I just want to mention that there are different types of septic systems. We have a newer one that has a front-end filter on it, and that filter can get clogged up fairly easily. Our state mandates inspections of this type of septic system every 3 years, but we have had the filter get clogged from light use much sooner than that. (My dh figured out how to clean the filter himself, but it's not a nice way to spend a Saturday afternoon!)
I believe a lot of the older septic systems (and perhaps those in other states) did not have these pre-filters.