 1985)

Particle

diameter

(mm)

Size Settling Velocity

10 Pebble 0.73 m/s

1 Coarse sand 0.23 m/s

0.1 Fine sand 1.0 x 10-2m/s (0.6 m/min)

0.01 Silt 1.0 x 10- m/s (8.6 m/d)0.0001 Large colloid 1.0 x 10- m/s (0.3 m/yr)

0.000001 Small colloid 1.0 x 10-13m/s (3 m/million yr)

*Spheres with specific gravity of 2.65 in water at 20oC

A double layer is formed around a colloidal particle (Fig.1).


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Figure 1: Electrochemical behaviour at the surface of colloidal particle

The layer to which the stabilizing ions are adsorbed is called the inner fixed layer or Helmholtz fixed

layer. Around the first layer a diffuse second layer is formed by ions of opposite charge. The psi

potential is the electrical potential between the interface of the colloid and the bulk of the solution.

The zeta-potential is the electrical potential between the rigid solution boundary and the bulk of the

solution and can be measured for colloidal particles.

The rigid solution boundary is defined as the boundary between the liquid which is immovably

attached to the colloidal surface and the body of the solution. For dilute solutions of electrolyte and

low potentials, the psi-potential and zeta-potential almost coincide.

Zeta-potential is defined by the following the following equation:

Z =D

qt4, in which:

Z = zeta potential

Q = charge difference between the particle and the medium

t = thickness of the layer around the particle through which the charge difference is

effective

D = dielectric constant of medium


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Instability of colloidal particles is caused by Brownian movement and Vander Waals forces of

attraction. Brownian movement makes particles come close to each other.

Figure 21: Forces fields between colloids of like charge

The net force is repulsive at greater distances and become s attractive only after passing through a

maximum net repulsive force called the energy barrier at some distance between colloids.The energy barrier is overcome before agglomeration of particles can occur. The energy required to

bring two particles together isinduced by Brownian movement or otherwise by relative movement in

the water. The processes are too slow, hence, in water purification; agglomeration is achieved by

chemically coagulating the colloids into clusters or flocs, which then can be removed by gravity

settling.

Coagulation

The coagulation process is set to achieve the destabilization of colloidal particles in suspension by

either of the following or both:-

a.

Reducing or neutralizing the charges on the colloidsb. Increasing the density of the counter-ion field, hence reducing the range of the repulsive effect.

Types of Coagulation

Perikinetic Coagulation:

It is the removal of hydrophobic colloids with use of electrolytes i.e. coagulants to form flocs. It is an

initial phase of coagulation process.

Orthokinetic Coagulation:-

It is achieved by stirring or rather movement in the water. Microflocs agglomerate with other flocs.


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Coagulation Process

Neutralization of charges is achieved by the addition of multivalent ions or colloids or both. Examples

of coagulants are Aluminum salts, Ferric salts. They are best applicable at optimum pH values of

between 6 and 7.

Note;-

At high pH values: colloids are negatively charged.

At low pH values: colloids are positively charged.

Reducing the Electrical field

- Pure water has low concentration of ions with low ionic strength.

- Increase in ionic strength of colloidal suspension tends to destabilize the colloids and hence

enhances coagulation.

Chemical Coagulation

Coagulation process removes color, taste, turbidity as well as microorganisms from water.

Commonly used coagulants are:-

1. Aluminum Compounds (Alum): Aluminum sulphates (Al2 (SO4)3. nH2O), where n ranges

between 12 and 16.

Coagulants may occur in granular or powdered form.

In order to achieve reasonable flocculation, l imeis added to the Alumto achieve required pH range.

Soda Ash and caustic soda could also be added.

Al2(SO4)3+ 6H2O 2Al (OH)3 + 3H2SO4

Ca (OH)2+ H2SO4 CaSO4+ 2H2O

(Lime)

2Na2CO3+ H2SO4 Na2SO4+ 2NaHCO3

(Soda Ash)

2NaOH + H2SO4 Na2SO4+ 2H2O

(Caustic soda)

Note

At low pH, soluble compound Al.OH.SO4is formed

At high pH = Aluminates ions AlO-2is formed.

The pH range depends on types of impurities present.

2.

Sodium Aluminates- (NaAlO2), an alkaline salt

Na AlO2+ 2H2O Al (OH)3 + Na OH

- Used alone or with alum.

- Does not give good coagulation compared with alum.


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3. Iron Salts

a. Ferric ion

b. Aluminum ion

Coagulation occurs at pH rage of between 7 and 8.5.

Examples

Ferric Chloride (FeCl3)liquid or crystalline form

Ferric Sulphate ( Fe (SO4)3)

Chlorinated copperasused for waters with lower pH

4. Lime

Either as burnt lime (CaO) or

Slaked lime Ca (OH)2)

Lime is commonly used to clean out scale in pipe.

COAGULANT AIDS

- Employed to improve coagulation

- Coagulant aids do not react with particles.

Examples

i. Clay i.e. Bentonite

Fullers earth

- used in water deficient in negatively charged colloids.

ii. Polyelectrolytes

Consist of long-chain macro molecules.

Consist of electrical charges or ionisable groupings.

It must be used with caution.

iii. Activated silica: used at a very small dose of 1mg of S 1O2/litre and at low temperature.

iv. Starch-based material

v. Tannin-based material

vi. Alginates

pH Adjustments

This is enhanced by use of the following

i. Sulphuric acid

ii.

Carbon dioxide -iii. Soda Ash (Sodium Carbonate)

iv. Lime (Calcium oxide/hydroxide)

v. Caustic soda (Sodium hydroxide)

Sulphuric acid and carbon dioxide and or carbonic acid are used to lower pH in alkaline waters. The

last three are used to increase hydrogen concentrations level (pH)

OPTIMUM COAGULANT DOSE

Things to consider:

Cost

Performance consideration: production of readily settleable flocs at reasonably shorttime.


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The estimate is done by Jar Test in the laboratory (Fig.3).

The jar test aims at:

To determine the best combination of coagulant.

To determine the needed pH adjustment.

To determine optimum pH coagulation

To determine if coagulant aids are necessary.

1 2 3 4 5

Figure 3: Jar test

Each beaker is filled with 1 litre of water

Then add in series 1ml -2ml3ml or 2ml-4ml etc. of 5g/l of Alum.

The concentration that gives the best coagulation is then chosen.

Example 1

Tests show that optimum coagulation with a particular water occurred when 1 litre of it was dosed

with 4 ml of a 10g/l alum solution and 1.6ml of a 5g/l suspension of lime. (Ca (OH)2).

Calculate the daily requirements of alum and lime to coagulate a flow of 350L/s

SolutionStep 1: Convert doses to mg/l.

Note that 1 litre of alum solution contains 10g of alum

1 litre of alum solution contains 10 x 1000 mg of alum

1000ml of alum solution contains 10 x 1000 mg of alum.

1ml of alum solution contains 10 mg of alum

Step 2: The optimum dose of 4ml will contain:-

4 x 10mg of alum

= 40mg of alum/litreLime dose.

Optimum dose 1.6ml of 5g/l suspension of lime

Note:

1 litre of lime solution contains 5g of lime as in 1 above

1 ml of lime solution contains 5mg of lime.

Optimum dose of 1.6ml will contain

1.6 x 5mg of lime

= 8.0mg of lime/litre


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Step 3: Conversion to Daily Requirements

1mg/l = 1g/m3

Daily requirement, volume, with a flow rate (Q l/s)

Take volume to, day (350 l/s)

350 x 60 x 60 x 24l

= 30240000l/d, change to m3/d by dividing by 1000l

= 30240 m3/ day

Multiply the mass of alum by the volume.

Multiply the massof lime by the volume

Alum 30240 x 40 mg = 1209600 mg/day = 1209.6g/day = 1.21kg/day.

Lime 30240 x 8 mg = 241920mg/day = 241.92/day =0.243kg/day.

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Coagulation Process