We were lucky, our drainfield was in the very early stages of failure. It would handle showers and toilet flushes just fine. But it flowed slow enough that if you did two loads of laundry in a row, the second load would usually dump 1/4 gallon of water or less out of the standpipe on final rinse. Never did have sewage backup into the home, or water in the yard.
We were sure it was the drainfield, but the first septic company we called dug a hole, and said it was bone dry, so he said the drianfield was fine. He noted the standpipe was only 9" above the trap, code is min 18", so I made it taller. When that didn't work, he said the vent was too far away, so I added that AAV device. You can see both in the pic. I doubted either of these would fix the issue, since we had no problems for over 20 years, and it didn't.
We called a different septic company to inspect, they just stuck a rod in several places on the other side of the yard, and hit standing water at less about 3'! They asked me why the first guy dug a hole? Turned out the first guy was 30' from the drainfield. He claimed he hit gravel, but that just isn't possible. He wasn't even close. I knew where the tank was, and assumed the tank went straight out. It actually goes about 20 feet to the left before draining into the field. The county keeps all this in thier records, and the septic companies are supposed to get that information before they even pump a tank.
