One reader who worked through the first Onsite Arithmetic problem made a couple of good points that should be clarified for subsequent problems and how you actually go about determining values in the field.

First, the magic sewage number of 7.5 gallons per cubic foot is rounded off from an actual value of 7.48 gallons per cubic foot. The number was rounded for ease of calculations. For normal household flows and residential tanks this gets us close enough to the actual value. So the numbers may not work out to the exact size of the tank but will be very close. As flows become higher, such as for commercial establishments like restaurants, bars, resorts, etc., using 7.48 gallons per cubic foot becomes more important because over time the flow discrepancy can become quite large.

Also, remember when calculating operating volumes that we are assuming the numbers reflect the inside tank dimensions, and in the case of septic tanks is measured from the invert of the outlet, which gives the operating volume of the tank. There will be free space in the tank over the liquid surface that is not included in the volume calculation. If all you have are the outside dimensions you can adjust the volume calculations by subtracting the wall thickness for the tank in question. This does come up in the field sometimes because it is easier to measure the outside dimensions in certain cases. Wall thickness will vary by product so it will need to be measured or supplied by the manufacturer. 

About the Author
​Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Send him questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.