Lava Rocks in Fosse Septique

Not a glamorous topic, but one that seems relevant to a lot of property owners in France.
Bought our holiday home last year and it came with a conforming fosse. Came with some advisories, but nothing too dramatic.
Due to circumstances we have not visited the house as much as we intended this year. When at the house, we have sometimes noticed a gassy odour in the garden. I have taken off what I would call in the UK, an inspection chamber. This is where apparently two soil pipes emerge from the house. In the pit there is another exit pipe.
The chamber appears to be part filled with rocks. These rocks obviously interfere with solid waste which emerges from the soil pipes. I have done a bit of googling, and as far as I can ascertain, the lava rocks are usually present at the point where the water leaves the fosse and enters the filter beds, not before the tank.
I have the SPANC report for the fosse which was done last year. The diagram shows the inspection chamber that I am referring to followed by two further access points, which I would imagine are above the tank.
I only really discovered this situation the day that I was leaving for the UK, so have not had a chance to investigate further.
Can anyone advise whether I am just missing the point here and this is a normal arrangement? I am really struggling to understand why the rocks are in the inspection chamber prior to the tank.

Hello Ron and welcome to the Forum.

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Screenshot 2019-09-07 at 09.38.21
Screenshot attached which shows the Fosse design. The tank is apparently plastic.

Looks like a “bac a graisse” to me (grease trap) since I know that these have to be fitted to cuisine/bathroom discharge before the water goes anywhere else.

Hopefully, someone will chime in with more knowledge about the “rocks”…

Thanks. Might be the answer. But as there appears to be only one chamber before the tank, it appears that all the waste goes through it. Sadly, not there now, so can’t investigate further until we are hopefully back next month.

My tank has a basket that contains these rocks in the second chamber. Last time I saw them was several years ago…never done anything with them.
However, where smells are concerned, I put a 100mm stink pipe in line, just before the entry to the tank. It’s stands at 4m above ground, somewhere in the hedge…so far, no stinkiness.

You did mention SPANC making advisories - what are these ??

It might help us if we have their thoughts in our heads… :thinking:

Our village is on a Reed-bed system. All the soil-pipes go to one big grease-trap which is certainly situated in-line and just in front of the reed beds. A Cantonier does “whatever” every Monday… I have no more details other than to say we get no smells from that area.

However, some houses did have smells where they had not fitted a stench-pipe… that was quickly remedied by those concerned.

Thanks for advice. There was a recommendation on the SPANC report regarding a breather pipe, so it is on my list of things to look at.
The thing I don’t get is why there are rocks before the tank. From my basic point of view, it is like putting rocks in a standard manhole in the UK. Not what you do. However it is not a normal UK style manhole as the outlet is not at the bottom of the pit. So it is some sort of trap. However, all the waste appears to go through it, as shown on the diagram, which is the bit I don’t understand. Surely solid waste should go straight into the tank.
But it does conform when inspected last year, so I am just struggling with the theory of it really.

These are the recommendations. It does say about a secondary ventilation pipe. This would help with any odours.
To be honest, the odour is only intermittent and is not a big problem by any means.
What I would really like to understand is the design of the system. I cannot find anything online that shows a diagram that appears to be the system that I have. The main thing that seems unfathomable, is why all the waste appears to land on a bed of rocks. I could be missing the point somewhere, but what is indicated on the diagram and my brief investigation have not enlightened me.

Screenshot 2019-09-07 at 10.18.16

The lava rocks act as a grease trap, as the liquid levels rise up the fosse the rocks are supposed to collect any grease that’s in the fosse. They recommend you clean/replace the rocks from time to time, (sorry can’t remember the time period), it’s easy to lift out the basket containing them & wash. You can easily buy fresh ones, if you prefer, from garden centres/bricolages.

You’ve got 1 year to fulfil the obligations you undertook when you purchased and to ensure that all is well ongoing… you will need to speak with your Mairie in any case, before doing the work on the rainwater pipes…

Makes sense to find out exactly what all the words mean. (I’ve got an idea, but I’m no expert.)

Talk the report through either with a SPANC representative or with a helpful person at your Mairie (who will chat for free).

A fosse which is not properly maintained will soon cause you grief, so do whatever is necessary now before it becomes expensive later on.

I have no real understanding of septic tanks…I put this one in a few months after we moved here. Hadn’t a clue what I was doing…still haven’t.
The old one is only 1000L, & judged to be too small. The bath, basin & bidet all emptied into the ditch in front of the house.
It’s been 17 years; it works; doesn’t smell; but doesn’t conform…can’t see why, but there you go

Thanks for advice. The rainwater was diverted by the previous owner before we took over.
I think the answer could lie in the fact that the property is not currently permanently occupied. The trap with lava in should probably be kept full up to the level of the outlet. Due to lack of use this trap has possibly evaporated and needs to be filled up to the outlet level. If not full to this level, any waste sits on top of the rocks. However, we were there for two weeks, so don’t understand why it did not fill up.
Still can’t understand the logic of solid waste collecting on top of the rocks. Maybe there is another outlet lower down and the top outlet is an overflow.
Grateful to hear from anyone who has a similar system.

Ron, where exactly are the rocks, are they in the actual fosse?

Hi Debby
On the diagram, the rocks are in the first square chamber. As you can see, it looks as though all the household waste empties into that pit/chamber.

and that is why I called it a Grease Trap (although the legend on the diagram does not call it that, as such). Mucky/sticky water has to go through a grease trap of some form to only allow “non-greasy” water going on into the wide world of the “fosse” and eventual soaking-away of the purified-water (?) into the good earth… :upside_down_face::thinking:

If all is done correctly it does work. In UK we were on well-water, with a septic tank/ fosse nearby (probably too near for today’s normes). Our Well was tested every 3 months (never a problem) and we only needed to empty the fosse itself once in 27 years… phew.

Looking at your diagram, it’s similar to our fosse, the square chamber has 2 round inspection hole. The 1st round hole, near the house, should just be an open hole into the tank with the inlet pipe. The second one should have the outlet pipe near the top & this is where the rocks should be. Solids enter the fosse, sink to the bottom where the ‘poo muncher’, (good bacteria), gets to work, as it fills up the liquids rise up it passes over the laver rocks to trap the grease before passing into the outlet pipe. If no traps there, the grease passes straight into the outlet pipe & over a period of time blocks the pipe causing problems! This is how both our fosse work, but maybe yours is different. If in doubt get the guys from SPANC to pop round & explain the system to you, they’re usually very helpful when not massively busy.

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Also meant to say, I hope my less than technical explanation made some sort of sense!

Yes, it would appear to be a grease trap. But still can’t quite understand why sewage appears to be deposited on top of the rocks via the inlet pipe. If there was a separate pre chamber for the kitchen water etc I would get it, but as on the diagram, everything seems to be deposited in the one chamber with the rocks in it.
I’m just struggling with that part of the design. It is a working system so it obviously has been designed properly, but I can’t understand the theory of it and the fact that raw sewage appears to sit on top of the rocks.

Yes makes perfect sense. The bit on mine that doesn’t make sense is the rocks being before the waste enters the tank in a chamber that also has sewage entering it.
I won’t bother boring anybody any more and will take advice from someone during next visit.
Thanks again for all the advice.