The IRS considers a septic system to be a capital improvement rather than an expense. So you'll depreciate it over 27.5 years rather than deducting it.
Here is a link to IRS Publication 527, Residential Rental Property, which discusses all aspects of this topic. Note table 1-1 beginning on page 5.
https://www.irs.gov/pub/irs-pdf/p527.pdf
