I am on a septic system and I am getting a bad odor back thru my over flow tube in the toilet tank. what is the cause and how do I repair it?

 

If it's a standard 2-part toilet and tank, I'd hazard a guess that you may have mold and mildew growing in the hollow passages inside the base (the water in the bowl normally keeps sewer gases from coming thru).

It _could_ be that the wax seal under the toilet has failed, and the smell is actually coming from under the base.

For the first one, I try pouring a couple of cups of bleach right down the overflow tube.

For the second, after after cleaning around the base and the floor around it, try sniffing around the floor level (sounds dumb, but that's how I found a bad wax seal on my own toilet. Water wasn't leaking in the bathroom, because there was a gap in the concrete slab around the flange, and the water was going under the slab, never to be seen.