It doesn't matter if you have septic or not. When I do my rag quilts I ALWAYS go to the laundromat and use their big machines. It is worth the cost as I don't mess up my machines.
Funny story: Once I took my rag quilt to the laundromat and was drying it in the dryer. A woman from another country was concerned and kept telling me it was "broken" as I kept on pulling out all the loose threads from the dryer filter. She understood, maybe, when she saw the finished dried quilt.
ali