Foul drain blocking - anyone seen this phenomenon?

[Karlos1544]

Hi all, new poster here and hoping for some advice. We have a manhole that blocks occasionally, so i periodically clear it to prevent that.

When i do i notice a very weird formation of 'waste' and i wonder if anyone has experienced the same. Waste is building up only around only one of the manholes inlets, and i cant figure out why.

I have attached a picture (BE WARNED - it is a picture of a partially blocked manhole, sorry) its the only way i can communicate what I'm seeing.

any help would be greatly appreciated!!!

thanks K

 

[oldbutnotdead]

My suspicion would be insufficient fall on that leg (or maybe even a slight uphill from source to manhole) causing a gradual buildup of solids. The outflow and the other incomers are obviously behaving themselves. Do you know what feeds into that particular leg- infrequently used toilet possibly?

 

[bernardgreen]

It might be that solids / grease / fat entering the chamber from the pipe marked green are being thrown across the chamber and are building up on the other side.

Is the any flow in the channel ( marked in red ) under the accumulated debris ?

 

[Karlos1544]

Thank you for this response, that might make sense. Problem is, i don't know where that leg comes from. We share that manhole and i only know the source of one leg (ours) unfortunately. I will do some further investigation to see where that comes from.

Bottom line is that a freeholder is trying to get us to pay to have this fixed, but the issue is with the plumbing in/from someone else's property, so its their responsibility in my view.

Thanks again for the response.

 

[Karlos1544]

@bernardgreen thanks for your response also. I will check if there is indeed flow in the chamber in red. If not, and stuff is being protected from the inlet you've marked green, perhaps putting something in place to direct it down the outflow would help?

 

[bernardgreen]

We share that manhole and I only know the source of one leg

Shared drains are the responsibility of the local water company ( or sewage service provider ) and they may be prepared to sort this out. ( with maybe a bit of pressure from you )

 

[oldbutnotdead]

Shared drains are the responsibility of the local water company ( or sewage service provider ) and they may be prepared to sort this out. ( with maybe a bit of pressure from you )

Yes that. If the pipe (that keeps getting blocked) coming into the manhole solely serves one dwelling then it is that dwellings' problem to fix. From the manhole onwards is a shared drain therefore (as bernardgreen said) now down to the water board to maintain/repair. Not your problem at all, polite letter to the freeholder explaining this (not your waste causing the blockage, shared manhole=water board) would be a starting point to deflect them

 

[oldbuffer]

I've had exactly that problem. Was caused by waste from first floor coming down soil pipe, entering chamber at right angle, straight across to side wall. All "moisture" forced out on impact, leaving paper etc stuck to wall. I had two solutions:

1. Temporary. Get wife to lift cover and hose it down every 3 months.
2. Permanent. Get man hole redone so that wast comes in at an acute angle heading in the direction of the sewer.

 

[Karlos1544]

you have all been really helpful thank you! the sticking to wall theory makes sense given the shape of the build up in that picture. I will investigate further and thanks for the heads up re the water company

 

[Hugh Jaleak]

Good grief, some of us look at that sort of thing, (and worse) on a daily basis. 'Oldbuffer' has hit the nail on the head, its the discharge from a WC entering the chamber with such velocity, it sticks to the opposite benching. I suspect there is either there is a stack near to that connection or the drain is on too steep a fall.

With traditional chambers, the slipper was often angled more sharply into the direction of flow and benching opposite shaped to prevent this happening. With these preformed plastic chambers, that doesn't happen. If it is an Osma chamber, Wavin do make channel covers to tidy up unused inlets, but I think both connections on that side must not be in use. Part numbers are 4D948 for the left hand, and 4D949 for the right hand.

 

[Ian H]

I agree with the above. Manhole is too close to the soil pipe.

If there was access on the stack and a manhole downstream i’d be tempted to get rid of the manhole altogether.

 

[Karlos1544]

@Hugh Jaleak thanks for the part numbers that could be a game changer! Will check it out

@Ian H yes that would be ideal, but we expect to be in this property approx 4 years so I think I’ll try some other options first.

Thanks all again, this forum is awesome!

 

[Hugh Jaleak]

Two separate discussions on the same subject yesterday, I commented on the other on a Facebook group. Drains used to be laid to a fall that would allow a 'Self Cleansing Velocity', seems to be a term a lot of Groundworkers today have never heard of, let alone put into practice. Falls of 1:40 for 4", 1:60 for 6" drains used to be the rule. Was judged as the optimum gradient, too shallow and the water would seep away, leaving the solids behind, too steep and the water could race off, again leaving the solids behind.

Too many drains laid now with ridiculous falls, went to one the other week, 225mm chamber, about 12" deep, full. Poked with a rod, soon emptied. Found it went into a 450mm chamber about 1.5m away, pipework between the 2 must have been on at least a 45° angle. Then when the detritus collects on the opposite benching and it blocks up, folks are asking why......