Here are the answers to today’s practice problem.    

To answer these questions I would convert the measurements in inches to feet:
60 inches ÷ 12 inches/foot = 5.0 feet
90 inches ÷ 12 inches/foot = 7.5 feet
72 inches ÷ 12 inches/foot = 6.0 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 gallon/cubic foot
5.0 feet x 7.5 feet x 1 foot/12 inches x 7.5 gallons/cubic foot = 23.43 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.
5.0 feet x 7.5 feet x 6.0 feet = 225 cubic feet x 7.5 gallon/cubic foot = 1,687 gallons

3. If the design flow is 300 gpd, what is the detention time?
Detention time is determined by dividing the operating volume by the daily flow.
So: 1,687 gallons ÷ 300 gallons per day = 5.6 days of detention

4. If the actual flow is measured as 400 gpd, what is the detention time?
If the flow is 400 gallons per day, it would be 1,687 gallons ÷ 400 = 4.2 days of detention

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?
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.
22 ÷ 72 = 0.30 x 100 to convert to 30 percent; so this tank should be pumped.

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?
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 then occupy 35 percent of the volume.