Additives are bad for your septic system, according to health officials in Washington State and at North Dakota State University, and while yeast won't harm your system as much as some others, it won't do it much good either. The most effective way to introduce beneficial organisms into your septic system is to use it normally for its intended purpose.
Yeast Can Be Harmful
One reason for adding chemicals to your septic system is to break down grease and prevent clumping in the tank, but these additives often end up flowing into the drainage field, where they can cause damage. Highly alkaline additives -- such as detergents -- interfere with the soil structure of the drain field, and the net result could be premature failure. Another reason for additives is to accelerate biological decomposition, which is the main rationale for adding bleach. The tank is already full of microorganisms, though, and adding more is like adding a spoonful of salt to the ocean -- it won't have much effect. Besides that, yeast can actually be harmful: It causes frothing in the tank that prevents grease and other solids from settling. These solids then flow out to the drain field and clog it.
The Benefits of Yeast are Limited
Yeast may effectively break down starches in your septic tank, but those represent only one of the five types of waste in the tank; the other four are proteins, fibers, greases and pectins. If you assiduously avoid putting animal products, vegetable oils, fibrous vegetables and other harmful, drain field-clogging items into your drains, a regular treatment with yeast may have limited benefits. Septic professionals regularly recommend against using it, but if you want to experiment, don't just pour yeast into the toilet. Instead, follow a simple procedure.
Procedure for Using Yeast
Pour about 3 cups of warm water into a bowl. Use tepid water from the tap.
Add 4 1/2 teaspoons of baker's yeast and 2 cups each of sugar and cornmeal.
The sugar and cornmeal activate the yeast.
Cover the mixture; wait for it to froth. Pour it into the toilet nearest the septic tank, and then flush twice.
Proper Septic Maintenance
No matter how many additives you treat your septic tank with, it has to be pumped at regular intervals to remove the sludge that collects at the bottom. You should have your tank inspected every two years, and the sludge layer must never be allowed to reach the level of the outlet pipe. If your tank has a filter on the outflow pipe to prevent solid matter from clogging the drainage field, clean this filter at least twice a year to prevent backups. Of all the septic maintenance tips, the most important one is to watch what you put in the drains.