Existing Sewer Network Model Query 1

[swazimatt]

We are looking at doing a development that will pump wastewater into an existing gravity network. We need to pump 12 l/s and the council has employed an international consultancy to carry out an assessment as they hold the model for the existing network.
They ran the model and informed us that we can discharge into MH 1 and would then have to upgrade the pipes from MH 1 to 7 to 300mm dia to avoid surcharge downstream and pipes being under 80% capacity. The pipes from MH7 onwards are already 300mm dia. I have proposed that instead of upgrading the pipes (about 500m) we extend our rising main and discharge into MH7 directly, the model has been re-run and we have been informed that this doesn't work as it is resulting in surcharging and pipes running full downstream of MH7.

I have queried this as it doesn't make sense. If i add 12l/s to MH 1 or 12 l/s to MH 7 it should have the same result downstream (the 12 l/s has been run as a continuous discharge for the peak wet weather flow scenario)

Am I missing something? I have done a lot of gravity network design and it doesn't make sense to me
Based on the quality of the reports from the consultant I suspect they have made an error. I have requested they update their report to explain why this happens.
The only thing i can think of is that they have allowed additional infiltration for the old pipe sections and assumed that when replaced there is no infiltration which is an odd assumption to make (all pipes get old eventually) and for the length of pipe should not be significant

 

[swazimatt]

Well that sucked! Sometimes you shouldn't scratch an itch!
Turns out when they looked at the initial scenario of pumping to MH1 and upgrading downstream they did not notice that this old network goes to 300mm dia pipes after MH7 and then at about MH 20 it reduces to a 225mm dia. When i queried it hoping the outcome would be that i could pump to MH7 they realised (only now! ... one of the world's larges consulting firms!!) that something wasn't right and have now updated the report and 500m of pipe upgrades has changed to two sections of 500m!

 

[TigerGuy]

How dare you ask questions for clarification

I always understood that downstream pipes are NEVER to be smaller diameter than upstream pipes.

 

[swazimatt]

yep, not sure how that happened. maybe the council had to replace some earthenware pipes and decided to upgrade them

 

[fel3]

TigerGuy…

You understand correctly. Unfortunately, "things happen." :-(

About seven years ago, I designed a small sewage lift station to bypass a problem area in the gravity sewer system of a small, nearby city. The problem was caused by a 300-foot reach of 6" sewer pipe that had been installed downstream of an 8" sewer pipe. Not only that, but the 6" pipe had been installed at an adverse grade of about 0.0040 so that it could meet the bottom of the downstream manhole. Because the city is very flat, fixing the downstream system to maintain true gravity flow would have required replacing something like a mile of pipe up to 15".

The history of this installation was murky. As best I could tell, it had been in place for at least 20 years. The City had apparently decommissioned an earlier lift station in favor of installing the 6" pipe. There were no drawings, so I suspect that City crews had done the work at the direction of the public works director, who was an urban planner with no background in civil engineering. I think he figured he could save the cost of operating and maintaining the old lift station, but he simply traded that for the cost of maintaining a funky sewer system. There is a reason that location needed a lift station.

Fred

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[TigerGuy]

Yep,

I saw a design that called for a larger pipe (30"?) discharging into a junction box that was then drained by twin 18-inch pipes, or something similar. At least that setup allows sufficient flow, as long as nothing larger than 16 inches comes out the 30-inch pipe into the box.

 

[fel3]

Back in the mid-1990s, I got involved with engineering support during construction for a major expansion project at the local wastewater treatment facility to increase the permitted capacity to 68 mgd. One day I was talking with one of the plant operators and I asked him what types of large items they got coming into the plant. He said (to paraphrase), "just about anything that will fit through a manhole and the larger pipes." This included kids' tricycles, lumber, small kitchen appliances (e.g. toasters), folding chairs, and so on. They also regularly got small bags of narcotics and small bags of money (probably from the same sources).

============
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[bimr]

they realised (only now! ... one of the world's larges consulting firms!!) that something wasn't right and have now updated the report and 500m of pipe upgrades has changed to two sections of 500m!

Just because it is a large engineering firm, don't ass/u/me they know what they are doing.

 

[swazimatt]

I know everyone makes mistakes but if you are tasked with reviewing a report and it says a+b=c but b+a=d you should be experienced enough to ask the question