I am also on a septic system for years and years and use a lint trap on the drain hose of my washer. Cheap way to ward off problems. I wash rag quilts all the time and never had an issue. You just have to make sure to keep an eye on it and change it when it's full.
I used to buy them at Home Depot but they quit selling them so I buy them online now.
http://www.bicwarehouse.com/lint-tra...filiate&aff=cj