I am hoping someone can offer some help in a situation that has me so very frustrated. I have one bathroom in my home, I might add that I live in a rural area, so I have my own septic system. For the past 4 days or so, when we flush the toilet, we have a horrible septic odor coming back into the house, in the basement that is. Now, before I get people telling me that the lines should be checked. They have been. All the lines are running smoothly, water is going where it should and so on. The septic has been pumped out just this spring. There is no odor coming from sink drains or anything else. The odor is coming back into the basement and smells horrible. With winter coming, we can't be leaving a window open to relieve the smell. The vent pipe is on the outside of the house and it also has no odor coming from it. We have tried adding more pipe to that and it hasn't helped. No lines or pipes are plugged, the toilet flushes properly. Any suggestions?