Page 239 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

bacteria, cysts, algae, spores, rotifers, fecal debris from larger than a given lower limit size, for example, 2 mm.
rodents, etc. Most of these particles also have negative Particle counting instruments require regular maintenance,
charges, as measured by ‘‘zeta potential,’’ and are removed for example, calibration and cleaning.
coincidentally with mineral colloids.

9.2.2.3 Natural Organic Matter and Color 9.2.3 TURBIDITY AND PARTICLE COUNTS IN AMBIENT
Natural organic matter (NOM) is comprised of humic and WATERS AND FINISHED WATERS
FAs, found in almost all surface waters, and are due to
Turbidity and particle counts vary from place to place and
breakdown of vegetation. Humic acids (HA) have molecular
seasonally. Despite the wide variation in source waters, the
weights ranging from several hundred to several thousand and
variation in finished waters is remarkedly narrow.
are comprised primarily of aromatic compounds (i.e., with
benzene-rings in various forms). FAs are similar in structure
but have lower molecular weights. A property of NOM is 9.2.3.1 Spatial Variation in Source Waters Compared
‘‘color;’’ its removal has been an ancillary objective of coagu- with Plant Effluents
lation since about 1900. About 1975, however, NOM was Table 9.2 lists raw and filtered water turbidity and particle
found to be a ‘‘disinfection by-product’’ (DBP) precursor counts for 10 water treatment plants in the United States. As
(Symons et al., 1975); after being determined carcinogenic, seen in Table 9.2, raw water turbidities vary from as low as
became a removal priority. NOM has a negative charge and is 0.2 NTU for Lake Mead near Las Vegas to as high as 49 for
removed partially by coagulation, for example, in the range Winnetka, Illinois. Particle counts vary from 3,200 #=mL for
10%–90%. Concentrations of NOM in surface waters range Lake Mead near Las Vegas to 500,000 #=mL for Winnetka,
from about 1–50 mg=L as dissolved organic carbon (DOC), Illinois. Filter effluent turbidities in Table 9.2 vary from 0.01
with a median about 4 mg=L. The molecular weight of most to 0.07 NTU with effluent particle counts as low as 41–720
organic color and humic substances is in the range, 700 < #=mL. The data show a representative range in turbidity levels
MW < 200,000 (O’Melia et al., 1979, p. 590). and corresponding particle counts for both raw water and
filtered water.
9.2.2.4 Total Organic Carbon
A measure of NOM is total organic carbon (TOC), which has 9.2.3.2 Seasonal Variation
two components, that is, particulate organic carbon (POC) The water quality characteristics of every ambient water show
DOC) In ambient waters, the DOC portion dominates, for a characteristic profile with respect to time. Average monthly
example, during spring runoff of a mountain stream, turbidities and alkalinities are shown in Figure 9.2a and b,
measurements were, TOC ¼ 9.6 mg=L; DOC 9.4 mg=L; respectively, for the Cache La Poudre River, a mountain
POC 0.2 mg=L (Carlson and Gregory, 2000, p. 559). stream in Colorado. These profiles illustrate the variability
that occurs for most water quality constituents over an annual
9.2.2.5 Turbidity
cycle which must be considered in water treatment.
Light scattering, that is, ‘‘nephelometry,’’ is a property of
colloidal suspensions. Thus, if a light source is directed into
such a suspension the light will ‘‘scatter.’’ A detector located 9.3 CHEMISTRY
at a right angle to the light source will show a reading that is
Coagulation is based on chemistry. In the early decades,
approximately proportional to the concentration of colloidal
c. 1900–1930, and through the 1980s, practice was based largely
matter (for a given suspension). An instrument arranged in
on empiricism. Theory began to evolve, c. 1920 and was assimi-
such fashion is called a turbidimeter. A turbidity unit is a
lated during each subsequent decade through the 1990s.
measure obtained by a turbidimeter from a standardized
suspension, called a nephelometric turbidity unit (NTU).
Natural waters have a wide range of turbidities, ranging
9.3.1 CHEMISTRY OF COAGULATION:EVOLUTION
from as low as 0.2 NTU for mountain streams in the winter
OF THEORY AND PRACTICE
to several hundred NTU for a muddy river. Values vary
seasonally and with rainfall events as well; for example, a Chemical treatment of municipal wastewaters, for example,
mountain stream may have turbidities of perhaps 50 NTU using lime or ‘‘sulfate of alumina,’’ was well established in
during spring runoff. Great Britain in 1870 (Metcalf and Eddy, 1916, p. 5). In 1885,
due to difficulties in filtration of Mississippi River water for
9.2.2.6 Particle Counts New Orleans, alum was tried which led to the modern practice
Particle counting technology evolved from about 1970 for of rapid filtration (as opposed to slow sand). Since the ionic
discrete samples to continuous ‘‘on-line’’ particle counting theory of solution was yet to be formulated, that is, by Svante
during the late 1980s becoming common in water treatment Arrhenius in his 1884 doctoral thesis, the early practice had
during the 1990s. These instruments can measure particle little theoretical basis. Later, in reviewing 1920s research,
concentrations in selected size ranges, for example, >2 mm, Willcomb (1932, p. 1418) was aware of the role of pH on
>5 mm, etc., as well as total counts, that is, all particle sizes alum coagulation.