Haven't read the entire thread, Tilly, so forgive me if I duplicate something…here in rural western U.S., everyone uses a septic system. Ours consists of a largish (8 feet long X 5 feet wide X 4 feet deep) cement tank buried in the ground (duh!) out of which flows a single plastic 4 inch diameter pipe which connects to a distribution box which, in turn, feeds a leech field made up of several courses of perforated PVC pipe buried in gravel, covered with tar paper and filled with top soil.
We used to pump ours once a year, but found doing so destroyed a goodly amount of the bacteria that breaks down solids and with our County Agents advice we only pump it once every 3 or 4 years now. Costs about $125 U.S.
Some of the systems are over 40 years old with nary a problem…
We do feed ours a commercially available bacteria starter about once a year, but probably nor really necessary...