Stagnant water in sewer pipe, water draining slowly due to silt deposits? (pics included)

raphael49 · 11 Jan 2021

Hi everyone. I am trying to fathom the sewer drainage system in my house. I lifted the sewage manhole cover outside my house (one is for outside guttering, the other internal), and see stagnant water which smells and is slow to drain

The main pipe exits here into two pipes, one big one above a smaller one. The smaller one was full of small black silt deposits, from where I do not know. I tried to remove them by hand, but could only reach so far. I do not know whether this small one is blocked off by design or whether its been blocked by some obstruction

The upper pipe is flowing freely, but because its elevated, sewage only flows into it once it rises few cm above the lower smaller one.

I just wanted some advice whether this was normal at all or what I can do?

Here's the visible upper pipe, smaller pipe beneath it is submerged

Manhole cover outside garage

Some kind of black rock like deposits?

Hugh Jaleak · 11 Jan 2021

Top picture is of an 'Interceptor' (or Buchan Trap as they're known in some parts.) This is a water filled trap designed in Victorian times to seal off the house drains from the main sewer. Top pipe is actually a Rodding eye, allowing access past the trap to clear the section of drain beyond if the need ever arose, it would have been fitted with a stopper originally, which is probably now long gone. Would look something like this.

The trap itself is blocked, hence the standing water. It needs to be cleared, a good poke with some drain roads whilst running water through the system may do it. This will allow the standing water in the system to drain, and prevent a possible blockage forming further upstream.

raphael49 · 11 Jan 2021

Thanks. I will get some deeper gloves and dig deeper. The rod didn't work because its some kind of rock material

Do you know how I can prevent a future blockage from happening? It is some kind of silt, I can grab some if I scrape the bottom of the pipe. It may be something entering from the hole in the back garden I really don't know. Cheers.

Ian H · 11 Jan 2021

It’s probably years worth of build up in the trap. Clean it out then have a look in next year.

Hugh Jaleak · 12 Jan 2021

DO NOT put your hands in there. even with gloves. You dont know what may be lurking and you certainly dont want to risk a laceration, its bacteria city down there. Use the screw attachment on the end of the rods, keep turning it clockwise to avoid it coming undone and bore into the material to loosed it up. If there's a reasonable flow of water, (tap running) coming through the chamber, sooner or later it should loosed up enough to start washing away.

If you're still struggling and the silt is loose, I'd recommend buying a cheap wet vac, (Wickes do a good one for about £40), and use that. (You then have it for other jobs too!) Use that to remove the material as you loosen it. (Turn water off or vac will fill very quickly!)

As Ian has said, it is years worth of build up, doubtful there is much you can do to avoid it, other than check the drain at intervals. May have taken 20 years to block like that though.

raphael49 · 15 Jan 2021

Thanks I've unblocked it now. I used a hinge scraper attachment to a drain rod and pulled out some kind of rock. I suspect it got in thru the back garden. I think this means I need to put a grill or mesh of some kind over it to stop large objects entering the sewer pipe. I discovered the black silt is coming from the paint that is peeling off sewer pipe, not sure if I can even stop this.

thank you both for your help

Hugh Jaleak · 16 Jan 2021

Good you've got it cleared. Not sure about the black paint, may have been some sort of sealer used in the past. If there's a decent flow through the system, silt shouldn't build up too quickly anyway, the water will carry it through.

Where is it you think objects might be entering the drains? Put up a pic is unsure of what it is, you may be able to block it off permanently if its an old vent or something. You really dont want any openings into the drainage system, apart from causing odour issues, if rats find it, they will use it as a front door.

Nige F · 16 Jan 2021

I reckon the "paint " is simply decades of crud from the high level of water when blocked. The rock could have been a lump of render from inside the manhole.

raphael49 · 17 Jan 2021

Nige F said:
I reckon the "paint " is simply decades of crud from the high level of water when blocked. The rock could have been a lump of render from inside the manhole.

I think you are right!

Hugh Jaleak said:
Good you've got it cleared. Not sure about the black paint, may have been some sort of sealer used in the past. If there's a decent flow through the system, silt shouldn't build up too quickly anyway, the water will carry it through.

Where is it you think objects might be entering the drains? Put up a pic is unsure of what it is, you may be able to block it off permanently if its an old vent or something. You really dont want any openings into the drainage system, apart from causing odour issues, if rats find it, they will use it as a front door.

Here is the rear garden drain cover. It's fed by the kitchen sink and dishwasher drain. The gully has a hole cut into it, as if the pipe was longer, which I don't think is necessary. Should this pipe go onto the train through the gully grid, or be left over the top to catch food, etc?

Hugh Jaleak · 17 Jan 2021

Ideally the pipe ideally needs extending slightly, (a 40mm coupling may be sufficient to give the required distance), to discharge below the grid, then a new grid fitting. Cutting just enough out to accommodate the pipe. Unlikely any 'lump' would have got into the system beyond via that point though.

Solvent Weld Straight Coupling 40mm Grey (toolstation.com)

160mm x 160mm Square Gully Grid Black (toolstation.com)

raphael49 · 18 Jan 2021

Hugh Jaleak said:
Ideally the pipe ideally needs extending slightly, (a 40mm coupling may be sufficient to give the required distance), to discharge below the grid, then a new grid fitting. Cutting just enough out to accommodate the pipe. Unlikely any 'lump' would have got into the system beyond via that point though.

Solvent Weld Straight Coupling 40mm Grey (toolstation.com)

160mm x 160mm Square Gully Grid Black (toolstation.com)

Thanks I will do this.

Unfortunately the trap is blocked again. There is definitely no rock, crud or render there, I just scraped out what seemed to mainly be human waste. I do not know what to do now, aside from somehow removing it and replacing it with a straight pipe. But this seems like something a DIYer wouldn't do by themselves. The bend seems too narrow. Perhaps it's exacerbated by the lockdown and the whole family being over, plus an additional bathroom/WC has been built in the garage (three in total in the house, plus possibly a fourth if we do the loft). I think it takes several flushes for the excremement to reach the trap from the WC in the garage (do not know if this is normal)

What do you think is the best course of action, in the short and long term?

raphael49 · 18 Jan 2021

I've just had a thought, I think the waste is getting pushed along the horizontal pipe under the house bit by bit. It's not all flowing to the trap after each flush. Perhaps if I adjusted the cistern I could force the cistern to empty entirely to encourage flow

Ian H · 18 Jan 2021

If you shine a torch down there does it all look clear beyond the trap?

dal5band · 18 Jan 2021

We had one of these traps cleared by a drain man years ago - he used a double ladel thing to grab stones and bits to get them out.

raphael49 · 18 Jan 2021

Ian H said:
If you shine a torch down there does it all look clear beyond the trap?
View attachment 219068

I am pretty sure it is clear, since I used this hole to push all the bunged up water through this morning, but I will check.

I am thinking of shoving my 2kw pressure washer nozzle into the blocked trap and hoping for the best.

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Stagnant water in sewer pipe, water draining slowly due to silt deposits? (pics included)

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