Page 353 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

11.4.4.2 P(paddle-wheel) with Units of the flocculation basin were, respectively, 5.6, 3.8,
Equation 11.19 may be solved by inserting the appropriate and 1.9 times the G value in the fourth compartment.
numerical data and units for each term. Table 11.5 delineates 2. As seen in Equations 11.13 and 11.19, any one of
the conversions for both SI units and U.S. Customary units, several independent variables may be imposed to
respectively. change G, i.e., V(compartment), n,or A(paddles).
3. The design should provide for variation in rotational
velocity, n, by the operator. At Cambridge, for
11.5 DESIGN
example, 2.0 n(compartment 1) 5.2 rpm, and
The design procedure for paddle-wheel flocculation basins was 1.1 n(compartment 4) 2.9.
established by Camp (1955) and remains largely the same as 4. The paddle wheels may be oriented with the axes
set forth in his guidelines. For baffled basins, the procedure either normal to the flow direction or parallel to it.
has remained empirical as established in the 1930s, but with 5. Paddle area for any one paddle wheel should range
more recent update of guidelines (e.g., Haarhoff, 1998). 10%–25% of the cross-sectional area of the basin.
6. Stators are advisable to mitigate the tendency for the
whole water mass to rotate with the paddles.
11.5.1 DESIGN PROCEDURE FROM CAMP
7. Peripheral speed of paddles may range from 0.1 to
Not much had been done with G in the first few years after 1.0 m=s (0.3–3.0 ft=s).
being introduced by Camp and Stein (1943). Camp’s 1955 8. The total basin detention period, u, may range from
paper brought G into the picture, however, for quantitative 30 u 60 min.
design, showing how to compute G for different kinds of 9. The Gu parameter should be distributed as uni-
flocculation technologies, e.g., paddle-wheel basins, baffled formly as possible among the different flocculation
basins, diffused air, and reciprocating blades. For each tech- compartments.
nology, Camp showed how to compute G from fundamentals, 10. Where there is conflict in the above guidelines with
e.g., Equation 11.19 for paddle-wheel flocculators. The 1955 the results of calculations based upon the G and Gu
paper also characterizes Camp’s work in that he (1) showed criteria, the latter should prevail.
how to apply fundamentals, (2) developed practical criteria,
e.g., for G and Gu, and (3) gave practical guidelines, e.g., for Some additional guidelines, also from Camp (1955), except as
tip velocity, area of paddles, tapered flocculation, etc. Thus, noted, are as follows:
Camp had delineated a design protocol for flocculation.
. If the width of a paddle is too wide, the water in front
11.5.1.1 Camp’s Criteria
is carried along by the velocity of the paddle (p. 10);
Camp (1955) recommended upper limits of G for a floccula- mixing does not occur to any extent.
1
tion basin ranging from 74 s for the first compartment . The blades of a paddle wheel should be relatively
to 20 s 1 for the third compartment. The upper limit of narrow and more in number (as opposed to wider and
G ¼ 20 s 1 in the third compartment was to minimize floc fewer in number). This is in accordance with mixing
breakup. The criterion for the total number of collisions, Gu theory and experimental findings, e.g., that the scale
had a wide range, i.e., 23,000 Gu 210,000. of the turbulence should be about the same as the size
The values adopted for G and Gu criteria represented limits of the floc desired (Section 10.3.1.2, Figure 10.4,
of design practice for 20 operating plants, as discussed in Figure 10.11; Section 11.4.3.5 and Figure 11.9).
Section 11.3.1.4, with data given in Table 11.1. As seen in This would favor smaller width blades, and larger
Table 11.1 (c. 1918–1931), 14 of the plants used baffle number, with larger blades in each successive com-
flocculation; only 4 used paddle wheels. After Camp’s partment (so that the scale of the turbulence increases
paper, paddle wheels were used in most designs. with floc size). A caveat is that in the third compart-
1
For flotation, G values are higher, e.g., G 70 s . Only ment the flow regime is likely to be laminar.
one or two compartments are used since a smaller floc, e.g., . Without stator blades, 0.15 A(blades) 0.20 times
d(floc) 10 mm, is desired. cross-sectional area to prevent rolling water. If A
(blades) 0.25, major rotation will occur.
11.5.1.2 Camp’s Guidelines . The P=V dissipative function is an average over the
Some guidelines for flocculation basin design, abstracted from basin volume; local variation, i.e., from point to
Camp (1955), are enumerated as follows: point, may be considerable.
. Since k ¼ 0at t ¼ 0, the startup power greatly
1. The turbulence intensity, G, should be ‘‘tapered’’ exceeds the equilibrium power, so the paddles must
along the length of the basin such that for the first be brought to equilibrium speed slowly.
1
compartment, G 70–80 s and for the last com- . Figure 11.13 shows power versus rpm for the paddle-
1
partment G 10–20 s . In an installation designed wheel flocculator at Cambridge, Massachusetts, as
by Camp in Cambridge, Massachusetts (Camp, determined by Camp (1955), showing an exponential
1955), the values of G in the first three compartments increase in power with rotational velocity.