Page 338 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Flocculation 293

coagulant and the passage of water through the sand.’’ Sedi- The research and the ensuing design for the Sacramento plant
mentation of the ‘‘coagulum’’ prior to filtration was a recog- (Langelier, 1921), established the guidelines for flocculation
nized part of the treatment train. Initially, the Sacramento practice. The outcomes of this work were as follows:
plant had ample sedimentation detention time but did not
have provision for thorough mixing of the alum after its 1. The jar test was invented as a means to explore the
addition. This led to dramatic breakthroughs in water treat- role of coagulation variables.
ment, described in Langelier’s words in an oral history inter- 2. The concept of flocculation was discovered.
view by Chall (1982, pp. 24–26): 3. The role of gentle mixing to promote floc growth
was established.
4. Paddle-wheel flocculators were developed.
It immediately occurred to me that I could demonstrate in our
5. An on-site pilot plant study complemented the
laboratory the advantages of thorough mixing in effecting good
laboratory work.
coagulation. I proposed this to Professor Hyde who readily
agreed with my suggestion. I devised an apparatus for the 6. The studies led to the first full-scale plant to imple-
demonstration. It consisted of six in-line, one liter, clear glass ment paddle-wheel flocculators, i.e., at the new water
jars, each provided with a slowly revolving paddle. The paddles filtration plant at Sacramento, California, put on line
were driven by a motorized shaft equipped with mitre gears. in January, 1924.
7. The importance of pH and alkalinity was discovered
The first test runs were made with muddy water from the
and principles were established that pH control was
Sacramento River, and from the beginning our results were
an important part of the coagulation process.
strikingly successful and far beyond our expectations. The
8. Langelier recognized the complexity of the
multi-jar feature made it possible to observe the effect of any
coagulation–flocculation process and that the jar
one variable; for example, the coagulant dosage, mixing time
test apparatus and a pilot plant were the means to
period, or rotational speed on the effectiveness of floc forma-
tion and subsequent clarification through sedimentation. isolate the variables.
In these early tests, the outstanding observation was that in the Figure 11.1 shows the design of the Sacramento WTP as
absence of prolonged agitation or stirring beyond that required modified by Langelier in 1919 to include paddle-wheel floc-
to instantly diffuse the added coagulant chemical solution, culators (but, as seen, did not include rapid mix). Langelier
visible coagulation did not occur for several hours and clarifi- also delineated the details of design, specifying that the
cation often required an overnight settling period. With pro-
velocities of the water should be up to about 2.0 ft=s (as
longed stirring, tiny flocs began to form within a few minutes
induced by the paddle wheels) and flocculation detention
and as time progressed up to about 10 min, the individual floc
times should be about 30 min. Langelier also compared end-
particles continued to grow in size and ultimately became
around baffling with the paddle-wheel design, in a separate
widely separated. When the stirring was stopped, the flocs
settled within a few minutes leaving a clear water above. pilot plant constructed for such purpose, and concluded that
After many repeated tests, it was concluded that adequate the paddle-wheel method was the most desirable because of
clarification with least chemical for a given water normally greater flexibility in operation.
required a mixing period of about 10 to 20 min.
11.3.1.3 Design Guidelines
Using synthetic waters, we noted a moderate increase in the To illustrate the evolution of practice, Leopold (1934,
required coagulant with increased turbidity, but more signifi-
p. 1070) described the ‘‘mechanical agitation’’ used for alum
cantly, and much to our surprise, we noted that the alum
floc formation in a 11.4 ML=day (3 mgd) plant at Winnetka,
demand increased more directly with the bicarbonate alkalin-
Illinois treating water from Lake Michigan. The plant was
ity of the water. This intrigued us very much and we tried
expanded to 22.8 ML=day (6 mgd) in 1932 with six floccula-
increasing the bicarbonate content of the water by adding
tion basins with combined detention period of 30 min. The
small increments of bicarbonate of soda.
mixer assembly was a 1.5 kW (2 hp), 5 speed motor, a speed
In all these tests, we noted that optimum flocculation occurred reducer, one set of bevel gears, and a vertical drive paddle
when a definite fraction of the total alkalinity had been neu- shaft. Each agitator had one paddle with a total surface area of
2
2
3
2
tralized. Without going into the chemical theory too deeply, 0.61 m (6.6 ft ) each or 125 m basin volume=m paddle
this indicated that optimum flocculation was occurring at a surface area (409 ft =ft ). The tip velocity of the paddle wheel
2
3
constant hydrogen ion concentration or pH, slightly above true was variable, e.g., 0.24 v(tip) 0.74 m=s (0.78 v(tip)
neutrality. We had arrived at an important conclusion in a
2.41 ft=s) and u (all floc basins) ¼ 30 min. Camp (1955)
roundabout way.
adopted the first two criteria in his recommendations for
design of flocculation basins; paddle-wheel area, however,
The first few series of the jar tests proved very convincing
was a fraction of the cross-sectional area of the basin, i.e.,
and he (Professor Hyde) immediately arranged for a small,
continuous flow pilot installation in Sacramento for the 0.15–0.20 to avoid rotation of the water mass (p. 11) instead
following summer. My recollection is that the Sacramento city of as a ratio with respect to the basin volume. In other words,
filtration plant, the first to use prolonged mechanical agitation to in advancing the concept of flocculation to practice-specific
induce flocculation, was placed in service in January, 1924. guidelines were the key, and evolved with experience.