What you want to know about your Septic System
The invention of the septic tank is credited to a Frenchman named John Louis Mouras, who, during the 1860s constructed a masonry tank with a glass viewing wall into which was directed various household wastes from toilets and sinks. The overflow ran into a standard cesspool of the time. Contrary to expectations, Mouras was thrilled to find that most solids when they settled to the bottom of the tank, digested in a few weeks and were liquified and carried away with the normal flow.
When the tank was opened after several years it was found to be almost free from solids.
This was later understood to be a process of anaerobic digestion relying on bacteria found in the human gut. This is why no chemicals are needed to start a new tank working. The septic tank is really an extension, another stomach if you will, of the human digestion process.
In a modern septic system the outflow of the tank is distributed equally over a large drain field where it percolates down into the soil where additional microbes working in the top few feet of the soil purify it further. This process is the same one that happens on the rest of your land where rainwater, rotten leaves, dead animals and their droppings are all composted and the water flowing through this process is recycled down over the years to rejoin the water table where well water is supplied.
As such it is a closed loop system (not unlike the "still suits" of the science fiction story "Dune") where you are ultimately drinking your own urine. And we've been doing it all our lives.
Septic overview with groundwater loop
Additonal information on plumbing: