I am currently experiencing some smells from the waste water system. (We are on mains drainage)
Upon investigating, it seems that the ‘grey’ water and ‘foul’ water are collected separately and the two (I suspect) meet somewhere in the drains after leaving the house. I am no expert on this matter and would like to know if this was current practice on the mid 1970’s when our house was built. Any ideas would be gratefully received
Damage to the pipework via roots?? Are you getting a smell in the toilet or bathroom indoors? This house built in 2022 has two big pipes coming out of the vide sanitaire under the house and then running separately to the main sewer for both of them to leave the estate.
our main drains were done in 2012 so no idea re yours in 1970… but I wonder if your stench pipe has become blocked (frelon/bird/bees nest) as this would normally carry any smells well above the house.. up, up and away 
if it’s blocked, smells can get into the house via basins/sinks anything where the gas can bubble up … aargh
French houses do not have stench pipes fitted as standard. Only with a septic tank do you have an outlet with a cap on the top just a few inches above the tanks
When we bought our house it was on the understanding that we must install a Tank of some sort as nothing was in place.
but the Maire said we could wait a couple of years for when Mains Drainage would be installed throughout the village.
Sure enough, Mains Drainage came along and every house was linked up to it and every house was fitted with its own stench pipe 
A few stench pipes weren’t tall enough and smells from the main drain did enter one or two houses… aaargh. but the problem/error was resolved, thank heavens.
Often they use air admitance valves over here in france rather than a very stall stench pipe. From what I have seen.
Doubtless this is yet another example of how things can and do vary across France. 
Not so in my experience.
I installed my septic tank system in 2012 and although being totally proficient in all things drainage, as an englishman abroad each stage was investigated by our local SPANC.
Since then I have corrected faults on systems for many clients following a SPANC insoection prior to selling a house and without exception it was a requirement to have an open ended vent pipe at high level to discharge smells.
The current hot weather is more than likely to cause wiffyness from a fosse wether or not it has a tall stench pipe.
It’s not unusual for “grey” waste to go via a grease collector (kitchen waste) (so one grey outlet plus one main soil pipe leaving the property)- equally older houses don’t always have them. But if you’'ve two pipes leaving - one will most likely go via a grease trap - the trap will be somewhere between the wall and the fosse (most likely at one end …but somewhere between is the only guarantee…?sorry).
If there’s a grease trap and your asking about pipes … It’s not been cleaned in a while
OP is on main drainage though,
Maybe you have a dried out trap somewhere - sink/shower/bath/toilet thats omly infrequently used.
As @Stella has mentioned, there is wide variance, especially with older properties. There are rules about how the system should be configured, and then there is the reality of how it actually is configured.
Our own system connects to mains drainage via jointly used pipework across our neighbours land. The ‘stench pipe’ function is taken care of by a 125mm rainwater downpipe that serves a number of different large roof areas. No way that it complies with the rules, but it works, and when there is a thunderstorm the whole system receives a darn good flushing.
I have no intention of selling my house, and neither has my neighbour any intention of selling his, so I suspect that the system will remain as it is for many years to come.
I hope the OP is not suffering from broken sewage pipes where connected to a WC in house like I discovered last summer where one of the toilets had been flushing waste directly onto the ground in the vide sanitaire under the house because the plastic waste pipe had come away from never being installed correctly. Luckily it was covered by the builder’s Deçenelle insurance and my pet plumber replaced the whole entire system with a lot more strength a few months back at a cost of a couple of euros under €3000 covered to him via me by the insurance. That all began with a whiff and then got worse and worse until my son managed to get under the house and discover it all and we cleared up the “debris” which by then had dried out. I had to resort to using only the other WC which is quite a hardship when you have been used to two LOL.
thanks for the many quality responses. I am now of the opinion that my house has a foul waste leaving the toilets and going out under the house never to be seen again, whereas the grey water is collected from all sources into a small regard, from which one single pipe feeds a grease trap containing three cascading cubicles, the final one opening to a large pipe which then disappears outside the house - no doubt joining up with the mains drain. I will investigate further, but the smell is much less today!
What I can’t understand is that the first two of the three ‘cubicles’ of the grease trap are full to the brim; indeed they have to be in order to cascade down into the subsequent one. It seems wrong to have these two containers permanently full of grey water but I don’t see how it could function any other way.
Our waste water became blocked but my various attempts got it flowing again. Just to make sure I shoved a hose pipe down and gave it a blast of water. The water flowed but started bubbling up from under the concrete patio outside the house.
Now out of my depth (no pun intended) I called a plumber who suggested a break in the canalisation between where it exited the house and the grease trap. Because I had no idea where the grease trap was, he proposed breaking through the concrete and digging down working from where waste exited the house towards the grease trap.
After about an hour or two of jack hammering and digging he called me to look at the problem. No broken pipe and no grease trap. The waste water just flowed straight into the foundations material. The expensive solution was to install a new grease trap.
Although the house was only built in 2012, he assured me this was not an uncommon problem.
Vendredi apri midi syndrome same as UK.
I thought that mostly applied to Dagenham Dustbins
Good grief… weren’t the drains checked at point of sale (let alone at point of doing the original work)
OK, not since Covid, but before and after 2012… I’ve been a translator in several situations where plumbing stuff was involved (Dordogne and Charente).
SPANC have been strict in their controls of how things should-be/were-actually done. They checked the flow of absolutely everything, to ensure that it all went to where it was intended (and certainly not into the foundations material 
)
Obviously areas of France will vary… but…
That is what causes our smells, usually 
As I remember, the fosse septique was checked but I don’t think the drain runs were verified.
The house was built by a local building company (who,surprisingly, no longer exist) for a very elderly Brit who was very vague about everything.