Page 241 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

available as a commercial apparatus, c. 1934, and soon
became the main tool in practice for assessing effective
Stumm, and O’Melia (1968) and then by Amirtharajah
coagulation. Langelier continued research in the field; for
and Mills (1982).
example, delineating the ‘‘zones’’ of coagulation (Langelier
and Ludwig, 1949), which, at the same time, advanced
theory. Box 9.1 outlines some of the history of these
developments.

BOX 9.1 EARLY STUDIES ON COAGULATION
In 1921, Professor Wilfred Langelier published a land-
mark paper, ‘‘Coagulation of water with alum by pro-
longed agitation.’’ The paper was related to the design
of a new filtration plant at Sacramento by Professor
Charles Gilman Hyde and, through the use of an in- Wilfred F. Langelier c. 1940, Professor of Sanitary Engineer-
house fabricated ‘‘stirring-device,’’ established such ing, University of California (from Langelier, 1982)
innovations as the ‘‘jar-test’’ and paddle-wheel floccu-
lation. In addition, his paper recognized the importance
of rapid-mix, water characteristics (e.g., turbidity, color,
alkalinity, colloids) and their effect on coagulant dos-
age, and laboratory studies. The issues of coagulant
dosage, coagulant mixing, and flocculation were
addressed through a systematic experimental program.
In 1942, Harvey Ludwig (see Box 1.2) presented a
Master of Science thesis on coagulation of turbid water,
building upon Langelier’s 1921 paper. The ensuing
paper (Langelier and Ludwig, 1949), with publication
deferred because of the war, assimilated principles of
colloid chemistry from the well-known soil scientist,
Professor Hans Jenny (see Ludwig, 1985, pp. 2–12). Harvey Ludwig c. 1945, graduate student under Langelier
The paper outlined the idea of double layer suppression, (used with permission by Dr. Ludwig)
the concept of ‘‘replaceable,’’ or adsorbed ions in the
double layer, with the increasing order of attraction
given as, Na ,K ,Mg ,Ca ,H ,Al ,Fe , the
þ
2þ
þ
2þ
3þ
3þ
þ
notion of ‘‘zeta potential’’ as the measure of negative 9.3.1.2 Color
repulsive force, the idea of charge neutralization as the Color was recognized early as colloidal and with negative
key to effective coagulation, and the role of a hydroxide charge (Saville, 1917). Coagulation of color was considered
precipitate (which they characterized as a ‘‘binder’’ by Black (1934, p. 1714), who noted that in the acid pH range
material). positive ions form and may react with negatively charged
The paper introduced the terms peri-kinetic floc- color colloids. In the higher pH range, he noted that aluminum
culation and ortho-kinetic flocculation to mean, and iron form ‘‘microcrystals,’’ which are seen as gelatinous
respectively, aggregation of flocs as permitted by neu- flocs. The latter have positive charge, and can neutralize and
tralization of zeta potential of colloids and aggregation precipitate negatively charged color colloids.
of flocs as caused by enmeshment with other flocs and
colloids (Langelier and Ludwig, 1949, p. 165; see also 9.3.1.3 Modern Theory
ortho-kinetic in the glossary). The basis for the Lange- The modern notion of coagulation was outlined by Moffett
lier and Ludwig paper was several thousand tests (1968, p. 1256), and in a preliminary form by Black (1948,
employing a variety of coagulant chemicals at different p. 143), as
dosages, different pHs and different alkalinities, with
12 different soils (the latter being used as the basis for 1. Hydrolysis of water reacts with Al 3þ to multi-
synthesizing 21 artificial raw waters used for the nuclear hydrolysis species
experiments). Conclusions from the study are encapsu- 2. Adsorption of the hydrolysis species at the solid–
lated by one of their many residual turbidity versus solution interface of the colloid resulting in the for-
coagulant dosage curves, which was a major underpin- mation of a ‘‘microfloc’’
ning of theory until extended by Black (1967), 3. Aggregation of the microflocs as ‘‘flocs’’ occurs due
van der Waals forces