is it septic tank safe?

Thank you for your inquiry. Whirlpool water softeners, when set up properly and operated efficiently, will have no negative effect on your septic tank performance.

Yes.

That's a big question that no one seems to have a solid answer for. I used it with a septic system with no ill effects. Some studies show the brine flush inhibits beneficial bacterial growth, some studies show it to be beneficial. In short - no one can say for sure.