Just had to install a new aerobic system. The county requires a permit, licensed installer and planner, and then a county inspection. They just about won't allow a leach field system any more. Our system is 1400 gallons, in 3 compartments. When the last, "clean" tank gets full, it's pumped out and sprayed on the back yard, with bleach added before spraying.
Why did we need to replace it? The original system was two tanks, with poor plumbing, poor drainage, etc. And the clay level is way too high to pass an inspection, anyway. I used the wayback machine on google earth to look at it over the years; the evidence is that it never worked right. You could see the outlines of the tanks as far back as you could go, and in 2011, when there were plenty of people living in the house, they had a "squelchy" area flowing away from the tank, just as we did.
Now, the inspector said that he has heard of people not having any problems and not pumping their tanks for 20 years, but he almost always discover that the water is going down a little ways, hitting a solid layer, and flowing with the underground water in the area, probably going into someone else's well.