Environmental Engineering Reference
In-Depth Information
We should point out that when this text refers to
water quality
our definition
is predicated on the intended use of the water (in our case, for potable use,
obviously). Over the years, many parameters have evolved that qualitatively
reflect the impact that various contaminants (impurities) have on water used
for potable purposes; the following sections provide a brief discussion of
these parameters.
Taste and odor
Under the Safe Drinking Water Act (SDWA), the U.S. Environmental Protection
Agency (USEPA) issued guidelines to the states regarding the Secondary
Drinking Water Regulations. These guidelines apply to drinking water con-
taminants that may adversely affect the aesthetic qualities of water such as
odor and appearance. Because these aesthetic qualities have no known adverse
health effects, secondary regulations are not mandatory, but most drinking
water systems comply with the limits. They have learned through experience
that, although the odor and appearance of drinking water are not problems
until a customer complains, one thing is certain—they will complain, and com-
plain quite often (Spellman, 1998). Disinfection itself often becomes a major
source of complaint. Probably the most often expressed complaint among
drinking water consumers is a “chlorine taste,” although the odor threshold
can be as low as 0.2 to 0.4 mg/L at the typical pH level (De Zuane, 1997).
Taste and odor are used jointly in the vernacular of water science. As we
just observed, drinking water taste and odor are not normally a problem
until a consumer complains; however, drinking water practitioners have
learned through experience that such problems may be an early indication of
a potential health hazard. Taste and odor are important for aesthetic reasons
(as a measure of the acceptability of water) and rarely have an impact on how
safe water is to drink, but they should not be ignored.
Objectionable taste and odor are more likely found at the raw water source
than at the consumer's tap. In general, water contaminants are attributable
to contact with nature or human use. Taste and odor in water are caused
by a variety of substances, including minerals, metals and salts from the
soil, constituents of wastewater, and end products produced in biological
reactions. Earthy or musty odors common in some water supplies are nor-
mally derived from natural biological processes. More offensive odors, such
as those caused by hydrogen sulfide gas (H
2
S), are not unusual in water sup-
plies. The rotten-egg smell of hydrogen sulfide gas may be encountered in
water that has been in contact with naturally occurring deposits of decaying
organic matter. Groundwater supplies sometimes have this problem; such
wells are commonly referred to as sulfur wells.
