Sedimentation in Sanitary Trunk Sewer

Question

Sedimentation in Sanitary Trunk Sewer

swre (Civil/Environmental)

(OP)

22 Oct 02 11:19

I am investigating the operation of an old trunk sanitary sewer system which includes several sections that are flat or at slightly negative slope. There is significant sedimentation in these sections, and operations staff spend a lot of $ cleaning it out anually.

There are a number of ways that I am thinking of looking at the problem. We have a pretty good calibrated model of the system that we could use to determine the maximum sedimentation that could be tolerated from the perspective of capacity reduction. We could also use the model to determine the frequency of scouring velocities. Perhaps is "self-cleaning" frequency is adequate enough if we just leave it alone. However, I would be concerned about hardening of the sediments which could prevent scouring if left for too long.

I'd appreciate any advice on how best to approach the problem, or any references that may be of interest.

Thanks.

Chris73 (Civil/Environmental) · Answer 1 · 2002-10-24 19:35:26

swre;
find out what kind of sediment you are dealing with. Mineral sediments won't bake to a hard cake alone. Neither organic because they will be biolog. degraded after a while. But organic particle can create in conjunction with mineral grains a concrete like compound. The only way to get this stuff out again is milling. That is why I would highly recommend to flush the sewer regularly unless you can certainly proof a recurrent natural flush (unlikely if not a combined sewer). The max sedimentation you mentioned can easily be 100% obstruction - I have seen many of those! You will find a bunch of methods in common literature to prevent or restrict sedimentation (flushing flaps, jets...) Make sure it works otherwise I will have a new project comming up for a sewer rehabilitation!
Cheers!
PS: By the way, if there is hydraulic capacity left and you are dealing with only short 'sacks' within the sewer, there are several rehab-methods you could thinks of to bridge these sections and recreate a consistant grade!

dicksewerrat (Civil/Environmental) · Answer 2 · 2002-11-03 00:06:17

Rehabilitating this line with atrenchless method of Cured-in-Place- Pipe may bridge or flatten the sags. It will also increase your velocty because the new pipe wall will have a much lower Manning's number. But you may still have to flush, jet the line evry now and then. 'Self-cleaning' velocities are helpful but not foolproof.
If there is an unusual amout of sediment, maybe there is a broken line upstream ready to collapse.

gtrainor (Civil/Environmental) · Answer 3 · 2002-12-13 23:18:48

I don't fully know the specifics, but have you looked at intercepting the sediment before the flat spots with a sediment trap manhole where the inlet and outlet are elevated a foot or two. This will allow a low flow condition where a lot of the sediment will drop out and stay in the manhole. You will still have to deal with sediment in the manhole but at least that's easier than in a pipe.

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Sedimentation in Sanitary Trunk Sewer