Posted by Jimmy on .
It is found that ozone reaction in water nearly follows firstorder kinetics in ozone concentration decreases by 50% in 12 minutes (t1/2 = 12 min). A water supplier wants to inject ozone into a pipe bringing water to thee water treatment plant to predisinfect the influent. The 3 ft diameter pipe is 3400 feet long with a steady flow rate of 10000 gal/min. What concentration of ozone in mg/L should be injected at the head of the pipe so that there will be an ozone concentration of 1 mg/L at the pipe's exit into the plant?

Engineering 
drwls,
Compute the average velocity of the water flowing in the pipe.
Get this from the relation
Q = A * V
where Q is the volume flow rate and A is the cross sectional area. I recommend that you convert Q from gallons/min to ft^3/min, to get a flow velocity in ft/s.
The time it takes the water to reach the plant is
T = L/V
Compute that time.
The ozone density will decrease by a factor
(1/2)^(T/t1/2)
Use that factor to determine the ozone density requirement at the source