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


Over the next couple months we will revisit a couple of problems to sharpen our calculation 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 90 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.

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

2. What is the operating volume of the tank?

3. If the design flow is 300 gpd, what is the detention time?

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4. If the actual flow is measured as 400 gpd, what is the detention time?

5. 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?

6. 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?

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Click here for the answers. 

Here’s a review of Simple Septic Math and our Pointers for Onsite Arithmetic


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