Coagulation and flocculation process in water treatment

Coagulation

Coagulation is defined as

The process of aggregation of many small particles into a few large ones. Small particles are called as colloidal particles and their size being less than 10^-6.

These colloidal particles have lesser velocity and more detention period. Detention period is a time taken by the particles to settle down. Coagulation is carried by the addition of certain chemicals which are called as coagulants.

Coagulation is a process in which coagulants are added for the purpose of rapid settlement of aggregates out of finely divided dispersed matter with slow or negligible velocity.

So a larger tank may be needed to complete this process. This involves more cost.

Purpose of coagulation

Coagulation purpose is to increase the settling velocity. When size of aggregates increases, their settling velocity automatically increases. Coagulation is completed in two steps.

1. Particle de-stabilization.
2. Particle transportation (Flocculation).

Particle de-stabilization

It involves use of chemicals. Stable particles aggregate very slowly. Most of the particles in water have a charge which may be negative or positive. Under most natural conditions, this charge is negative. If the chemicals we added, have the same charge as that of particles. Then they don’t combine with each other.

The charge of the particles may result from the various processes. For example ionization of the surface group. Many solid surfaces contain ionization groups. For example hydroxide ion etc.

When the charge on the particle results from ionization of the surface, it depends on the pH value of the solution.

1. At low pH-value, surface will be positively charged.
2. At high pH-value surface will be negatively charged.
3. At some intermediate stage, the charge may be zero.

The suspension of particles is stable when there exist a potential barrier. When two particles approach each other, the net repulsive force exist. If repulsive force is dominant, then the system is stable. So we have to de-stabilize the system by adding the chemicals which are called coagulants.

There are four methods by which chemicals can destabilize.

Compression of diffuse layer

The diffused layer can be compressed by addition of counter ions in the solution. Sodium, calcium and Aluminum ions are the counter ions for negatively charged surfaces. These counter ions are absorbed on the surface of particles. In this mechanism, the charge of particle is not changed. Electrostatic interaction is important. The precipitation of particles depend upon the change of ions.

Schulze-Hardy rule

According to this rule, critical concentration of coagulants is inversely proportional to the z⁶

Where

z = Charge on counter ions.

Adsorption and charge neutralization

The coagulant species that are to be adsorbed on the surface of the particles, should have a charge opposite to that of the particle. Upon adsorption, the charge of the particle is neutralized.

Enmeshment in precipitation

When large quantities of certain coagulants such as Al +++ and Fe+++ are added then precipitates formed are Aluminum hydroxide and Ferric hydroxide.

Adsorption and inter-particle bridge

Organic chemicals called polymers are also used for destabilization. It has been observed that negatively charged polymer can destabilize negatively charged particles. For example Alum.

Particle destabilization is carried out in rapid mixing tanks. Rapid mixing occurs in short time normally 1 min or even less.

Polymers may be used separately or in combination with inorganic coagulants for economy purposes.

Coagulant aids

They help in destabilization process of Coagulation. Generally these help to produce denser sludge. These include

1. Activated Silica.
2. Bentronite clay.
3. Poly electrolytes.

Flocculation

It is the process of slow mixing to provide inter-particle contacts.

The gentle mixing is usually done mechanically although hydraulic mixing is sometimes required. Typical detention time is 1 hour. The rapid mixing and flocculation tank together bring about aggregation and comprises the coagulation process. No material from water is removed in these tanks. Solids are removed in subsequent setting and filtration facilities.

The rate of flocculation depends upon

1. The number of particles present.
2. Their volume with respect to the volume of water.
3. Velocity gradient in the basin.

Tanks used for flocculation are called as flocculators. These are of two types.

1. Baffled flocculator.
2. Mechanical flocculator.

Velocity gradient

The intensity of mixing of the particles is defined as velocity gradient. It is the slope of the relative velocity between the two fluid elements.

The velocity gradient G is an important parameter for the design of flocculators. It is the change in velocity per unit length.

When G is higher, flocculation will be rapid and lesser time will be required to achieve flocculation. However if G is too high, it will result in excessive shear force and tend to shear apart the flow particles. To encounter this, tapered flocculation is done with the help of tapered flocculators.

Tapered flocculation

In tapered flocculation, high velocity gradient is used at inlet and low velocity gradient is used at outlet.

It can be done by gradual decrease of thickness of flocculator. In mechanical flocculators, it is done by decreasing paddle velocities. It is used to avoid break-up of particles.