"The Drains" - advice required

I have my own mini sewage farm down the side of my house comprising a little concrete sarcophagus 45cm x 25 X 35 deep, divided into two chambers by a 25mm thick brique cloison. (Probably a 1965 “Grease Trap” that has been functioning well for at least the last 15 years). I had a malfunction caused by the build-up of “undesirable” materials blocking the outflow from the second chamber to the village sewer. I reckon that it was probably because it had not been mostly “digested” in the first chamber, the partition having (suddenly) broken up.

The brique is easily replaced. However, there is not enough of the original left for me to work out what had been the size and the position of the opening that would be below the waterline – probably crucial to the efficiency of whole process. Can any kind soul out there point me to the website that I just cannot find, or furnish me themselves with some advice? I would be most grateful.

I guess SPANC could advise you on that :thinking:

SPANC is for assainissement non collective. If your commune organises it’s own communal assainissement , then first stop is probably your maire.

Hi Pete,
I think your question can be answered by applying a certain amount of logic.
The concept is to effectively create a U bend so that grease and other floating materials remain on the inlet side of the divider. Therefore the top of the opening in the divider should be below the level of the bottom of the outlet pipe, and the top of the solid part of the divider should be higher than the top of the inlet pipe.
The area of the opening in the divider should be at least equal to the cross sectional area of the outlet pipe in order to prevent overflowing.

Furthermore, the inlet and outlet pipes should not be directly in alignment with each other in order to prevent any direct flow of high speed effluent crossing the space and going directly from the inlet pipe to the outlet pipe in the event of breakage of the divider. In an ideal world the inlet and outlet pipes are in adjacent sides of the concrete ‘regard’ so that there is a 90 degree offset between the two. Where this is not possible then a lateral offset of at least the diameter of the pipes between the closest sides thereof should be just fine.

The whole point of this grease trap ‘regard’ is to collect grease, and other floating debris which should not have been put into the drainage system in the first place, such as small plasticised items or paper facial tissues. It is therefore necessary to inspect it at least annually for a good clean out, or perhaps every six months if it carries a large volume of effluent on a regular basis.
It’s not exactly a pleasant job but it is good for the roses. (Minus any plastic of course.)


Hi Gents

Thank you very much for your input on my problem. Bit of a hiatus though when, coincidentally, the main sewer in my village blocked up sending the “used water” of half the village, pumping up through my little problem box. Luckily, it only effects about 2 sq. metres of our property before flowing off into a ditch. Wait a few days until the “residue” dries then scrape it off for disposal. It happens 4-6 times a year. It wasn’t like this in Bournemouth! I reckon this definitely qualifies me as a “France Survivor”!

Anyway, Robert your thinking was 100% in line with mine – difference being that I have zero expertise or knowledge, so nice to hear confirmation. I did however, by following links on the internet, find myself on a catering industry site in Australia avoiding “fatbergs”. They were recommending exactly the same design (but 4x capacity) to restaurateurs . Their advice for the opening between the chambers was ½ to 2/3 area of the inlet. Seemed small on my application. At the same time there was little scope for making it as big as 100% and having much divider left. Having observed the flow it could never be anything like max. So I plumped for 100 sq cms for a pipe cross section of 133. I can easily redo it if need be. Whatever, it will never be as bad as our 6x annual events! After a few days it seems to be working OK.

I try, Graham and Mark, from experience, to avoid officialdom if possible. I guess there would be an even chance that this 1965 hand-made affair didn’t then, or doesn’t now, meet H&S regs. Like Bernard Cribbins discovered . . . .

“Don’t dig there, dig it elsewhere,
You’re digging it round and it ought to be square!”