Page 367 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

Required that of Table 11.6 except that the paddle-wheel diam-
Calculate the overflow velocity of a tank that will eter is to be 3.0 m instead of 3.9 m.
remove all particles larger than d(floc) 0.88 mm. Required
Reference is Section 11.4.2.1. Set up a design algorithm for four compartments.
11.4 Examples of G and u Parameters in Practice
Hint: Apply a trial-and-error solution using Table 11.6.
Given=Required 11.10 Design of Paddle-Wheel Flocculation System for a
For a nearby water treatment plant employing paddle Flotation System
wheel flocculators, calculate (a) G, (b) u, and (c) Gu.
Given
Do this for a full range of flow conditions, and sum-
A two-compartment flocculation basin is to be
marize in tabular form, and compare with values for 3
designed for Q ¼ 0.263 m =s (6.0 mgd) at T ¼ 208C
comparable plants in Table 11.1. What range in oper-
for a flotation system.
ator control is possible for G.
11.5 Floc Description Required
Set up a design algorithm with appropriate criteria for G.
Given=Required
Describe the floc formed at the different stages of the floc Hint: Apply a trial-and-error solution using Table
basin. Alternatively, describe the floc formed during a jar 11.10.
test at different times of rapid mix and flocculation. 11.11 Utilization of a P versus R Plot
11.6 Air Bubbles for Flocculation Given
Given Figure CD11.14 is a photograph of a floc basin, which is
A basin is underlain by a grid of diffusers spaced at a part of a water treatment pilot plant at Colorado State
300 mm. Let the basin be 3.0 m in depth. University. To give an idea of size, the floc basin was
constructed of four 1219 mm 2438 mm (4 ft 8 ft)
Required
acrylic sheets to form the top, bottom, and two sides
Calculate G as affected by the flow of air, Q(air).
with 1219 mm 1219 mm (4 ft 4 ft) sheets forming
Arrive at a design for a flocculation system.
the ends. The paddle wheels were oriented with vertical
2
Hint: F D ¼ C D Agv =2g, P ¼ F D v,and V(basin) ¼ Qu.
shafts, each with a direct current motor with adjustable
For C D , use the relation given by Fair, et al. (1968,
speed. The paddle wheels were 762 mm (30 in.)
p. 25-3), Section 6.2.2, for the laminar range through diameter 940 mm (37 in.) long. The blades were
the transitional range, i.e., C D ¼ 24=R þ 3=R 0.5 þ 0.34.
25.4 mm (1 in.) wide. The first compartment had five
11.7 Approaching Uniform Turbulence With Paddle-
blades on each of four arms, spaced at radial distances
Wheel Flocculator
given in Table CD11.7; the third compartment had
Given only two blades per arm. For the first compartment,
A paddle-wheel flocculator is to be designed. the motor was mounted on a 150 mm ‘‘lazy Susan’’
Required ball bearing plate (designed for use with rotating
The paddle wheel should be designed with blade widths shelves for a kitchen cabinet). A 460 mm brass rod
and spacing such as to approach uniform turbulence. was attached to the motor with end restrained by a
hook attached to a force gage. Without restraint, the
Hint: A spreadsheet may be set up dividing the paddle
motor would rotate freely without turning the paddle-
wheel into concentric volumes that are equal. The
wheel shaft. The force of the lever arm at distance 460
sizing of the blades and their spacing should result in
mm from the shaft was measured by the force gage. The
equal G values for each concentric volume.
product of the force times the lever arm distance was the
11.8 Design of Paddle-Wheel Flocculation System—
torque exerted on the paddle wheel. The torque times
Power Required
the rotational velocity in radians per second was the
Given
power dissipated by the blades of the paddle wheel
A flocculation basin is to be designed for Q ¼ 0.263 due to form drag. This power varied with rotational
3
m =s (6.0 mgd) at T ¼ 208C with other data the same as
velocity. Table CD11.7 shows data collected on two
that of Table 11.6 except that the paddle-wheel diam-
separate occasions and the associated calculations.
eter is to be 3.0 m instead of 3.9 m.
For different rotational velocities, calculations were
Required made for power dissipated, P; turbulence intensity, G;
Calculate the power, P, required by the paddle wheel. power number, P, and Reynolds number, R. The related
Hint: Apply a trial-and-error solution using Table 11.6. plots are shown in Figure 11.15: (a) G versus n; (b) G
11.9 Design of Paddle-Wheel Flocculation System— versus R; (c) P versus R; and (d) P versus n. The
1
Design Algorithm range of practice, selected as G 100 s , is indicated
in each plot.
Given
A flocculation basin is to be designed for Q ¼ 0.263 Required
3
m =s (6.0 mgd) at T ¼ 208C with other data the same as Discuss the significance of the plots.