المحاضرة 4-9

EXAMPLE:
Laboratory data were obtained on the settling of a municipal wastewater as in Table (1). Design a settling tank to produce a maximum effluent suspended solids of 150 mg/l and with flow rate 4000 m3/d.
Table (1)
Initial solids, 393 mg/l
Time (min) Depth (cm)
60 120 180
Removal %
5 26
10 27 36 40
20 39 28 35
40 50 44 38
60 60 48 50
120 70 64 60
Initial solids, 550 mg/l
Time (min) Depth (cm)
60 120 180
Removal %
15 31 22 15
20 46 31 -
40 63 42 24
60 71 60 45
90 73 61 -
120 75 67 59

Solution
With the values given in Table (1), Figure (1) can be drawn as shown:





























Figure (1)



And Table (2) can be constructed by calculating the percent removal, the velocity, and the overflow rate of different times as shown:
Table (2)
Initial solids, 393 mg/l
Time (min) SOR (m/d) Removal of SS %
27 96 41.6
42 61.71 49.5
60 43.2 57.3
118 21.97 65.8
Initial solids, 550 mg/l
Time (min) Velocity (m/d) Removal of SS %
20 129.6 36.7
30 86.4 46.8
47 55.15 56.5
68 38.12 62.5
96 27 70.8

By plotting the SS removal versus overflow rate and time, Figs. 2 and 3 can be drawn:































Figure (2)
















Figure (3)

From Figs. 2 and 3:
For 393 mg/l concentration :



Overflow rate SOR = 31.4 m/d
Detention time = 74 min

For 550 mg/l concentration :


Overflow rate SOR = 22 m/d
Detention time = 104 min

For design consideration:

Surface Overflow Rate(SOR) = 22/1.5=14.67 m/d
Detention time (t) = 104/60 x 1.75=3 hr
For flow rate Q = 4000 m3/d

Area required for settling tank (A):
A = Q/SOR=4000/14.67=273 m^2
Use tank with 19 m in diameter ( A = 283 m2)

Effective depth = (t x Q)/A=(3 x 4000)/283 x 1/24=1.8 m