My septic system is grandfathered in, now that sewer is available. If the system fails then replacement isn't an option, rather I'll have to hook up to sewer. When I applied for a permit to replace my septic tank, the water-sewer utility wanted the county to deny the permit. The county took my side, arguing that I was maintaining an existing system rather than fixing a failed one. I got the permit.
I really wanted to install an in-tank aerobic treatment unit, my own little sewage treatment plant. My reasoning was that the output from the ATU wouldn't degrade the leach field at all, since it's suitable for surface discharge. The county permitting office gave me info on the system they recommended (for sites that needed one). But the contractor flat-out refused to do it. He never really gave a reason. I think he didn't want to install a potentially troublesome device unless it was absolutely required.
