@RESIPSA123Where did you see the Rev. Proc.?  Would be great if you could share the citations for reference (I mean that in a respectful way).

I am with @Terry53029 and @IRonMaN that septic system has a class life of 39 years on the basis it is a necessary structural component relating to the operation or maintenance of a building as defined under §1.48-1(e).  In fact, that has consistently been the IRS position.  Caselaw also supports that and considers whether the asset in question is (1) accessory to a business, (2) permanent, or (3) "ornamentation", taking into account the type of building of which the asset is a part.  It should not, therefore, be treated as a land improvement.