Here are the answers to the questions posed in Onsite Arithmetic.

A rectangular septic tank has the following dimensions: 7 feet long; 4 feet wide; and 5 feet deep.

 

1. What is the operating volume of this tank in gallons?

Answer
This time all measurements are in feet so we do not have to convert inches to feet because we already know the dimensions in feet.

The calculation is: length x width x depth = 7 feet x 4 feet x 5 feet = 140 cubic feet

Now we use the magic conversion number of 7.5 gallons per cubic foot to get the answer in gallons: 140 cubic feet x 7.5 gallons/cubic foot (the cubic feet cancel leaving the answer in gallons) = 1,050 gallons of operating depth.

 

2. What are the gallons per inch of tank volume?

Answer
To determine gallons per inch, the following calculation is used: length x width x 1 foot/12 inches x 7.5 gallons per cubic foot. So 7 feet x 4 feet x 1 foot/12 inches x 7.5 gallons/cubic foot  = (28 cubic feet ÷ 12 inches) x 7.5 gallons/cubic foot = 17.5 gallons per inch (rounded up) because the feet cancel out.

 

3. If the flow is 600 gallons, what is the detention time in this tank? Is the tank large enough?

Answer
If the flow is 600 gallons per day and we want at least two days of detention time in the tank to allow for settling, it means the tank is not large enough. (Operating gallons divided by flow is 1,050 gallons/600 gallons per day = 1.75 days.) At a minimum it would need an operating volume of 1,200 gallons.

 

4. If the sludge depth is 10 inches and the scum depth is 5 inches should this tank be pumped?

Answer
If the total sludge and scum depth is 15 inches and the tank operating depth is 5 feet we convert the feet to inches in this case 5 feet x 12 inches/foot = 60 inches. Determining the percentage of the tank is then done by 15 inches ÷ 60 inches x 100 = 0.25 x 100 = 25 percent. This tank is right at the point where it should be pumped.

Now let’s say the total was 12 inches and last year when you were there it was 8; what would you tell the homeowner? I would say to them, “In less than a year this tank will need to be pumped. Since I am here now it actually costs you less than having me come back in six months to clean the 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