the bacteria will come back naturally if it is killed off. it is pretty difficult to kill off all the bacterial in a 1000 gallon septic system, all the goop **** and crud stuck in the nooks and crannys of every pipe hold bacteria. by the time the acid or bleach or whatever sanitizer you use regularly gets in there it is pretty diluted...forget yeast, you need bacteria which is not the same as yeast. flush some road kill if in doubt.