COAGULATION - FLOCCULATION

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COAGULATION - FLOCCULATION

The purpose of water clarification is to remove the suspended particles and colloidal materials from water supplies or from wastewater. Dense suspended solids such as sand can be easily removed by sedimentation. Problems in clarification begin with the less dense suspended particulate matter and the colloidal materials. Small, less dense particulate matter may be removed by sedimentation only with extended detention times not available in many situations. Colloidal materials form very stable water suspensions. Colloidal particles may consist of clay and silt, color bodies, precipitated iron or manganese oxides, and bacteria and algae. Treatment processes such as lime softening may produce colloidal calcium carbonate or other precipitates requiring clarification. Often, many colloidal species are formed during the secondary biological waste treatment process. The need for water and wastewater clarification exists in most every type of situation involving water renovation.

Coagulation and flocculation are the mechanisms by which particulate and colloidal materials are removed from water in the process of clarification. Coagulation can be defined as charge neutralization which results in the destabilization of suspensions of particles in the colloidal size range (1-500 millimicrons) allowing agglomeration to occur. Colloidal particles have a negative electrical charge. This net negative charge results in stable suspensions due to the repulsive forces between each particle. As the halo of net negative charge surrounding each particle is neutralized, it is stripped away reducing the effective particle diameter. One may easily visualize the particle agglomeration that may occur through natural particle collision, helped by slow mixing, once the charge is neutralized and the effective diameter of the particles is reduced.

Theoretically, the term coagulation may also be applied to the neutralization of a net positive charge surrounding particles suspended in water. However, in most waters requiring charge neutralization, it is a net negative charge that must be neutralized. Therefore, the term coagulant is applied only to cationic chemicals.

The extent of particle agglomeration due to coagulation cannot be predicted. In some circumstances very small numbers of particles may agglomerate to form microfloc. The coagulated material may or may not be agglomerated enough to achieve good settling without flocculation. A high density macrofloc may not need further treatment, while microfloc and low density macrofloc will likely require flocculation for optimum clarification.

Flocculation can be defined as the mechanism by which microfloc or low density macrofloc particles are further agglomerated resulting in rapid settling floc bodies and enhanced finished water quality.

Inorganic coagulants have been used to clarify water for years. Trivalent ions such as aluminum and ferric iron coagulate colloidal suspensions by charge neutralization and by promoting agglomeration. Therefore, in addition to their coagulating ability, they are also capable of further flocculation through their ability to form hydrated gelatinous hydroxides, at appropriate pH levels. These gelatinous hydroxides entrap destabilized particles as they sweep through the water under the force of gravity.

Polymeric coagulants, or that class of polyelectrolyte with relatively low molecular weight (compared to flocculant polymers) and a high cationic charge density, are finding wide application as coagulants in water clarification. Polymeric coagulants also have some ability to cause flocculation through a mechanism quite dissimilar to that of the inorganic coagulants. Flocculation by polymer coagulants is brought about via their molecular weight (M.W.) which is very high as compared to inorganic coagulants.

Molecular weight, for the purposes of this discussion, may be pictured as representing polymer chain length. The greater the M.W., the greater the chain length and the greater the flocculating ability. Longer chain length allows bridging, or attaching to, greater numbers of particles. However, coagulant polymers have relatively low molecular weights and flocculating ability when compared to the high molecular weight cationic, nonionic and anionic flocculant polymers. Therefore, polymeric coagulants have the ability to enhance flocculation only to a limited degree.

Many water and wastewater treatment facilities using either inorganic or polymer coagulants may require enhanced flocculation to meet water quality requirements. Enhanced flocculation may be brought about via the higher molecular weight cationic, nonionic, or anionic flocculants. Flocculants may be pictured as working mainly by their ability to bridge microfloc particles producing larger, more dense, and faster settling floc while also producing a clearer, solids free, supernatant.

It is difficult to predict whether a cationic, nonionic, or anionic flocculant will produce the best result. Due to this non-predictability, there can be no substitute for exhaustive jar testing when it comes to selecting coagulants and flocculants for an economic system of water clarification.

The preceding discussion was slanted toward gravity sedimentation carried out in clarifying equipment of various designs. This same model can be used when developing a chemical treatment system for enhanced clarification via dissolved air flotation, D.A.F.. First, there must exist solid, particulate material in the form of microfloc to separate from the aqueous phase. These particles can generally be produced or maximized by charge neutralization and destabilization of the suspension with coagulant materials. Once the cationic demand is satisfied, further flocculation of the particles may be obtained with polymer flocculants. These materials are used to produce floc to which the minute air bubbles will attach and float to the surface intact.

In facilities presently using inorganic coagulants to satisfy a high cationic demand, it may not be economically possible to replace the inorganic with a polymeric coagulant. However, partial replacement of the inorganic with polymer may be beneficial in terms of chemical handling ease and lower solids production. Total replacement of inorganic coagulants with polymer material is becoming commonplace with the continuing development of coagulants with higher charge density.