Page 366 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Flocculation 321

where pressure vessels, called the ‘‘contact flocculator’’ and
C is the concentration of primary particles (microflocs) at a ‘‘depth filter,’’ respectively (Section 12.7.2.2). Coagulant
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distance, z within the floc blanket (#=m ) is injected on the suction side of a centrifugal pump that
C o is the concentration of primary particles (microflocs) pressurizes the system. Tank diameters range from 508 mm
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entering the floc blanket (#=m ) (20 in.) for a 30 gpm model to 1372 mm (54 in.) for a 220
1
k is the coefficient of collision frequency (m ) gpm model.
z is the distance from the bottom of the floc blanket (m) The Culligan Multi-Teche system was developed during
v is the velocity through blanket, defined as Q=A (m=s) the period 1976–1979 responding to a need that some
3
where Q is the flow through blanket (m =s) A is the area 60,000 small water systems would fall under regulations
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of blanket as defined by its confining dimensions (m ) emanating from PL93-523, i.e., the 1974 Safe Drinking
Water Act. Its genesis was an idea from Professor E. Robert
Tambo and Hozumi (1979, p. 446) found that k¼ 4.37, 28.8 m Baumann, a consultant with Culligan for a ‘‘contact floccu-
for d¼ 1 mm, and 2 mm, respectively, for v¼ 0.03 m=min. lator’’ comprised of a bed of gravel which produced ‘‘con-
trolled turbulence,’’ and hence flocculation, followed by a
11.6.2.2 Design Practice, Equipment, Operation filter bed.
Design for solids contact units is largely a matter of equip-
ment selection, although some design latitude may be worked 11.7 SUMMARY
in based on overflow rates and detention time. Empirical
guidelines, based upon available equipment, is the usual The flocculation process evolved largely as an empirical
basis for design, complemented by experience with actual practice since being delineated by the 1919 work of Langelier
installations. Overflow rates of 1220–2440 m=h (500–1000 in Sacramento for the first paddle-wheel units. Design prin-
2
gpm=ft ) are given for alum or ferric treatment and 2928– ciples were articulated by Camp in a 1955 paper that were
2
4148 m=h (1200–1700 gpm=ft ) for lime treatment (Burns and adopted almost universally in practice. Turbulence theory was
Roe, 1971, pp. 6–7). adopted by Kaufman and Argaman in the late 1960s to lay the
The key operating variable related to floc formation is the foundation for flocculation kinetics, which was developed
level of the sludge blanket. The operator must monitor the further in the 1980s and 1990s.
blanket depth to keep it deep enough that good flocculation
occurs, but shallow enough that floc will not be carried out of
the basin. How often and how much sludge is to be wasted is PROBLEMS
readily determined by the operator after sufficient experience
11.1 Flocculation Discoveries of Langelier at Sacramento
with a given system.
Given
Excerpts of Professor Wilfred Langelier’s oral history
11.6.3 SUPER-PULSATORSe interview with Malca Chall (1982) given in Section
11.3.1.2.
The super-pulsator is a proprietary unit of Ondeo-Dergemont
(formerly Infilco Dregemont), which functions as a solids Required
contact basin. Its shape is rectangular, namely, circular. Identify discoveries of Langelier, c. 1921, as related to
Following the flow path, a coagulant chemical, e.g., alum, is the design of the Sacramento WTP that were important
added to the raw water pipeline which enters a vacuum cham- in developing knowledge of coagulation and=or floc-
ber and then to a distribution manifold (a pressure conduit) culation or that affected practice.
from which perforated distribution laterals distribute water 11.2 Distinction between Coagulation and Flocculation
uniformly below parallel-plate separators (which function to Given
hold the sludge blanket). The sludge between the plates must Excerpts of Professor Wilfred Langelier’s interview
be agitated to give a homogeneous sludge concentration. The with Malca Chall (1982).
sludge blanket is agitated by means of a nonsteady flow created Required
by the head of water in the vacuum chamber provided when How does Langelier deal with the distinction between
a vent valve above is opened. In effect, a hydraulic ‘‘pulse’’ ‘‘coagulation’’ and ‘‘flocculation?’’
occurs to agitate and expand the suspended solids contained 11.3 Settling Velocity–Biological Floc
between the plates. A unique feature is that short-circuiting of Given
flow cannot occur, i.e., the entire sludge blanket functions in An activated sludge floc has an average ‘‘longest
the solids contact coagulation process.
dimension,’’ d(floc-avg) 1.0 mm. Assume a standard
deviation for the floc size distribution is 0.12 mm,
which means about 0.15 fraction of the floc has a
11.6.4 CULLIGAN MULTI-TECHe
size smaller than (1.0–0.12 mm), i.e., d(lower std.
The Culligan Multi-Teche system consists of a bed of dev.) 0.88 mm, and, by the same token (for clarifi-
coarse media followed by a filter media, each in separate cation), d(higher std. dev.) 1.12 mm.