Suspended solids may consist of large solids, settleable by gravity alone without any external aids, and nonsettleable material, often colloidal in nature. Removal is generally accomplished by coagulation, flocculation and sedimentation. The combination of these three distinct processes is referred to as conventional clarification. This requires also three distinct unit processes:

  • High shear, rapid mix for coagulation;
  • Low shear, high retention time, moderate mixing for flocculation;
  • Liquid-solids separation.

Coagulation is the process of destabilization by charge neutralization and is necessary for the removal of the colloidal-size suspended matter.
Flocculation is the process of bringing together the destabilized, or “coagulated” particles to form a larger agglomeration, or “floc”.
Sedimentation refers to physical removal of this agglomeration from suspension. 

COAGULATION

This process can be accomplished through the addition of inorganic salts of aluminium or iron. These inorganic salts neutralize the charge of particles causing turbidity and also hydrolyse to form insoluble precipitates, which entrap particles. The selection of the coagulant metallic salt depends on initial raw water alkalinity and pH. Typical aluminium and iron coagulants are acid salts that lower the pH of the treated water by hydrolysis. Although the best clarification performance may not always coincide with the optimum pH for hydroxide floc formation, iron and aluminium hydroxide flocs are best precipitated at pH levels that minimize the coagulant solubility. For instance, with aluminium sulphate the optimum coagulation efficiency and the minimum floc solubility normally occur at pH 6,0 to 7,0; iron coagulants can be used successfully in the range of pH 5,0 to 11,0.
The most common coagulation process is a sequence of mixing raw water together with the coagulant salt in a tank having adequate volume and calibrated mixing velocity, in order to prevent the flocs from being destroyed.


FLOCCULATION


Floculation is the agglomeration of destabilized particles into large particles called flocs. This step can be enhanced by addition of water-soluble  high-molecular-weight organic polymers. The term polyelectrolytes refers to all water-soluble organic polymers used for clarification, whether they function as coagulants or flocculants. These polymers can be classified as anionic, cationic and non-ionic. For any given particle there is an ideal molecular weight and an ideal charge density for the best performed coagulation, there is also an optimum charge density and molecular weight for the most efficient flocculation. This means, because suspensions are normally non-uniform, that specific tests are necessary to find the coagulants and flocculants with the broadest range of performance.


SEDIMENTATION


Sedimentation is the liquid-solids separation and takes place in a process unit where the flow velocity is reduced dramatically in order to favour the physical removal of agglomeration from suspension. Originally, conventional units consisted of a rectangular basin where water movement was horizontal with plug-flow through this unit, particularly used for municipal water or large industrial plants. Since this unit requires more land space per unit of water capacity, sedimentation now takes place in circular units. In order to reduce the land space sometimes lamella pack is used to increase the virtual sedimentation surface; the result is a compact unit that can be made as pre-fabricated package reducing dramatically construction costs (for civil work) and minimizing the installation operation.


Further Information


- Theory of Clarification:

PART I

PART II

PART III


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