The septic pipes won't freeze for the same reason the well pipes don't freeze they're below the frost line, or should be anyway.

A traditional Maine home will often have frozen pipes, especially in the north. I say that because those houses were built "Good enough" for most of the time, when real cold sets in they're not good enough because there really isn't enough insulation.

Any house built in the last 30 years ought to have no problem with -20F. My parent's house was built in the early '70s and I don't remember one incident of freezing pipes ever...