Page 80 - Water and wastewater engineering

GENERAL WATER SUPPLY DESIGN CONSIDERATIONS 2-23

3. Microbiological: Microbiological agents are very important in their relation to public
health and may also be significant in modifying the physical and chemical characteristics
of water.

4. Radiological: Radiological factors must be considered in areas where there is a possibility
that the water may have come in contact with radioactive substances. The radioactivity
of the water is of public health concern in these cases.

Physical Characteristics
Color. Dissolved organic material from decaying vegetation and certain inorganic matter cause
color in water. Occasionally, excessive blooms of algae or the growth of aquatic microorganisms
may also impart color. Often the color in water is not true color but apparent color that results
from a colloidal suspension. Tea is an example of apparent color. While color itself is not usually
objectionable from the standpoint of health, its presence is aesthetically objectionable and sug-
gests that the water needs appropriate treatment.

Taste and Odor. Taste and odor (T&O) in water can be caused by foreign matter such as
organic compounds, inorganic salts, or dissolved gases. These materials may come from domestic,
agricultural, or natural sources. Algae are frequently the source of T&O in surface water supplies.
T&O can also result as a byproduct of chlorine disinfection. Drinking water should be free from
any objectionable taste or odor at the point of use.

Temperature. The most desirable drinking waters are consistently cool and do not have tem-
perature fluctuations of more than a few degrees. Groundwater and surface water from mountain-
ous areas generally meet these criteria. Most individuals find that water having a temperature
between 10 C–15 C is most palatable. Municipal drinking water is not treated to adjust the tem-
perature. However, the temperature of the water may be an important water quality objective for
a client and may be an important consideration in the selection of the water source.

Turbidity. The presence of suspended material such as clay, silt, finely divided organic mate-
rial, plankton, and other particulate material in water is known as turbidity. The unit of measure
is a nephelometric turbidity unit (NTU). It is determined by reference to a chemical mixture that
produces a reproducible refraction of light. Turbidities in excess of 5 NTU are easily detectable
in a glass of water and are usually objectionable for aesthetic reasons.
Clay or other inert suspended particles in and of themselves may not adversely affect health,
but water containing such particles may require treatment to make it suitable for disinfection. In
general, turbidity reduces disinfection efficiency by consuming the disinfectant and shielding the
microorganisms. Following a rainfall, variations in the groundwater turbidity may be considered
an indication of surface or other introduced pollution entering the aquifer.

Chemical Characteristics
Arsenic. Arsenic occurs naturally in some geologic formations. It is widely used in timber
treatment, agricultural chemicals (pesticides), and the manufacture of computer chips, glass, and
alloys. Arsenic in drinking water has been linked to lung and urinary bladder cancer.