Environmental Engineering Reference
In-Depth Information
TAble 7.1
Water Hardness Classifications
Calcium Carbonate Concentration (mg/L)
Classification
<75
Soft water
75 to 150
Moderately hard
150 to 300
Hard
>300
Very hard
waters are classified according to degree of hardness, as shown in
Table 7.1
.
Hardness is primarily a function of the geology of the area with which the
surface water is associated. Waters underlain by limestone are prone to hard
water because rainfall, which is naturally acidic because it contains carbon
dioxide gas, continually dissolves the rock and carries the dissolved cations
into the water system.
Standard Methods
(APHA-AWWA-WEF, 2005) recom-
mends measuring hardness as follows:
Hardness = 2497(Ca) + 4.118(Mg)
(7.3)
where total calcium (Ca) and total magnesium (Mg) are expressed in mg/L.
Hardness can also be measured using the
EDTA titration method
(APHA-
AW WA-W E F, 2 0 0 5).
Solubility
Solubility is a term often used in connection with water treatment, labo-
ratory analyses, and chemical and physical studies of water, even though
solubility is not a general, physical, or chemical parameter. To understand
solubility, you must also understand the concept of a water solution, which
is a homogeneous liquid comprised of a solvent (the substance that dissolves
another substance) and a solute (the substance that dissolves in the solvent).
Simply stated, solubility is defined as the mass of substance contained in a
solution that is in equilibrium with an excess of the substance.
Summary
The biological and physical characteristics of water represent two thirds of
the parameters critical to understanding the elements drinking water tech-
nologists may encounter. The final third that makes up the whole—chemical
parameters—is addressed in
Chapter 8
.
