Some "experts" recommend yogurt as a starter for anaerobic digestion. But it is unclear whether it is supposed to pass through the alimentary system first!
Try a humm buster on the vent if it is a vent issue. Works a treat
Try to avoid allowing bleach and other products that say 'kills bacteria' down the toilets/sinks - it is the bacteria in the fosse that stop the smell - or at least avoid them until it is well established. We used Eparcyl until the fosse was well established but now only need it very occasionally, such as when visitors come and insist on using biological washing powder or after I had a hefty dose of anti-biotics for pneumonia.
I use Eparcyl when mine gets whiff. I've never gotten into the habit of weekly treatment, but it reminds me when its due. You can get Eparcyl at any supermarche.
I've never fed ours with anything I haven't fed myself with & we have no low level smells. It's vented at the head of the stack & at the tank exit with both vents terminating at the ridge. Could smell up there but I'm not going up to find out ;-)
if it's a new one then it should be vented to roof level and so no smell even if the bacteria haven't started working. Mine stinks but that's not surprising as it doesn't meet any norms and has no ventilation...! :-O
Eparcyl once a week and there should be no problem. If it is working properly then the smell will soon subside. If bleach or any other chemicals go into them and kill the bio function then you will know it. It takes a lot of rotten eggs to achieve what you get then :-(
Not unusual. It takes time for them to settle down to quiet digesting. Sometimes it happens when an older well functioning fosse is cleaned out. Your friend might also like to check the ventilation into and from the fosse. If the ventilation pipe which should vent itself above the gutter line has been misplaced and sits in a dead area smells can be blown back to the ground level with consequent nuisance. If its been installed correctly according to French Normes it shouldn't pose a continuing problem.
Ours is in use daily, and no it doesn’t smell, but we do “feed” it on a regular basis with the eparcyl foss treatment…I would say a new system should not smell but it does need to get digesting…suggest they go back to the installer with questions