I just bought an old farmhouse an am going thru all the repairs that need to be done and have a puzzle.
when a large load of waster is flushed down the pipes (i.e. bathtub emptying) the basement fills with septic gas. found a cleanout cover rusted out (and now fixed) but still having the gas.
I'd say that when the water enters the tank, the gas gets pushed back, after the fix it has dropped noticeably, but it appears it has now been directed to the tank vent.
still some gas in basement, and a fairly good stinker from the septic vent, have i been living in the city soo long to forget that this is normal?