Some good advice about septic systems:
1) Don't talk to anyone in or around the Town Hall about the condition of your septic system.
2) No one can inspect the condition of your septic system without digging up the tank and the leach field.
3) One can replace either the tank or the leach field, and then 2 years later replace the other.
4) The only way to know if your system is not working is that the water stops going down the drains.
a) The vent pipe is plugged. (run a snake down it)
b) The drain pipe is plugged. (run a snake down it)
c) The tank is full. (have a company come out and drain it)
d) The leach field is plugged. (have a company dig up and replace the field)
5) Anything that is put down the drain, will will eventually have to be removed from the tank.
6) Grease clogs up your leach field.
7) Building Inspectors want new buyers to replace their septic systems: Refer to #2.
A State-approved site plan is about $5000.00. Refer to #1.
If you replace part or even all of an existing system, you don't need any approved design or state permits. Just call it a "repair" and do whatever you want. Gotta love New Hampshire!
Here's a local company that manufactures "innovative septic technology." http://presbyenvironmental.com/
Leach beds can fail. The ground around them becomes saturated and plugged with black gunk. Then the liquid doesn't percolate out, and the system backs up. This can be caused by improper construction, abuse, or neglect. In a modern system, the tank should be pumped periodically, depending on use, maybe as often as every 5 years or up to 10. If the tank isn't pumped, the "indigestible" solids that are supposed to settle in the tank start going into the leach system and plug it up.
Before about 1967 (give or take a couple years), there were no state subdivision regulations or lot (percolation) testing requirements. Any system built before the regulations were in place is built to whatever design the installer thought was good enough. Some of them are great, and some really suck. I have seen chalets that were built as vacation homes where the entire "septic system" consisted of a "dry well" about the size of a barrel made out of dry-stacked concrete blocks and covered with a scrap of plywood and 6 inches of dirt. And of course even a system installed last week might be a disaster waiting to happen if it was a very poor "repair" job.
One thing a prospective buyer can do is to call around to the local "honey wagon" outfits to see if any of them know anything about the system. They might know at least what kind of tank is in the ground; whether it is concrete, steel, or good old-fashioned railroad ties. They would also know if it had been regularly pumped, and whether it had caused problems in the past.
The bottom line for any home buyer is: Caveat Emptor!