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Systems/ATUs + Get AlertsTo 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.

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