Fosse management, ah, a subject dear to many a brits noses
Well, to start, lets assume that it's a system that meets all the current requirements. Mr Spanc (I love that name) or whoever has been delegated to do the work of inspecting your installation will visit you every 4 years, at which point he will want to see a receipt for emptying the fosse by an approved muck sucker dated less than 6 months ago. So effectively a fosse has to be emptied every 4 years, now that’s not a very good way of doing it but it's easy to understand and there's no ambiguity about it such as half full.
Really it's down to the house holder to be sensible, if it's a holiday home it will need a lot less emptying than a system that is used to it's maximum capacity. Look in the manholes take out the prefilter and at a last resort try & "feel" the level of the "mud" There is no economy in leaving your tank until it overflows with "stuff" as by the time it overflows the damage has been done and the drainage system is blocked. That's going to cost a whole lot more than a 250euro pump out of the tank.
Start by finding your fosse and taking of the lid furthest from the house, if there's only one, that’s the one. You're going to need marigolds now. Inside this lid you will find a prefilter (if it's not check under the other cover, the pipework might not go logically you might well have to push/pull the outlet pipe to release the basket and hopefully there's a handle to lift the prefilter by, if this is a whole new experience, just to find the fosse then this prefilter probably won't come out very easily.....
Take this to a dark satanic corner of your land (preferably down wind) and hose it off/out. If you have what appear to be rock in it and this is the first time that you have seen them in 10 years rush out to brico & get a sack of pouzelaine, it's not expensive and whats more, if you buy it in B&Q it's called larva rock! (and times the price) so you can put new rocks on your BBQ as well. No don't try the ones out of the prefilter
If you have a concrete lid to the fosse you can tell if it’s ventilated properly by looking at the underside. If there is white deposits then you need to improve the ventilation, after all the bacteria needs oxygen to work & break down the solids. There should be 2 vent pipes going to your roof line, both 100mm diameter, one comes from the tank to allow the gases to escape and the other at the end of the upstream to admit air. The ends of these vents should keeps out leaves/birds/cows etc but allow free movement of gases.
If you have a grease trap (oops I nearly typed crease trap ) most older ruins, sorry I mean properties, have one. Modern stuff, well if the fosse is less than 10metres from the house there is no requirement to install one, but when one is installed it should be less than 2metres from the house. Anyway, if you have one, how do you know? smell after doing the dishes? close to the house? Look for a round concrete cover a rectangular plastic cover, anything that is reasonably close to the house.
If you have never cleaned it and it should be done every 3 months but I'm as guilty as you and have left it, erm, years. If it's been that long you will need a shovel! The solids from this can be put in a sealed bag with the normal household rubbish. I advise doing it in the winter, the smell isn't so bad!
We all know about these fosse activators, but do they do anything? Do they work? If you go and have a read of various spanc forums (shows you what an exciting life I lead eh!) they all say that it’s not needed, but won’t do any harm though using these products is no alternative for regular maintenance & emptying.
Bleach, well we all know what a toilet bowl looks like if it’s just left to its own devices, oh, perhaps it’s only men that know that… I’m sure that bleach will kill 99% of all germs but one litre of bleach in 3000litres of germs is going to have its work cut out! So I say use it, but just like wine – in moderation.