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Water Quality Characteristics and Drinking Water Standards
Chapter 10
The following definitions apply to both primary and
CONSTITUENTS
secondary US standards:
To be palatable, water must be significantly free from color,
1. Maximum contaminant level (MCL): The highest
turbidity, taste, and odor, of moderate temperature in summer
level of a contaminant that is allowed in drinking
and winter, and well aerated. At least four human perceptions
water. MCLs are set as close to MCLGs (maximum
respond to these qualities: the senses of sight (color and
contaminant level goals) as feasible using the best
turbidity), taste, smell (odor), and touch (temperature). If
available treatment technology and taking cost into
the pleasant sound of running water is considered one of its
consideration. MCLs are enforceable standards.
qualities, the sensory appeal of water becomes complete.
2. Maximum contaminant level goal (MCLG): The level
of a contaminant in drinking water below which there
10.4.1 Color
is no known or expected risk to health. MCLGs allow 10.4 PHYSICAL CHARACTERISTICS AND
for a margin of safety and are nonenforceable public
Dissolved organic material from decaying vegetation and
health goals.
certain inorganic matters cause color in water. Specifically
3. Maximum residual disinfectant level (MRDL): The color is of vegetable provenance, like the meadow-tea in
highest level of a disinfectant allowed in drinking the running brook of New England. However, water may
water. There is convincing evidence that addition of also become discolored by industrial wastes, natural iron
a disinfectant is necessary for control of microbial and manganese, and the products of corrosion. Occasionally
contaminants.
excessive algae blooms or the growth of aquatic microor-
4. Maximum residual disinfectant level goal (MRDLG): ganisms may also impart color. Color itself is not usually
The level of a drinking water disinfectant below objectionable from the standpoint of health. Its presence is
which there is no known or expected risk to health. esthetically objectionable. To appeal to visitor as well as
MRDLGs do not reflect the benefits of the use of native, the color of given water should be low. Yet the accus-
disinfectants to control microbial contaminants. tomed color of a water supply may have been quite high in
5. Treatment technique (TT): A required process the past, without eliciting comment. The recommended limit
intended to reduce the level of a contaminant in drink- for color is 15 color units in the United States and Canada,
ing water. US Environmental Protection Agency’s and the MAL for color is 20 color units in Mexico.
(US EPA) surface water treatment rules require sys-
tems using surface water or groundwater under the 10.4.2 Turbidity and Particle Count
influence of surface water to (1) disinfect their water
and (2) filter their water or meet criteria for avoiding The presence of suspended matter such as clay, silt, finely
filtration so that the contaminants are controlled at divided organic materials, plankton, and other inorganic and
the allowable levels. organic materials in water is known as turbidity. Following a
rainfall, variations in the raw water turbidity may be consid-
The following are similar terms used by different coun- ered an indication of surface or other introduced pollution.
tries: (1) maximum contaminant level (MCL) and maximum Turbidity may also come from eroding clay banks, or from
residual disinfectant level (MRDL)—United States; (2) max- industrial wastes, products of corrosion, and growths of other
imum acceptable concentration (MAC)—Canada; (3) max- microorganisms. Nephelometers are now the standard instru-
imum allowable limit (MAL)—Mexico; and (4) guideline ments for measurement of low turbidities in water and give
value (GV)—WHO. For simple comparison of the US drink- results in terms of the nephelometric turbidity unit (NTU).
ing water standards with international drinking water stan- Turbidities in excess of 5 NTU are easily detectable in a glass
dards (WHO, Canada, and Mexico), the “maximum allow- of water and are usually objectionable for esthetic reasons.
able concentration” represents MCL, MAC, MAL, and GV Turbidity is the traditional parameter for water qual-
in this chapter; while “recommended concentration” and ity control in water treatment. Turbidity is used to indi-
“recommended limit” represent the drinking water standards, cate water quality and filtration effectiveness (e.g., whether
which are not legally enforceable. disease-causing organisms are present). Higher turbidity lev-
The US Primary Drinking Water Standards and the US els are often associated with higher levels of disease-causing
Secondary Drinking Water Standards are summarized in Sec- microorganisms such as viruses, parasites, and some bacte-
tion 10.8. The Mexico Drinking Water Standards are dis- ria. These organisms can cause symptoms such as nausea,
cussed in Problem 10.14. The drinking water standards listed cramps, diarrhea, and associated headaches. Particle counts
and discussed in this book are for reference only. An engineer which are measured by particle counters are gaining pop-
in any country, any state/province must know how to search ularity to evaluate water treatment efficiencies and finished
for the most current applicable drinking water standards from water quality. A particle count of 50–100 particles/mL (in the
the literature for his/her specific water engineering project. size range of 2.5–150 μm) represents a finished water quality