Did you know your septic system has to be designed by either a licensed septic designer or a Professional Engineer? Mr. Nick was able to design ours (because he is a PE) but if you're in a situation where you have to pay for someone to do it wouldn't you like to know, just what am I paying for? For starters, a septic system is the vehicle that takes your waste (you know, what you're putting down the toilet) and sends it through the appropriate venues to make it suitable to be released back into the ground. If your septic system isn't designed right then you are sending toxic juju back into streams, rivers, your neighbors well, your well ... you get the picture.
So, how does it work? Basically a septic system takes the solid waste and sends it through a waste tank which allows all the solid particles to float to the bottom. The liquid waste is then pumped into your drainfield as gray water. Your drainfield is filled with pipes full of small holes (think a soaker hose under the ground) that emit the gray water into the gravel (which you have to put there). That water eventually filters through the gravel and into the earth where it reacts with the oxygen already in the soil. This reaction creates gases that evaporate out of the ground. Meanwhile the now "clean" (I say it like that because I still wouldn't drink it) water filters back into the soil.
If you don't design the system correctly the drainfield can clog up and grow bacteria and algae. Ideally you should only have to pump your septic tank (you know, where all that solid waste is being stored) every 3-5 years.
Basically you are asking the ground to process and clean your waste, making it okay to go back into the environment. Which is why the county monitors all instances of septic design against county and state guidelines. Which is why a professional needs to handle this part of your home building.
You remember how gross it is when Bear Grylls drank his own urine right? Just think of what would happen with the other stuff...