In my basement laundry room is (to my knowledge) the only drain pipe in the whole basement, which hooks into the end of the main line as it exits the house and heads to the septic tank. When we moved in a year ago the drain was loosely plugged by an expanding ring cap. Never having owned a house before I didn't think much of it, but soon enough I learned what it was and why it was there, when a liter or so of black sludge backed up and forced the cap off and nearly ruined the carpet outside the room. Needless to say the smell was horrid. We cleaned it up as best we could and I snaked it five times (and two more from a toilet downstream [installed right onto the main drain line]), but nothing was draining, so we called a plumber.
The plumber took a much larger snake to it and eventually got it flowing again (after ripping out the toilet and snaking it from both directions) but couldn't identify the source of the clog. Meanwhile all the material that was coming up looked like mostly-decomposed food (though nothing we actually eat strangely enough) mixed with the sludge. We also noticed that running any water in the house except the aforementioned toilet caused the sludge level to rise in the drain.
Now the kicker - despite flushing the drain out with buckets of water every few days, a year later it has happened again. We took the same actions to clear it up, but still have no idea what the source is or how to fix it.
The drain itself smells awful, and has since day one (they had it masked with an air freshener when we moved in) so I'm sure it's an ongoing problem, but I'm clueless how to fix it without literally ripping up the basement to replace the pipe.
Can anyone offer advice on how to solve this issue or at least theorize on what the cause is?