Smaller sewer mains connecting to Main trunk line below the HGL of the Main Trunk Line. 1

[CivilG]

To whom it may concern,

Is there ever a problem with a smaller sewer gravity main dumping into a larger sewer trunk line below the HGL of the larger sewer trunk main. In other words, if the smaller sewer main is surcharged above its pipe crown from flow from the main trunk line will there be problems with flow coming out of the smaller sewer line. Assume all sewer connections and laterals up stream on the smaller sewer main are high enough. I am only concerned with the connection at the main trunk line.

Thanks,
Civil G.

 

[dcornelius]

There should be a manhole at the connection to make maintenance easier on the small line. There will be issues with the small pipe coming in below the level of flow in the trunk line.

 

[jgailla]

I would probably put a lift station in. Sanitary surcharging is not a good idea.

 

[GeoEnvGuy]

Sh!t flows downhill possible problems to consider

The main line will deposit into the smaller line and block it.
The small line will have stagnant water at the connection and not have enough velocity to move sh!t into the main line.
The small line will have water back up into the small line until enough head is available to clear the blockage at the end. This can work for a while but not the long term.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

 

[bimr]

One should modify the design so that situation does not occur.

 

[nackra]

some districts that I have done work for require match crowns to reduce surcharge issues in the smaller lines

 

[RSD90]

Over a period of time the sludge/settle-able matter shall settle and solidify due to surcharge. This will result in septic conditions and shall also reduce the pipe opening which will result in choking of connecting sewer. Though all other connections are high enough than the main line but if the connecting sewer gets choked, the water would then revert back to the connections. Hence the system would not be a sustainable one and shall keep asking for regular maintenance.

Matching crowns of both the pipes and keeping the HGL of small sewer connection above the HGL of the main sewer are perfect solutions to it.





Sarvesh Rahate
Water & Wastewater Engineer