Page 152 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Sedimentation 107

0

0.5
16 40 77 100 100 100

1
09 28 50 78 100 100
Depth (m) 1.5 07 21 36 63 87 100

2
90%

2.5 05 17 30 53 76 80% 93
70%
40% 50% 60%
30%
20%
10%
3 05 15 27 48 67 85
3.5
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Time (min)
FIGURE 6.14 Percent removals in flocculent settling, plotted from Table 6.3.

1:98 m 1:37 m
d n d n 1
DP n, n 1 R(2:44 m, 50 min ) ¼ 30% þ 10% þ 10%
R(d o , T) ¼ P o þ DP o, n þ 2:44 m 2:44 m
d o d o
1:07 m 0:85 m
d n 2 10%
(6:19) þ 2:44 m 10% þ 2:44 m
þ DP n 1, n 2 þ
d o 0:67 m 0:46 m
10%
2:44 m 2:44 m
þ 10% þ
where 0:15 m
n is the number of increments of equal percent removals þ 2:44 m 10%
from P o to 0. ¼ 57%
P o is the iso-percent removal line intercepting the depth,
d o , at a given time, T (min) The corresponding calculation in U.S. customary units is
d o is the depth of the level at which calculation is started
(m) 6:5ft 4:5ft
10%
d n , d n 1 , . . . is the median depths between each succeeding R(8:0ft,50 min) ¼ 30% þ 8:0ft 10% þ 8:0ft
two iso-percent removal lines, e.g., P n 1 and P n 2 (m) 3:5ft 2:8ft
10%
8:0ft 8:0ft
þ 10% þ
2:2ft 1:5ft 0:5ft
10%
10% þ
10% þ
þ
Example 6.5 Calculation of Percent Removal at 8:0ft 8:0ft 8:0ft
One Coordinate Point for Flocculent Settling ¼ 57%
Problem statement Discussion
Calculate the percent removal at depth 2.44 m after deten- Note that R is the total percent removal, i.e., includes
tion time T ¼ 50 min. partial removals for particles in which v s < v o .
Method
Figure 6.14 shows the plot for the results of the settling
column test given in Table 6.3. To obtain the total percent 6.6 HINDERED AND COMPRESSION SETTLING
removal at depth 2.44 m and 50 min detention time, i.e., R (TYPE III AND TYPE IV SUSPENSIONS)
(2.44 m, 50 min), follow the steps given below:
Hindered settling (Type III) and compression settling (Type
1. Draw a horizontal line at 2.44 m (8.0 ft) depth. IV) occur in the final clarifiers that follow activated sludge
2. Draw a vertical line at T ¼ 50 min. reactors. The analysis of the settling characteristics of such a
3. Calculate removals by Equation 6.19 as illustrated, suspension is important to the performance of final settling
i.e., basins and is reviewed in this section in several steps.