We have a 50 year old home in Stowe, Vermont. About 7 years ago it was completely rennovated.
Our problem began about 8 years ago when tailings from the (2nd floor)garbage disposal began occasionally appearing in the downstairs standing shower drain. About a year ago we moved into the house full time and the problem has reoccurred more frequently. Sometimes, septic toilet waste emerges from the downsatirs drain.
We have had the septic tank pumped out and the drain problem occurred a week later. We then had the line from the shower to the septic tank cleared and a camera determined that the line was not broken. Our problem persists. Our technician thinks that the problem could be in the drainage field. However this is confusing because the shower is able to quickly drain large volumes of water.
It's possible that you do not have proper venting of the waste system.
By the way you should not have a garbage grinder on a septic system.