Page 67 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

22 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

BOX 2.1 ON WATER QUALITY By the 1980s, the idea of water quality had moved
well beyond the traditional notions prevalent in the
In the 1880s, notions of water quality were limited by
the knowledge in the two of its science ‘‘mother’’ 1950s. Also, the spectrum of contaminants was very
broad and might well have included more than 100 in
fields—chemistry and bacteriology. Inorganic constitu-
a typical analysis. Because of both perceived health
ents were expressed as concentrations of various salts,
risks and analytical capabilities that included more com-
e.g., calcium sulfate, sodium chloride, etc. Many years
pounds and at lower levels, the number regulated
would elapse before inorganic chemical water quality
increased to several thousand contaminants.
would be expressed in terms of cations and anions, e.g.,
as Ca ,SO 4 , etc. The ion theory had not yet taken
2
2þ
hold, being proposed only in 1887 by Svente Arrhenius
(1859–1928), professor of chemistry, University of 2.1.2 STATE OF WATER
Uppsala, but not accepted until years later. Regarding
The state of a volume of water, as defined here, refers to its
microbes, the science of microbiology was just being
water quality. Characteristics of water quality that may com-
defined, based upon the work of Pasteur in 1861, Lister
prise its ‘‘state’’ include temperature; concentrations of vari-
in 1867, and Koch in 1876 and 1882 (Prescott et al.,
ous kinds of particles; concentrations of dissolved materials;
2005, p. 8). By about 1882, the science of bacteriology
and parameters such as turbidity, pH, color, conductivity, etc.
had an identity.
The idea of ‘‘state’’ (defined usually, in the field of physical
Courses in ‘‘sanitary chemistry’’ that evolved from
chemistry, as pressure, temperature, volume) comes from the
this background were focused largely on wet chemistry
field of physical chemistry and is the same as a water quality
analysis of such constituents as alkalinity, hardness,
‘‘profile.’’ Both water quality ‘‘state’’ and water quality ‘‘pro-
nitrate, chloride, nitrogen, biochemical oxygen demand
file’’ are terms adopted for use in this text.
(BOD), etc. The Langelier Index, proposed by Professor
The term water quality ‘‘state’’ adds the notion that energy
Wilfred Langelier in 1936 (see Langelier, 1936),
is involved. For example, if we reduce the concentration of a
brought some degree of rationale from equilibrium
substance, as done by a unit process, a state change results and
chemistry to the problems of deposition of calcium
energy is required.
carbonate and corrosion in pipes. The index was applied
empirically to handle problems of practice. Much about
bacterial growth and enumeration of bacteria was 2.1.3 CRITERIA
understood by the early twentieth century. Such was,
A water quality criterion refers to a contaminant level, which
in-a-nutshell, the state of knowledge of water quality
when not exceeded, will not impair a given beneficial use of
about 1950.
water. A great deal of research and deliberation is involved in
By the early1950s, the stage was being set for
establishing a criterion for a particular contaminant. Seldom is
themodernera. Thebook Water Quality Criteria the result definitive, and considerable uncertainty may be
(McKee, 1952) was published by the State of Cali- associated with any numerical value determined.
fornia, microbiology fundamentals were assimilated
in academic studies, and Werner Stumm at Harvard
2.1.4 STANDARDS
introduced the idea of equilibrium chemistry as a
means to model the behavior of natural systems. A criterion becomes the basis for a standard, which is a
By the late 1950s, instruments such as atomic absorp- codified criterion. Water quality standards have evolved
tion, gas chromatography, polarography, fluorescence, over the decades of the twentieth century. Usually, standards
TOC analyzers, mass spectrograph, etc. were are normative in character, i.e., dependent not only on effects
introduced. on uses but on economic and cultural factors.
By the 1960s, ideas from chemistry theory, e.g.,
thermodynamics, kinetics, redox reactions, acid–base 2.1.4.1 Kinds of Water Quality Standards
reactions, complexation, etc., became assimilated into Water quality standards have been developed for a variety of
the nomenclature of aqueous chemistry. Analyses of situations. The first in the United States were in 1914 and
water could include a complete spectrum of organic applied to drinking water on ‘‘common carriers’’ that crossed
compounds along with the traditional ones. Total interstate boundaries. These standards evolved, incrementally,
organic carbon (TOC) was a parameter used to supple- to the USPHS Drinking Water Standards of 1962. They are
ment BOD but has yet to supplant the latter. Instrumen- useful to review for the following reasons: (1) the standards
tal methods were displacing wet chemistry and provide an overview of some of the notions of basic standards
providing the means to analyze for virtually any con- for drinking water quality; and (2) the 1962 standards were a
taminant, and at microgram per liter levels. starting point for those that have evolved pursuant to PL93-523,
the 1974 Safe Drinking Water Act, and its ensuing amendments.