Thanks to its antibacterial properties, chlorine bleach can suppress the bacterial activity which breaks down solid waste in a septic tank, potentially leading to bigger problems.
Why Bleach Can Cause Problems
Beneficial bacteria inside a septic tank convert solid waste into a liquid effluent that flows freely through the tank outlet pipe into the underground drain field. When enough chlorine bleach is flushed down the household drain, the antibacterial activity of the bleach may inhibit this process.
If the bacterial activity in a septic tank is suppressed, waste matter remains semi-solid instead of liquefying. Clogs may form as undissolved waste migrates into the septic tank outlet pipe. In turn, a clogged tank outlet pipe could lead to a backup of raw sewage inside the house, ultimately requiring professional repair and expensive cleanup.
How Much is Too Much?
The effect of bleach on septic tanks depends on relative concentration. The American Cleaning Institute reports that the typically modest volume of chlorine bleach present in household waste water will not significantly suppress septic tank bacteria levels.
In fact, over a gallon of bleach down the drain every day -- the amount present in about 15 average-size loads of laundry -- would be required to impact septic bacterial activity and impede free flow of sewage.
Ordinary household amounts generally pose no problem.