To answer these questions I would first convert the measurements in inches to feet:

60 inches ÷ 12 inches/foot = 5 feet
108 inches ÷ 12 inches/foot = 9 feet
72 inches ÷ 12 inches/foot = 6 feet

 

1. What are the gallons per inch for this tank?
To determine gallons per inch, take the length x width x 1 foot/12 inches x 7.5 gallons/cubic foot

Note: the actual gallons per cubic foot are 7.48 but we round off to 7.5 for ease of calculation.

5 feet x 9 feet x 1 foot/12 inches x 7.5 gallons/cubic foot = 28.12 gallon per inch since the units cancel to gallons per inch

 

2. What is the operating volume of the tank?
Operating volume is the length x width x depth x 7.5 gallons/cubic foot

5 feet x 9 feet x 6 feet = 270 cubic feet x 7.5 gallons/cubic foot = 2,025 gallons

 

3. If a 2,000-gallon septic tank is required, is this tank large enough?
Since the operating volume is approximately 2,000 gallons the answer is yes. If 7.48 gallons/cubic foot is used, the volume is 2,019 gallons so still above the 2,000 gallons required.  

 

4. If the design flow is 900 gallons per day, what is the detention time?
Detention time is determined by dividing the operating volume by the daily flow

So 2,000 gallons ÷ 900 gallons per day = 2.2 days of detention

 

5. If the actual flow is measured as 700 gpd, what is the detention time?
If the flow is 700 gallons per day instead it would be 2,000 ÷ 700 = 2.8 days of detention

 

6. If the scum is 6 inches deep and the sludge is 16 inches deep should this tank be pumped? What if it was just pumped a year ago?
A septic tank should be pumped when the combination of sludge plus scum is more than 25 percent of the operating volume. The combined total is 22 inches in a 72 inch deep tank; so 22 ÷ 72 = .30 x 100 = 30 percent; so this tank should be pumped.

 

7. If the tank had been at 12 percent last year and is at 23 percent this year, what recommendation would you make in terms of pumping the tank?
Even though it is at 23 percent I would recommend the tank be pumped because if solids accumulate at the same rate for the next year, they would occupy 35 percent of the volume.

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

• Onsite Arithmetic: A Basic Tank Problem
• Onsite Arithmetic: A Cylindrical Tank Problem
• Onsite Arithmetic: Pump Tank Math
• Onsite Arithmetic: Setting Alarm Floats
• Onsite Arithmetic: O&M Problem