I just want to add my personal experience with having my water softener backwash going into the house drain that goes to a septic tank.
When we bought this house a year ago, the sellers replaced the d-box and did a pressure backwash of all the laterals in the septic field. At the same time, we decided to put in a filter on the outlet of the septic tank (we payed about $850) to prevent any solids from going to the field.
That filter worked too well, after 4 weeks of moving in, my wife and I decided to check the filter to see how well it was working. To our surprise, the filter was so full, that it actually floated up and it started allowing solids through. So we decided to check every 3 weeks from that point forward to clean the filter. Every three weeks we opened up the tank, cleaned the filter (most of the time, it was so clogged up that the water level in the tank was over the outlet), put it back in, and closed the tank. We did this from August until May. Not fun.
I had done a lot of research, and I found the papers referenced here and in other forums saying that the backwash was bad, good, or that it would have no effect to the biology of the septic tank. Since some of the reading I had done said that the backwash of a water softener could alter the bacteria in the tank causing the solids to not settle I wanted to give this a try (I had no idea what else could be causing the filter to get clogged so quickly...) Based on this (and tired of opening the tank every 3 weeks to clean the filter), I decided to re-route the water softener backwash to the outside. I live in NJ so I wanted to prevent the backwash from causing issues if the outside temp was below freezing so I also purchased a Freeze Drain by Raybend (from Amazon) and decided to use that on the outside. My setup is basically a trap, 2 feet of 2" pipe going to the outside, 90 degree angle, to the Freeze Drain, to 4 inch PVC to the garden. I haven't gone through a winter with this setup but I'm hopeful that it will work OK.
Since that May date, we have been opening the tank regularly (between 3-4 weeks) to clean the filter and check how well it is working. I have to say that after re-routing the backwash to the outside, it has been night and day and getting progressively better. We just opened the tank today (Aug 20) and the filter was almost empty. At this point, we will open it again in 3 months to see how it is doing.
Based on my personal experience, I can attest that routing a water softener backwash into a septic system is detrimental for the operation of the system. I highly suggest routing the backwash to the outside of the house if you are using a septic system.