# Onsite Arithmetic: A Basic Tank Problem

Test your septic math skills — or practice for your next certification exam

### Interested in Systems/ATUs?

Systems/ATUs

Here is another series of problems to help maintain and sharpen our sewage math skills. For the first problem let’s look at what should be an easy septic tank example.

During a service call to a residence you have not visited before you measured a septic tank with the following dimensions: 60 inches wide by 108 inches long by 72 inches deep from the invert of the outlet pipe. Perform the following calculations and make the proper interpretations about the need to pump the tank.

1. What are the gallons per inch for this tank?

2. What is the operating volume of the tank?

3. If a 2,000-gallon septic tank is required, is this tank large enough?

4. If the design flow is 900 gallons per day, what is the detention time?

5. If the actual flow is measured as 700 gpd, what is the detention time?

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?

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?

For an example calculation and some septic math tips, check out: