What Coagulation Is All About

The term coagulation refers to the first step in complete clarification. It is the neutralization of the electrostatic charges on colloidal particles. Because most of the smaller suspended solids in surface waters carry a negative electrostatic charge, the natural repulsion of these similar charges causes the particles to remain dispersed almost indefinitely. To allow these small suspended solids to agglomerate, the negative electrostatic charges must be neutralized. This is accomplished by using inorganic coagulants, which are water soluble inorganic compounds), organic cationic polymers or polyelectrolytes. The most common and widely used inorganic coagulants are:

• Alum-aluminum sulfate-Al2(S04)3

• Ferric sulfate-Fe^SOJj

• Ferric chloride-FeC 13

• Sodium aluminate-Na2Al204

Inorganic salts of metals work by two mechanisms in water clarification. The positive charge of the metals serves to neutralize the negative charges on the turbidity particles. The metal salts also form insoluble metal hydroxides which are gelatinous and tend to agglomerate the neutralized particles. The most common coagulation reactions are as follows:

A12(S04)3 + 3Na2C03 + 3H20 = 2A1(0H)3 + 3Na2S04 + 3C02

A12(S04)3 (NH4)2S04 + 3Ca(HC03) = 2A1(0H)3 + (NH4)2S04 + 3CaS04 + 6C02 A12(S04)3 K2S04 + 3Ca(HC03) 2 = 2A1(0H)3 + K2S04 + 3CaS04 + 6C02 N^AIA + Ca(HC03)2 + H20 = 2A1(0H)3 + CaC03 + Na2C02 Fe(S04)3 + 3Ca(OH)2 = 2Fe(OH)3 + 3CaS04 4Fe(OH)2 + 02 + 2H20 = 4Fe(OH)3

The effectiveness of inorganic coagulants is dependent upon water chemistry, and in particular — pH and alkalinity. Their addition usually alters that chemistry. Table 2 illustrates the effect of the addition of 1 ppm of the various inorganic coagulants on alkalinity and solids concentration.

Table 2. Coagulant, Acid and Sulfate - 1 ppm Equivalents.

1 ppm Formula or Chemical

Increase

Increase

ppm Total Solids Increas e

A12(S04)318H20

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