Just out of interest - what criteria does one use to judge when a
spetic tank does require pumping out? The interpretation of the above
thread is that a cess pool requires regular pumping - yearly? - but
that a septic tank rarely requires this treatment. I suppose it does
depend on the size of the septic tank and the domestic throughput, but
that still brings back the question -- when?

I had mine done after fully 15 years recently and the operator wasn't
too impressed ! What seemed to be upsetting him was that the crust was
hard and therefore required more work to process.

Rob