My house is not attached to the town septic system, instead I have installed a BioCycle septic system. Everything that goes down drains in my house goes into my BioCycle where it works its way round some chambers where natural bacteria work their magick on it. When a certain amount of liquid is in my BioCycle the excess liquid comes out onto my vegie garden.
Due to recycling all liquids used in my house I have a list of products that are suitable. I contacted the people who check my BioCycles ph every three months and asked them about the soap I make. They rang me back to say it's all safe. Plus due to there being a limited amount of soap entering my systems on any day ... any risk is further reduced.