Working on septic systems, once operational, is not only nasty but quite humbling. Anybody who thinks their s**t doesn't stink is quickly dissuaded. But it also is potentially dangerous. Beyond the obvious bacteriological potentials, like nasty bacteria getting into wounds, eyes, mouth, respiratory system, the biggest danger is asphyxiation in a confined space. This is especially true of large commercial sysytems. Methane and other gases displace oxygen and anyone entering quickly succumbs. It is often the case that the second person presumes the first had a heart attack or something and climbs down in to help and also dies. And then the first emergency services rescuer, although confined space training is much better now. I remember 3 or 4 of these instances [nation-wide] during my twenty years in the field.
Best left to the professionals, for whom I acquired a lot of respect. They truly earn their money, IMO.
Septic tanks usually do not need to be replaced unless they are found to be leaking into groundwater. They are supposed to be water tight, nowadays. However the baffles within them do wear out. Baffles divert water flow downward to keep floaters from going straight across from ingress to egress and out into the field. Depending upon design, these may or may not be replaceable. If not the tank has to be replaced.
I do not believe that a field can be remediated by tilling. Typically the field is dug out and replaced by new media. With the price of the work compared to the price of sand, I don't think it's cost effective to try to re-use the old stuff. But I am speculating and have never seen any research on any thing like this.
Mike is right. A well maintained residential gravity system can last forever if they are properly maintained. Typically though, they will go 15 to 20 are not pumped and the field gets ruined.
