Coagulacion

acio3 BASIC PHYSICAL-CHEMICAL PROCESSES IN WATER TREATMENT
1. COAGULATION-FLOCCULATION
above but of smaller size and with a settling rate that is extremely slow. They are also responsible for turbidity and colour. 1. 1. 1. Suspended solids and colloids . Dissolved substances (less than several nanometres) 1.1.1.1. Definitions These are usually cations or anions. Part Water contains manycompounds, which of the organic matter is also dissolved. can be classified in three categories (see Gases are also present (O2 , CO2 , H2 S, etc.). page 5). . Suspended solids 1.1.1.2. The role of These products may be mineral in origin coagulation-flocculation (sand, silt, clays, etc.) or organic (products The coagulation-flocculation processes resulting from the decomposition of plant facilitate theremoval of SS and colloids. or animal matter, humic or fulvic acids, for This occurs in a final stage of solids-liquid example). Added to these compounds are separation: settling, flotation or filtration microorganisms such as bacteria, plankton, (sub-chapters 3, 4 and 5). algae and viruses. These substances, in In order to remove dissolved substances, particular, are responsible for turbidity andeach type of substance requires a specific colour. treatment, which may or may not be . Colloidal particles (less than 1 micron) preceded by coagulation-flocculation, or These are SS of the same origin as the even a solids-liquid separation process.

1.1. GENERAL

Chap. 3: Basic physical-chemical process in water treatment

1.1.2. Colloidal suspensions 1.1.2.1. Stability of colloidalsuspensions - The necessity for coagulation

Table 35 lists a number of materials and organisms with their size and an indication of the time needed for these particles to settle vertically through one metre of water, at 20°C, under the influence of gravity alone.

Table 35. Settling time for various particles. (According to Stokes' law)
Particle diameter mm µm Å 10 1 10-1 10-2 10-3 10-4 10-5 10-6104 103 102 10 1 10-1 10-2 10-3 108 10' 106 105 104 103 102 10 Type of particle Gravel Sand Fine sand Clay Bacteria Colloid Colloid Colloid Settling time through 1 m of water 1 second 10 seconds 2 minutes 2 hours 8 days 2 years 20 years 200 years Specific Area m2.m -3 6.102 6.103 6.104 6.105 6.106 6.107 6.108 6.109

The table also shows that the smaller the particle, the larger its specificarea. Thus colloids are particles that cannot settle naturally and for which surface area factors are most important. These factors determine the stability of colloidal suspensions. In fact, colloids are subject to two major forces: - Van der Waals attraction, which relates to the structure and form of colloids as well as to the type of medium (EA ), - the electrostatic repulsive force, which relatesto the surface charges of the colloids (EB). The stability of a colloidal suspension depends on the balance between the forces of attraction and repulsion, the energy level of which is: E=EA +EB

This relation is shown in the diagram in figure 32:

1. Coagulation - Flocculation

In order to destabilize the suspension, it is necessary to overcome the energy barrier Es. To accomplish thisand, thereby, promote the agglomeration of the colloids, it is necessary to reduce the electrostatic repulsive forces. This destabilization is brought about by coagulation 1.1.2.2. The double layer theory In raw water, colloids invariably carry a negative charge (imperfections in the crystalline structure, ionization of peripheral chemical groups, etc.). In order to neutralize this negative surfacecharge, positive ions, which are present in the raw water or are introduced into it, come together to form a layer around the colloid. Various theories have been put forward (Figure 33): . The Helmholtz theory: A layer of positive ions covers the entire surface of the colloid and ensures the neutrality of the entire mass (bound layer). . The Gouy-Chapman theory: The layer of positive ions is...