Science of Water : Concepts and Applications

364 The Science of Water: Concepts and Applications
where
u = overfl ow rate, m3 (m2 d) or gpd/ft
Q = average daily fl ow, m3 /d or gpd
A = total surface area of basin, m2 or ft2 l and w = length and width of basin, m or ft
√ Key Point: All particles having a settling velocity greater than the overfl ow rate will settle and
be removed.
Hudson (1989) points out that rapid particle density changes due to temperature, solid concentration,
or salinity can induce density current, which can cause severe short-circuiting in horizontal
tanks.
Example 10.66
Problem:
A water treatment plant has two clarifi ers treating 2.0 MGD of water. Each clarifi er is 14 ft wide, 80 ft
long, and 17 ft deep. Determine: (a) detention time, (b) overfl ow, (c) horizontal velocity, and (d) weir
loading rate assuming the weir length is 2.5 times the basin width.
Solution:
Step 1. Compute detention time (t) for each clarifi er.
Step 2. Compute overfl ow rate u.
2 mgd 1,000,000 gal 1ft1d Q
2 d 7.48 gal 24h
3
5570 ft /h
3
92.8 ft /min
() a
Step 3. Compute horizontal velocity V.
Step 4. Compute weir loading rate u w .
V 14 ft 80 ft 17
ft
t
3
Q 5570 ft h
1,000,000 gpd
(b) u 893 gpd ft
14 ft 80 ft
Q
lw
3
Q 92.8 ft min
(c) V
0.39 ftmin wd 14 ft 17
ft
Q 1,000,000 gpd
(d) uw
28,571 gpdft 25 .
w 2.514 ft