In my working life, I've moved many times. Four of those times, my homes had septic tanks. We have a container on the back of the master bath toilet labeled "old yeast" and periodically we sprinkle 1-2 tablespoons into the toilet and flush. It has never seemed to hurt the septic tank and we hope it helps the resident microbes.

I suspect city sewer systems would NOT appreciate yeast going into their systems.

Periodically, I make sourdough. I usually use the discard in a recipe rather throwing it away, but have never worried about a little going down the sink drain.

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