I'm with Mr. Yote.  It should be depreciated over 27.5 or 39 years.

The septic system is a permanent integral part of building structure, the lack of which will most likely breach building ordinance for being fit for occupation.  As a result, the class life is the same as the building to which it is attached.

See also AmeriSouth XXXII Ltd. et al. v. Commissioner, (T.C. Memo. 2012-67) for a detailed analysis.