Mar 11, 2012

Coagulation, flocculation & settling

Coagulation Process
in water treatment process Coagulation is the process that involves the addition of chemical coagulant(s) to reduce the repulsion forces between particles or the neutralization of the charges on the particles. Traditionally, metal salts such as aluminum sulfate or alum, ferric sulfate, ferric chloride, and ferrous sulfate have been utilized as coagulants. In recent years, polymers have been used in conjunction with, or in lieu of, metal salts to enhance the coagulation process.

When a metal salt is added to water that has sufficient alkalinity, it will hydrolyze into complex metal hydroxides of the form Meq(OH)p (Me = metallic ion). The actual hydroxide formed depends on the pH of water and the coagulant dosage. for aluminum hydroxide. 

The dosage of metal salts used in coagulation is usually slightly in excess of the amount required for reduction of the zeta potential. The excess metal salts hydrolyze into hydroxides which are extremely insoluble in water. As the hydroxide precipitate forms (Al(OH)3(s), Fe(OH)3(s), and accumulates, small colloidal particles are
entrapped or enmeshed in the sticky flocs settle. This process by which colloids are swept from suspension is known as sweep coagulation.

Chemical agents for coagulation

The coagulants most widely used for removing turbidity, color, taste, odor, bacteria and surface charge of particles in water treatment are compounds of iron and aluminum and polymers.
Aluminum sulfate, the most commonly used coagulation in water treatment, is most effective between pH ranges of 5 and 7.5. Ferric chloride is effective down to pH 4.5, and ferrous sulfate, effective only above pH 9.5, are sometimes used. Although they generally produce a dense, rapid-settling floc, they can cause color problems if the precipitate is not removed completely.

Alum dosages may range from 5 mg/L to 50 mg/L, depending upon the turbidity and nature of the water. At low turbidity and high dosage, Al(OH)3 is almost certain to form so that the predominant turbidity-removal mechanism is sweep coagulation. At high turbidity and lower dosages, adsorption and charge neutralization will be the predominant mechanism.

Flocculation Process

Flocculation is defined as the aggregation of destabilized particles into larger particles known as flocculent particles or “floc”.  The aggregation of colloidal particles takes place in two separate and distinct phases: (1) the repulsion force between particles must be overcome; this requires that the particles be destabilized; and (2) contact between the destabilized particles must be induced so that aggregation can occur. As mentioned earlier, the destabilization step is achieved by addition of chemicals to modify the electrochemistry properties on the particle surfaces. This coagulation process step is virtually instantaneous, in milliseconds to seconds,
following addition of the coagulant in rapid mix tanks. The aggregation step on the other hand, requires more time for development of large flocs, by gentle stirring in the flocculation tanks.

Flocculation Systems

There are two general groups of flocculation units: hydraulic flocculators, and mechanical flocculations. The hydraulic flocculators utilize cross-flow baffles or 180o turns to produce the required turbulence. The main design objective is to achieve gentle, uniform mixing that will not shear the floc. They are only used when flow rate is relatively constant and are rarely used in medium- and large sized water treatment plants. The mechanical flocculators typically used are paddle-wheel mixers, walking beam flocculators, flat-plate turbines, and axial flow propellers or turbines

There are three groups of hydraulic flocculators: baffled channels, hydraulic-jet flocculators, and coarse-media flocculators. Of these, baffled channels are the most commonly used. Hydraulic flocculators generally can produce good floc and often without much short circuiting. The main disadvantages are inflexible mixing and head loss across the basin.

to be continued

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