We have just had a new 4000ltr fosse septic installed for our Chambre D'Hote, it's passed the SPANC46 check with no problems but about 2 weeks ago we started smelling a bad drains smell.... Has anyone else had any issues with fosse smells?

I know it's not the best subject in the world but I really don't want my customers to have to put up with the 'orrible odour, so any advice would be welcomed.

Thanks,
Jen

You may need to go back to the installers. Is all new from house going our via foul pipes? Our fosse does have a cabbage small at times, not found cause or solution as yet, but fosse is 50 years old. Looking forward to mains in a year or so.

Fosse smells can be normal depending on the wind and design of the Fosse, Are you using the products with it "Flush down the loo stuff". Remember if you have Antibiotics then you need to use more product in the fosse as Antibiotics kill the good fosse bacteria.

But check its ventilation, Get the builder back if your not happy.

First things first, where exactly are the smells coming from as it could be anything

Thanks all, the smells may well be coming from the vent as they're definitely that side of the house.... Hubby is reluctant to call the chap back who installed it though, but it may come to that. We can smell it as we sit on the terrace that will be our customers terrace, which is the main problem, and the vent is up on the roof quite a way up, so I wouldn't have thought we could smell it from that. But perhaps that is the issue, it needs to be even further up to the roof (we have a 3 floor house built into the side of a small hill so the first floor (which is actually the ground floor from the front) has the vent on top of that. Does that help or merely confuse?!
Is there a really good product maybe that might help eliminate the odour?
Thanks again, all comments much appreciated.
Cheers,
Jen

It sounds like you've already found the problem - the vent pipe has to go past the top of the roof (about +40cm) - no idea what it is in english, in French it le faitage
Also you can also check if nothing has fallen into it (it does happen!) and if it's not blocking the ventilation as this would also be a cause of nasty smells. Also is the vent pipe diameter sufficient?

Just further speculation here.
Being as you are built into the side of a hill and a three floor house, you could be subject to a "down-draft" in certain wind and air conditions.

This is a phenomenon where the hill 'lifts' the air current upwards causing a draw down effect on the leeward side of the hill. (I knew that post-grad diploma in air pollution 30 years ago would come in useful one day )

This may (as pointed out already by monsieur le plumber) affect the height of any stack, be it vent or chimney, you have on the house.

I'm not saying it is that, but adding it to the melting pot.