1. If the chlorine chemical dose is 3 mg/L and the feed in lbs/day is 280, what is the flow in MGD?

2. A rectangular basin 12 feet long and 9 feet wide is 6 feet deep. It treats a flow of 90,000 gallons per day. Determine the detention time in hours.

3. Determine the required flow in gal/day for a settling tank that is 30 ft diameter, and 12 ft deep with a detention time of 3 hours.

4. How many feet in 30 inches?

5. How many cubic feet in one cubic yard?

Answers:

1. If you know how to work the Davidson Pie Chart you have this one, if not the formula is:

Flow, MGD = (Chemical feed, lbs/day) / (Dose, mg/L X 8.34 lbs/gal)

= (280 lbs/day) / (3 mg/L X 8.34 lbs/gal)

= 11.2 MGD

2. The formula for detention time is (Tank Vol, gal X 24 hrs/day) / (Flow, gal/day)

First thing is we need to figure the Volume of the tank and volume is always in cuft or ft3 which means multiply 3 numbers together = 12 ft X 9 ft X 6 ft = 648 cuft

From cuft we can determine the gallons because there are 7.48 gal/cuft and since we are going from cuft to gallons we multiply = 648 cuft X 7.48 gal/cuft = 4,847 gallons Now we just need to fill out, the formula and work it:

(4847 gal X 24 hrs/day) / (90,000 gal/day) = 1.29 hrs

3. The formula for detention time is (Tank Vol, gal X 24 hrs/day) / (Flow, gal/day)

First thing is we need to find the Volume of a circular tank and volume is always in cuft or ft3 which means multiply 3 numbers together = 3.14 X R, ft X R, ft X depth, ft

R or Radius, ft is 1/2 the diameter = (30 ft) / (2) = 15 ft

3.14 X 15 ft X 15 ft X 12 ft = 8,478 cuft OR using Diameter is:

0.785 X 30 ft X 30 ft X 12 ft = 8,478 cuft

Now convert cuft or ft3 to gallons by multiplying by 7.48 gal/cuft

8,478 cuft X 7.48 gal/cuft = 63,415.4 gal

Okay – now comes the rub – you are asked to find the flow in gal/day when you are given the Detention time in hours. If you know how to work the Davidson Pie chart you have this, if not:

(Tank Vol, gal X 24 hrs/day) / (Detention time, hr)

(63,415.4 gal X 24 hrs/day) / (3 hours) = 507,323.5 gal/day

4. There are 12 inches per foot. So:

(30 inches) / (12 inches/foot) = 2.5 feet

5. There is 3 ft in one yard, but remember it is 3 yards not just one. So:

1 cuft X 3 ft/yd X 3 ft/yd X 3 ft/yd = 27 ft3