Page 229 - Water and wastewater engineering

6-6 WATER AND WASTEWATER ENGINEERING

Surface of particle A Surface of particle B

Repulsion curve no.1
Repulsive forces Repulsion curve no.2 forces Repulsive
Interaction energy 0 Net energy curve no.1 0 Interaction energy

Attractive forces Net energy curve no.2 forces Attractive

Van der Waals attraction
Distance between particles
FIGURE 6-4
Attractive and repulsive forces that result when two particles are brought together. Repul-
sion curve no. 1 and net energy curve no. 1 result when no coagulant is present. Coagulant
reduces the repulsion to curve no. 2.

• They have a high charge density.
• They are insoluble in the neutral pH range.

The inorganic chemicals commonly used in the United States are listed in Table 6-1 . They
are classified as hydrolyzable metal cations. In the United States, the predominant water treat-
ment coagulant is aluminum sulfate or “alum.” It is sold in a hydrated form as Al 2 (SO 4 ) 3 · x H 2 O
(where x is usually 14), because it is the least expensive coagulant (MWH, 2005).
Polyelectrolytes such as polydiallyldimethyl ammonium chloride (poly-DADMAC) and epi-
chlorohydrin dimethylamine (epi-DMA) are the typical organic coagulants used in water treat-
ment in the United States (MWH, 2005). Their chemical formulae are summarized in Table 6-2 .
They are water soluble and cationic.

Physics of Coagulation
There are four mechanisms employed to destabilize natural water suspensions:
• Compression of the electric double layer,

• Adsorption and charge neutralization,
• Adsorption and interparticle bridging, and
• Enmeshment in a precipitate.

Although these mechanisms are discussed separately, in practice several mechanisms are
employed simultaneously.