Environmental Engineering FOURTH EDITION

Water Treatment 137

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Figure 7-2. Charges on a suspended particle, as explained by the double-layer theory.

the particles do not repel each other and, on colliding, stick together. A stable colloidal
suspension can be destabilized in this way, and the larger particles will not remain
suspended.
Alum (aluminum sulfate) is the usual source of trivalent cations in water treatment.
Alum has an advantage in addition to its high positive charge: some fraction of the
aluminum ions may form aluminum oxide and hydroxide by the reaction

AI3' + 30H- + Al(0HhJ..

These complexes are sticky and heavy and will greatly assist in the clarification of
the water in the settling tank if the unstable colloidal particles can be made to come
in contact with the floc. This process is enhanced through an operation known as
flocculation.
A flocculator introduces velocity gradients into the water so that the particles in a
fast-moving stream can catch up and collide with slow-moving particles. Such velocity
gradients are usually introduced by rotating paddles, as shown in Fig. 7-4. The power
required for moving a paddle through the water is

CDA~V~

p=-

2'

where

P = power (N/s or ft-lb/s),

A = paddle area (m2 or ft2),