Yeast is very good for septic systems and in fact yeast is a natural way of maintaining your septic system.
The yeast when put down your sewer line into your septic system helps to keep bacteria alive and that's good because the bacteria is what helps to break down solids in the septic system which keeps you from having to have the septic tank pumped as often.
To keep your septic system maintained naturally with yeast Flush ½ cup of instant dry baking yeast down the toilet, the first time.
Then to continue maintaining your septic system add ¼ cup of instant yeast every 4 months, after the initial addition of the yeast.