Our septic tank system consists of four settling tanks in line draining into a soakaway. There should be a siphoning system between the tanks, but that disappeared many years ago which mean that more sediment than should ends up in the soakaway, which will eventually block up. So far, fingers crossed, it works reasonably well. We can't see our outflow as it is somewhere underneath a concrete yard. Emptying our tank now costs about £100 . As others have said, the tanks are always full, that is the way the system is designed. We were told to have it emptied once a year, but sometimes we 'forget' until a patch of moist, very green grass appears round one of the manholes...
We take no particular care of what goes down the drains.
Our neighbours tanks, which curiously are on our land, need emptying far more often, but they have five children and a hot tub. Their soakaway has drainage fingers leading away under our garden. I wish I had known about this when we moved in and I would have planned round them as plants growing over a finger grow incredibly well. Ten foot sweet peas anyone?
Another 'use' for a septic tank system is to huddle cold frames round them (without covering the manholes of course). Both hot bath water and the fermentation taking place lead to a slightly warmer microclimate.
Gillie