Page 166 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

4.2 CHAPTER FOUR

BASIC WATER CHEMISTRY

Water to be treated is known variously as raw water, feedwater, or source water. Water
that has been treated is known as treated water, product water, or solute. Impurities that
dissolve in water are called electrolytes. The dissolved impurities dissociate (separate) to
form negatively and positively charged atoms called ions. The positively charged atoms
are called cations because they migrate to the cathode electrode; the negatively charged
atoms are called anions because they migrate to the anode electrode. The ions in solution
act almost independently. For example, magnesium sulfate dissociates to form positive
magnesium ions and negative sulfate ions. Ions are often referred to as salts in reverse
osmosis water production.
When a compound dissolves in water, the molecules of the compound separate, disperse
among the molecules of water, and then are held in suspension. When the solubility limit
is reached, those same compounds become suspended solids because there is no chemical
reaction. Dissolved materials cannot be removed by filtration.
Some types of liquids cannot be dissolved. They break down into extremely small par-
ticles and then disperse into water, but do not dissolve. These liquids are called immiscible
liquids, and the resulting small particles are known as colloids. Colloidal material is com-
posed of suspended particles at the upper end of the size range for ions and molecules, from
0.001 to 1.0 μm. Other liquids, such as oil and grease, cannot separate into smaller particles
and become dispersed, but simply remain in suspension. If they are lighter than water, they
float on top. If heavier, they sink below the surface.
All acid compounds in water chemistry consist of hydrogen combined with an acid radi-
cal. Since the acid radical moves around as a unit, it is convenient to view an acid radical as
an integral anion unit. When a metal radical and an acid radical combine, they form a class of
chemicals called salts. When a metal cation and a hydroxide anion combine, a base results.
The valence of any element is the measure of its chemical combining power compared
to that of a hydrogen atom, which has the assigned value of 1. An element with a valence of
+2 can replace two hydrogen atoms in a compound; with a valence of −2, it can react with
2 hydrogen atoms. When atoms combine to form compounds, a cation must combine with
an anion and, in addition, the valences of the two basic elements must be equal in order to
form the compound.
Most of the basic chemical reactions in water treatment consist of rearranging cations
and anions using their valences. As can be seen in Table 4.1, hydrogen has a valence of
1 and sulfate has a valence of 2. When combined to form sulfuric acid, two hydrogen atoms
are required to form the combination, resulting in the formula H SO . The chief exception
2
4
is the case where carbonates and bicarbonates are destroyed by heating or aeration, giving
off carbon dioxide.
The term equivalent weight is the weight in pounds of any element that could combine
with 1 lb of hydrogen. Since the valence of an element is proportional to its combining
power, the equivalent weight is based on its valence. This is illustrated in Table 4.1.

WATER IMPURITIES

Natural or source water is never pure. Water picks up impurities as it comes into contact
with the ground or, when percolated through the earth, mineral formations. It also contains
dissolved gases and dust picked up by rain, snow, and hail or by surface water in contact
with the air above the water level. Water is classified as surface water when it comes from
sources such as lakes and rivers and as ground water when it comes from streams, wells, or
other aquifers originating underground.

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