I've managed everything from outhouse pits to municipal treatment plants and everything in between, among other things.
No matter how you treat raw sewage, municipal or individual septic system, the by-product of properly digested sewage is sludge. Sludge is dirt, for all intents and purposes and it displaces the liquid capacity of the two tanks to accommodate the outflow of the residence. The untreated sewage does one of two things once the tanks are full of sludge.
1. It backs up into your house which probably means not only the tank is "stuffed" but the leach field as well.
2. If the leech drains are open, it allows raw, un-digest sewage to flow into your drain field, along with everything that is bad for every living thing.
Crap is no different than any other vegetable or animal waste, digested in a mammal or not. Crap does carry serious diseases but digests the same as Celery cuttings. You need liquid space with all the natural bacteria that eats everything organic, in the first chamber. The digested sludge goes to the bottom, and the digestion process turns the sewage into reasonably pure water which rises to the top and spills over into the second chamber. Further settling and some digestion occurs there and then the water at the top enters the drain field almost clean enough to drink!
Don't hold your nose when thinking about your septic system. Understand how it works and keep it healthy. It's a beautiful thing and bacteria do all the heavy lifting.