Have just measured it and was surprised how small it is – a little over 2.5 cubic metres. It’s one metre deep.
The top of the sludge is at the same level as the water was 20 years ago. I think there must be an outlet at the bottom somewhere.
Smell – there was no smell before I plunged a batten down to measure the depth – now there’s a smell of rotten eggs.
You agitated it so hardly surprising. Give it a day and it will settle down again no doubt.
My fosse septique is probably similar to the one in the diagram – ‘modern style’ - only slightly smaller. There must be an outlet somewhere to drain out water, so maybe it drains into the other half of my garden somehow, and presumably the sludge remains until it fills the tank, then to be emptied away.
Like most people, I add my contribution to the tank once a day – so that’s about 2.35 cubic metres of poo over 30 years, if my maths is correct.
Getting interested now in design – might get it emptied just to see how it works!
Am getting a methane gas detector and want to poke around the soil in the other half of the garden. Methane can be lethal. Useful as well for detecting butane leaks from cooker and gas hot water heater.
If the law says it doesn’t comply, then my get-out clause, sort of, is that there’s a mains sewer in the chemin, just outside my garden gate – connection at my expense.
If it’s that size and hasn’t been emptied in that long, sounds like it may be draining into the ground.
I’m no expert in septic tanks, but I think this particular type is designed so that clean effluent flows into surrounding soil, where it is simply absorbed, leaving the solid matter behind to be held and emptied when it fills the tank.
The solid matter can’t flow out – it would block the inlets and outlets of the tank.
Any experts out there?
That’s what I thought too. 
Just wondering what the “P.C.C” or “PCC” refers to, please?
I think it means ‘plain cement concrete’…
If you are interested its called beton de propreté in french
I’ve got my methane gas detector, being a gadget guy, and it works! We all have certain amounts of combustible gas inside us, and when I breathed into the gas detector nozzle it set off the alarm. And poking around under wet decaying leaves it alarmed again. So too around the edges of the manhole cover to my septic tank. The greater the amount of combustible gas, and there are numerous such gases, the higher the pitch of the alarm.
Added Edit – when I switched on one of the burners of my butane gas cooker hob, all 6 warning lights blazed and the noise alarm was at its highest pitch.
