Mike, the strain we used was Champagne, a good all-around wine yeast. I may be wrong on the temperature, it was over 25 years ago, but the consensus was we killed the yeast because the must was too warm. We were able to save it by adding another packet of yeast a couple days later.
I'm sure beer yeast is more robust, and bread yeast is probably more robust still.
But my point was, yeast has limited utility in a septic system because of what it consumes (sugar and starch), and the limited temperature range it lives in. Bacteria, which does the real work in the system, can function from just above freezing up to around 160 degrees F.
In the higher elevations here, septic tanks that are not working properly can freeze solid in the winter. Bacteria produces heat, and will keep the septic tank from freezing in sub-zero weather.
