97Principles of Coagulation........................
Untreated surface waters contain clay, minerals, bacteria, inert solids, microbiological organisms,
oxidized metals, organic color producing particles, and other suspended materials. Some of the
microbiological organisms can include Giardia cysts, pathogenic bacteria, and viruses. Oxidized
metals include iron and manganese. All of these materials can inhibit disinfection, cause problems
in the distribution system, and leave the water cloudy rather than clear. The purpose of coagulation
is to remove these particles.
The ability of particles to remain suspended in water is a function of both the particle size and
specific gravity. Turbidity particles can range in size from molecular to 50 microns. Particles which
are greater than one micron in diameter are considered silt, and settle out due to their relatively
large size and density without the need to coagulate in a matter of seconds or minutes.
Colloidal material ranges in size from 0.001 to one micron in diameter. These materials require
days to months for complete settling. Since detention times in the water treatment process are
generally less than twelve hours, the rate of settling of these colloidal particles must be increased
in the water treatment process. This is accomplished in the coagulation process when tiny
particles agglomerate into larger, denser particles
which will settle more quickly as shown in the picture
on the right.
These tiny colloidal particles have a very large surface
area to mass ratio, and this factor is important in
keeping the particles suspended for long periods of
time. In fact, the surface area to mass ratio is so high
that electric charges and ionic groups become
important in keeping the particles suspended. Two
types of colloids exist.
These are hydrophobic or water hating colloids, and
hydrophilic or water loving colloids. Hydrophilic colloids form suspensions easily, and can be
difficult to remove. These colloids can, however, react chemically with the coagulants commonly
added to water under proper conditions. Examples of hydrophilic colloids would be organic color
forming compounds. Hydrophobic colloids do not easily form suspensions. The reactions between
hydrophobic colloids and the coagulants commonly added to water are largely physical rather than
chemical. Examples of hydrophobic colloids would be clays and metal oxides.
The Coagulation Process
Coagulation is accomplished by the addition of ions having the opposite charge to that of the
colloidal particles. Since the colloidal particles are almost always negatively charged, the ions which
are added should be cations or positively charged. The coagulating power of an ion is dependent
on its valency or magnitude of charge. A bivalent ion (+2 charge) is 30 to 60 times more effective
than a monovalent ion (+l charge). A trivalent ion (+3 charge) is 700 to 1000 times more effective
than a monovalent ion.
Typically, two major types of coagulants are added to water. These are aluminum salts and iron
salts. The most common aluminum salt is aluminum sulfate, or alum.