WHAT FACTORS ARE CONSIDERED IN A SEPTIC DESIGN?
Once a site investigation per AAC R18-9-A310 determines suitability for a septic installation, a design begins with determining the amount of effluent that will be discharged daily from the structure. For a residence, Pima County bases its calculations on the number of bedrooms and the number, and type, of plumbing fixtures in the home. This information is used to determine the necessary tank size, per AAC R18-9-A314(4). It is also used as a factor in determining the necessary size of the disposal field. The other factor in the discharge calculation is derived from the soils evaluation. Based upon the soils’ ability to drain and treat the liquid effluent, a soil absorption rate (SAR) is assigned. AAC R18-9-A312(D) describes the SAR process. WHEN IS A PERCOLATION TEST ACCEPTABLE? WHEN IS AN ASTM SOILS ANALYSIS RQUIRED? These two methods of examining the subsurface conditions of a proposed septic site determine the native soils’ ability to treat and remove liquid effluent. Per AAC 18-9-A
