Flat sewer regarded to steep sewer and upsized

[kolkata321]

Hi All,

Please can you comment on the following design solution-

A small village serviced by an exisiting 225mm sewer which drains to a wet well of the inlet of the treatment works. The final 600m section of the main is laid very flat~ 1 in 450 slope. Multiple incident of sewage flooding occured in some private properties due to backing up of sewage. The properties are located 600m upstream of the well. These properties drains to the flat section of the sewer.

Based on spreadsheet calculations and surveys (CCTV, Manhole, Flow monitor), it was confirmed that the existing 225mm mm sewer at the downstream section of flooding properties was undersized resulting in surcharging during storm condition. As a result the sewage backs up from the main sewer into the existing 150mm lateral sewer serving the properties and causing flooding. It was concluded that the main sewer was laid at a slack gradient ~1 in 450; as a result the sewer was susceptible to frequent deposition of debris and grease in dry condition which exacerbated the flooding situation. The existing sewer was laid relatively flat and hence did not comply with engineering standards requirements of 0.75m/s self cleaning velocity.

Due to the hydraulic incapacity problems of the existing system and because of operational problems due blockages in the existing sewers, it was recommended re-grading 600 m length of the main sewer and upsizing from 225mm to 300mm up to the wet well.

The sewer at the wet well was lower 1.72 meters, provided just 0.5m above the pump start level. There is a overflow in the wet well at a high level. In its original condition the exisiting sewer from the village is slightly higher than the strom overflow. WIth the change in the design the new sewer is about 1.75m below the overflwo pipe.

My concern is why the exisiting sewer was laid at such a flat gradient-I can not say that they were lazy! must be reasons behind this.

By regrading the sewer and upsizing the sewer, we are inviting more flow and at a faster rate. During storm conditions when the inflow is greater than the pumped flow, there will be more spill from the overflow unless adequate storage has been provided in the 300mm sewer.

Members, please advice in anyway can be justify the above design. Any thoughts about the above design.

Also, can anyone advice any spreadsheet to calculate storage volume of sewer considering the gradient of the sewer.

Regard

 

[beej67]

Changing the pipe slope and size doesn't change the flow, just the velocity and the driving head. Continuity relationships /etc.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

the original designer did not do a lifecycle cost analysis. they saved capital cost by installing the sewer in a shallow trench. they ignored maintenance cost for cleaning and flushing and flooding cleanup. suggest you disconnect the storm sewer from the sanitary sewer

 

[BenJohnson]

It is anyone's guess why they laid the original inflow line so flat. Could the inflow sewer been constructed before the pump station, and they put in a sewer line that would gravity flow into the treatment plant? Anyway, it sounds like you have that problem solved.

Your problem now is the pump station. The pump station must be sized to handle maximum inflows. The wet well is provided to hold sufficient influent during normal flows so that the pump(s) do not rapidly cycle on and off. Neither the wet well or the upstream sewer pipe should be used as a method for storing storm flows until the pump can "catch up" with the heavy flows. Sewage that just sits in a pipe for even a short time will deposit solids that normal flows may not scour clean. Hence, more maintenance work.

Your wet well and sewer pipes should be used to transport waste water to the plant - not act as storage detention basins.

Your task is to increase the pump station capacity so it can handle storm inflows. This will require a top to bottom analysis. Maybe you need two pumps with second pump set to turn on when water in the wet well reaches a higher level than the first pump can handle. Then you must look to see if the pipe from the discharge side of the pumps can handle the extra flows. Then a hundred other details must be checked, like can the existing electrical capacity to run two pumps?

It sounds like you might benefit from a source control study of an old sewer system. Why are you getting such high inflows during storms? Manholes that leak? Cracked sewer pipe? Or residents who connect their outside drains into the sanitary sewer. A thorough CCTV of your upstream sewer collection system could reveal a lot of problems with storm flows entering the sanitary system. I suspect your treatment plant would be a lot more stable with fewer storm flows in the waste stream.

 

[bimr]

[ul]
[li]Sometimes sewers are constructed at less than optimum grades in order to achieve greater horizontal reach with the sewer. You can install a flat sewer a longer distance than a sloped sewer. A flat sewer also avoids the capital cost of an intermediate lift station. The downside of this approach as you point out, is that with flat sewers and low flows, maintenance costs are increased as the sewers will need more frequent cleaning. Problems with hydrogen sulfide may also occur with flat sewers.[/li]
[li]Sewers are not usually used for storage. As beej67 suggests changing the pipe slope and size doesn't change the flow, just the velocity and the driving head.[/li]
[li]The volume of the sewer is simply the area times the length. The slope of the sewer will effect the volume slightly but is inconsequential.[/li]
[li]It is recommended that you develop population estimates in order to properly size the sewer. This will be required to permit the sewer.[/li]
[li]Have you considered the use of a lift station and force main instead of the gravity sewer?[/li]
[li]The overflow at the wet well seems to be a Sanitary Sewer Overflow (SSO) which is probably illegal. It will be difficult to permit a SSO as most environmental agencies are working to eliminate SSO's. You need to remove the stormwater or increase the capacity of the wet well pumps to eliminate the SSO.[/li]
[/ul]

 

[dicksewerrat]

Could you put some type of flow valve on the pipe at the wet well? Red valve comes to mind.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[rconner]

What pipe material is involved?

 

[Hoaokapohaku]

Maybe this question is a little obvious, but was the sewer originally designed to take the storm water? Or, is the additional flow caused by storm water intrusion into the sanitary system that was not accounted for in the design? If so, the problem might be solved by finding and stopping the source(s) storm water intrusion.

This actually sounds like a construction problem to me. A little over 20 years ago, I was responsible for oversight on a couple projects with similar size sewers at the same and even slightly less than slope than the slope on the subject sewer (as little as 0.16%). On those projects, the choice was either very small slopes, or pump stations, which the owner wanted to avoid. For a sewer to work properly at that kind of slope, the installation must be virtually PERFECT, meaning that the pipe invert elevation at every point must be exactly correct with NO measurable variance at any point along the pipe length. Similarly, the pipe horizontal alignment must be dead straight. This also means that the trench must be properly graded to allow consistent thickness of bedding, and the bedding must be properly compacted at every location to ensure there is never any settlement of the pipe. This is especially important if the type of pipe used is flexible, such as PVC. Extreme care must be taken to ensure that the perfectly laid pipe doesn't move during backfill and compaction. The pipe cushion on the sides of the pipe must be properly compacted as well. This is often a problem because the contractor does not excavate the trench wide enough to allow room for proper compaction equipment, or corners are cut on compaction. If the material on the sides of the pipe are not properly compacted, subsequent cushion and backfill will deform or even partially break and crush the pipe. Any construction defect creates a place that can trap debris and cause problems such as you describe. I remember at the time thinking I don't know how this could be accomplished with "old school" methods. Using trench and pipe lasers, it really wasn't very difficult, but it does take good methodology and meticulous attention to detail. Some contractors are very good at this kind of construction, usually ones that specialize in this kind of work. Those are the exceptions; most other contractors aren't used to working with the close tolerances required and may not take the required amount of care to produce an acceptable result.

I've paid attention to those sewers over the years because I doubted at the time if those design slopes would be adequate. They have performed problem free.

 

[bimr]

The minimum practical slope for construction is generally considered to be 0.08%