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The best way to kill the odor is to partially fill the sink with very hot water then add a couple cups of bleach then let it down the drain. I don't know why you would want to put yeast down a drain
I was told by my septic pump co. to mix 1/2 cup of yeast to a gal of warm water and put it down a different drain every mon. to keep pipes clean. Now I find that a lot of people do this regularly but it seems everyone has a different ratio to mix. Maybe the ratio doesnt matter. My girlfriend tells me she has always only used 2 tbl spoons to a gal. I was hoping someone actually knew the right ratio and chemically what it does.