Here are the answers to the practice problem for the month of September.

I have a cylindrical (round) tank that has a diameter of 4 feet and is 5.5 feet deep below the invert of the outlet. (Hint: remember )

1. What is the operating volume in gallons?

Answer
For the volume of a cylindrical (round) tank the volume is found by calculating the area of a circle multiplied by the depth.

So:  gives the area of a circle and then we multiply that by the depth to get the volume in cubic feet and then use our magic number of 7.5 gallons per cubic foot to give us the answer.

3.14 is the value for pi and the radius is one half the diameter so it is 2 feet.

3.14 x 2 feet x 2 feet x 5.5 feet x 7.5 gallons per cubic feet = 518 gallons for the operating volume.

2. What are the gallons per inch?

Answer
For gallons per inch you take the area of the circle and multiply it by 1 foot per 12 inches x 7.5 gallons per cubic feet.

Area of a circle: 3.14 x 2 feet x 2 feet = 12.5 square feet x 1 ft/12 inches = 12.5 cubic feet per 12 inches = 1.05 cubic feet per inch x 7.5 gallons per cubic foot = 7.8 gallons per inch because the cubic feet cancel out.

3. If the flow is 600 gallons, is it large enough?

Answer
The tank is not large enough to be a septic tank for a flow of 600 gallons per day because there is less than a day of detention time. A tank this size could be suited for a pump tank.

4. If sludge plus scum equals 15 inches does it need to be pumped? What if sludge plus scum equals 24 inches?

Answer
The tank is 66 inches deep which equals 5.5 feet.

15 inches ÷ 66 inches x 100 = percent of the tank = 22.7 percent. If it’s a septic tank it would not quite be at the 25 percent to pump. However, if the tank is a pump tank it should be pumped; plus, it means that the septic tank should be cleaned more often because you do not want to see sludge accumulations in the pump tank.

This article is part of a series of practice problems for installers:

• Onsite Arithmetic: Is This Tank Large Enough?
• Onsite Arithmetic: Cylindrical Tanks
• Onsite Arithmetic: Setting Floats
• Onsite Arithmetic: Flow
• Onsite Arithmetic: Flow - Part 2
• Onsite Arithmetic: More O&M Practice
• Pointers for Onsite Arithmetic