Science of Water : Concepts and Applications

106 The Science of Water: Concepts and Applications
Some of the common ions found in water are:
Ion Symbol
Hydrogen H +
Sodium Na +
Potassium K +
Chloride Cl −
Bromide Br −
Iodide I −
Bicarbonate HCO 3 −
Water dissolves polar substances better than nonpolar substances. This makes sense when you
consider that water is a polar substance. Polar substances such as mineral acids, bases, and salts
are easily dissolved in water—while nonpolar substances such as oils, fats, and many organic compounds
do not dissolve easily in water.
Water dissolves polar substances better than nonpolar substances but only to a point. Polar substances
dissolve in water up to a point—only so much solute will dissolve at a given temperature, for
example. When that limit is reached, the resulting solution is saturated. When a solution becomes
saturated, no more solute can be dissolved. For solids dissolved in water, if the temperature of the
solution is increased, the amount of solids (solutes) required to reach saturation increases.
WATER CONSTITUENTS
Natural water can contain a number of substances (what we may call impurities) or constituents
in water treatment operations. The concentrations of various substances in water in dissolved,
colloidal, or suspended form are typically low but vary considerably. A hardness value of up to
400 ppm of calcium carbonate, for example, is sometimes tolerated in public supplies, whereas 1 ppm
of dissolved iron would be unacceptable.
When a particular constituent can affect the good health of the water user or affect the environment,
it is called a contaminant or pollutant. These contaminants, of course, are what the water
operator works to prevent from entering or removes from the water supply. In this section, we discuss
some of the more common constituents of water.
SOLIDS
Other than gases, all contaminants of water contribute to the solids content. Natural water carries
many dissolved and undissolved solids. The undissolved solids are nonpolar substances and consist
of relatively large particles of materials such as silt, which will not dissolve. Classifi ed by their size
and state, chemical characteristics, and size distribution, solids can be dispersed in water in both
suspended and dissolved forms.
Size of solids in water can be classifi ed as suspended solids, settleable, colloidal, or dissolved.
Total solids are those that are suspended and dissolved solids that remain behind when the water is
removed by evaporation. Solids are also characterized as volatile or nonvolatile.
The distribution of solids is determined by computing the percentage of fi lterable solids by size
range. Solids typically include inorganic solids such as silt and clay from riverbanks and organic
matter such as plant fi bers and microorganisms from natural or human-made sources.
√ Important Point: Although not technically accurate from a chemical point of view because
some fi nely suspended material can actually pass through the fi lter, suspended solids are defi ned
as those that can be fi ltered out in the suspended solids laboratory test. The material that passes
through the fi lter is defi ned as dissolved solids.