In article ,
Chris Lewis wrote:
To summarize responses: septic tanks need periodic pumping, however,
the "right" interval varies a lot from one installation to another.
In some, it could be decades. And in another, as short as a year.

I have seen a tank that was just fine when it was pumped after 15
years. The owner attributed its robustness to the fact he ran the
washing machine drain directly to a separate leach line (in the desert,
not in a lawn), bypassing the septic tank and therefore never pouring
chlorine bleach into the tank.

Lee