sedimentation tank where chemical coagulant is added to the water to facilitate the
settling of suspended particles having a diameter of less than 50 μm (Baker 1948).
The primary treatment will be beneficial in enhancing the efficiency of any treatment
plant as it reduces the significant amount of pollutant prior to reaching the secondary
treatment. The primary treatment generally reduces 90–98% of settleable solids and
60–80% of suspended solids along with 30–50% oxygen demand from the waste and
hence reduces the load of other treatment subunits (Peavy et al. 1985). Coagulation
is invariably achieved by using different metal salts that hydrolyses in water. These
metal salts adsorb the colloidal particles and form larger flocs. These flocs, therefore,
can easily be settled down after becoming larger particles under the effect of gravity
and later known as settled sludge. However, the settling of these colloidal particles is
not a facile task as they carry a net surface charge that causes to repel the particles.
Thus, before understanding the coagulation/flocculation process, it is required to
understand the colloids and their characteristics.
13.2 Colloidal Stability and Its Removal
Colloidal particles are an aggregate form of atoms/ions or molecules having density
close to the water with a smaller diameter that prevents them not to settle down under
the effect of gravity (Ghernaout et al. 2015). Thus, they are stable in nature if they are
suspended and do not agglomerate naturally as their stable phase possesses randomness showing Brownian movement (Peavy et al. 1985). The most critical factors
responsible for the stability of colloidal suspensions are surface charges and their
excessive large surface to volume ratio resulting from their minute size (Sawyer et al.
2003). The surface charge plays a primary contribution to particle stability as surface
phenomenon overruled mass phenomena. The surface phenomenon encompasses
the accumulation of electrical charges around the particle surface (Peavy et al. 1985).
These charges developed as a result of either the molecular arrangements within
crystals or the loss of atoms due to abrasion of the surfaces or other different factors
such as ionic strength of the surrounding medium, etc. Surface waters mostly possess
negatively charged colloidal particles on their surface. The charged particles present
in the solution phase cannot have a net charge, so the ions present in vicinity of the
particles in solution will affect the surface charge of the particles. The particle
charged must be counterbalanced by ions of opposite charge present in the solution.
This electroneutrality results in the formation of an electrical double layer. The
counter ions must be possibly configured as it is shown in Fig. 13.1.
The electrical double layer comprises of (1) a fixed or bound layer that contains
oppositely charged ions to attract the negatively charged surfaces and (2) a diffuse
layer, consisting of a mixture of charged ions as shown in Fig. 13.1. In between these
two layers, a thin layer, named Shear surface, is present that separates the two layers.
This arrangement of charged particles produces an electric potential between the
surface of the colloidal particles and the bulk of the solution, which is stronger at the
Stern layer and decreases exponentially with increase in distance from the colloids
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S. Sonal and B. K. Mishra
settling of suspended particles having a diameter of less than 50 μm (Baker 1948).
The primary treatment will be beneficial in enhancing the efficiency of any treatment
plant as it reduces the significant amount of pollutant prior to reaching the secondary
treatment. The primary treatment generally reduces 90–98% of settleable solids and
60–80% of suspended solids along with 30–50% oxygen demand from the waste and
hence reduces the load of other treatment subunits (Peavy et al. 1985). Coagulation
is invariably achieved by using different metal salts that hydrolyses in water. These
metal salts adsorb the colloidal particles and form larger flocs. These flocs, therefore,
can easily be settled down after becoming larger particles under the effect of gravity
and later known as settled sludge. However, the settling of these colloidal particles is
not a facile task as they carry a net surface charge that causes to repel the particles.
Thus, before understanding the coagulation/flocculation process, it is required to
understand the colloids and their characteristics.
13.2 Colloidal Stability and Its Removal
Colloidal particles are an aggregate form of atoms/ions or molecules having density
close to the water with a smaller diameter that prevents them not to settle down under
the effect of gravity (Ghernaout et al. 2015). Thus, they are stable in nature if they are
suspended and do not agglomerate naturally as their stable phase possesses randomness showing Brownian movement (Peavy et al. 1985). The most critical factors
responsible for the stability of colloidal suspensions are surface charges and their
excessive large surface to volume ratio resulting from their minute size (Sawyer et al.
2003). The surface charge plays a primary contribution to particle stability as surface
phenomenon overruled mass phenomena. The surface phenomenon encompasses
the accumulation of electrical charges around the particle surface (Peavy et al. 1985).
These charges developed as a result of either the molecular arrangements within
crystals or the loss of atoms due to abrasion of the surfaces or other different factors
such as ionic strength of the surrounding medium, etc. Surface waters mostly possess
negatively charged colloidal particles on their surface. The charged particles present
in the solution phase cannot have a net charge, so the ions present in vicinity of the
particles in solution will affect the surface charge of the particles. The particle
charged must be counterbalanced by ions of opposite charge present in the solution.
This electroneutrality results in the formation of an electrical double layer. The
counter ions must be possibly configured as it is shown in Fig. 13.1.
The electrical double layer comprises of (1) a fixed or bound layer that contains
oppositely charged ions to attract the negatively charged surfaces and (2) a diffuse
layer, consisting of a mixture of charged ions as shown in Fig. 13.1. In between these
two layers, a thin layer, named Shear surface, is present that separates the two layers.
This arrangement of charged particles produces an electric potential between the
surface of the colloidal particles and the bulk of the solution, which is stronger at the
Stern layer and decreases exponentially with increase in distance from the colloids
304
S. Sonal and B. K. Mishra