Science of Water : Concepts and Applications

340 The Science of Water: Concepts and Applications
Example 10.27
Problem:
Assuming the fl ow is steady and continuous, a fl ash mix chamber is 6 ft long, 4 ft wide, with water to
a depth of 3 ft. If the fl ow to the fl ash mix chamber is 6 million gallons per day (MGD), what is the
chamber detention time in seconds?
Solution:
First, convert the fl ow rate from gallons per day (gpd) to gallons per second (gps) so that time units
will match:
Then calculate detention time:
6,000,000
69 gps
(1440 min/day)(60 s/min)
Detention time, s
volume of tank, gal
flow rate, gps
3
(6 ft)( 4 ft)(3 ft)(7.48 galft )
69 gps
7.8 s
Determining Dry Chemical Feeder Setting, lb/day
In adding (dosing) chemicals to the water fl ow, a measured amount of chemical is called for. The
amount of chemical required depends on such factors as the type of chemical used, the reason
for dosing, and the fl ow rate being treated. To convert from mg/L to lb/day, use the following
equation:
Example 10.28
Chemical added, lb/day (chemical, mgL)(flow, MGD)(8.34 lbgal)
Problem:
Jar tests indicate that the best alum dose for a water sample is 8 mg/L. If the fl ow to be treated is
2,100,000 gpd, what should the lb/day setting be on the dry alum feeder?
Solution:
Example 10.29
Problem:
lbday ( chemical, mg/L)(flow, MGD)(8.34 lbgal) ( 8 mgL)(2.10 MGD)(8.34 lbgal) 140 lbd Determine the desired lb/day setting on a dry chemical feeder if jar tests indicate an optimum polymer
dose of 12 mg/L and the fl ow to be treated is 4.15 MGD.
(10.25)