Page 316 - Water and wastewater engineering

LIME–SODA SOFTENING 7-33

The GLUMRB design guidance is less than 4.2 m/h. Therefore, the unit is acceptable for
overflow rate.
b. Next check the detention time for softening using the volume of the basin.

V 580 m 3
t 0.078 d or 1.88 h
3
Q 7,400 m /d
The GLUMRB design guidance is one to two hours. Therefore, the unit is acceptable for
detention time.
c. The unit is next checked for mixing time by calculating the volume of the truncated cone
and calculating the detention time in the cone. The volume of a truncated cone is

h
r
V [( ) 2 (r r ) ( ) ]r 2 2
12
1
3
where h height of the truncated cone

r 1 radius at bottom of cone
r 2 radius at top of cone
For the trial tank the values of h, r 1 , and r 2 are taken from Table 7-4 .

(3 6 m ) 2 2
.
V [(6 5 . ) (6 5 . m )(2 m ) (2 m ) ]
3
223..4 m 3
The mixing time in the cone is

223 4 . m 3
t 0030 d or 43 46 min
.
.
3
,
7 400 m /d
The GLUMRB design guidance is a minimum of 30 minutes. Therefore, the unit is ac-
ceptable for mixing time.
d. The weir length is checked by computing the length of the weir taking into account that
there are eight weirs and that water enters both sides of each weir.
⎛ 12 mdiameter 1 3 . mmdiameter column⎞
L (8 weirs )(2 sides/weir ) ⎜ ⎝ 2 ⎟ ⎠
85 6 . m

3
With a flow rate of 7,400 m /d, the weir hydraulic loading rate is

3
7 400 m /d 3 3
,
.
0
.
WL 86 45 m /dm or 0060 m /min m

85 6 . m