Ours are about 30 years old, and there is apparently a lot of clay in our soil...
The current field is saturated, and the tank isn't draining out into the field.
We're having a new field installed with a valve at the tank so that when the new field fails, the owner at that time can have it switched to the old (rehabilitated?) field.
I'm not into the current technology at all, I just know that the tank is full and not draining....
Not An xpert on the subject- sorry.
But I am trying to learn...
And I might stay at a holiday inn..