Maximum grade change in wastewater gravity line

[swazimatt]

I have at some stage read something about the effects of having a large grade change on gravity sewers, especially from steep to flat. Resulting in turbulance and possible H2S release. I am in a position to design a 300mm dia gravity main in a planned development. The development will take a long time to be fully operational and this pipe will have two pumpstations discharge into it eventually. In the meantime i would like to design and issues out of the system

I unfortunately cannot remember where i read it and was hoping someone could direct me to a source. I remember a previous job where i had to spread the grade change out over a few manholes where the terrain was transitioning from steep to flat and i believe it worked out quite well in the end

 

[bimr]

If a sewer enters a manhole at an elevation considerable higher than the outgoing pipe, it is generally not satisfactory to let the stream merely pour into the manhole because the turbulence associated with the drops or falls will release hydrogen sulfide from the stream resulting in odor and corrosion problems. Drop manholes are usually provided in this case.

 

[JedClampett]

Look into Vortex Inserts. We used them to reduce odors in a sewer line down a hilly street.
I don't know why drop manholes weren't used.

 

[bimr]

"When a smaller sewer joins a large one, the invert of the larger sewer should be lowered sufficiently to maintain the same energy gradient. An approximate method for securing these results is to place the 0.8 depth point of both sewers at the same elevation."

Link

 

[swazimatt]

Hi All
the issue is not a drop in height it is a big change in grade. The terrain is reasonably steep (8%) but the gravity main does run along the contours at some places. To keep pipes reasonably shallow we are following the terrain grade as much as possible on the sloped portions resulting in grades of about 8% but where we transition to minimum grade either to go under a little hump or turn to the "contoured" roads there will be a big change in grade which will result in a hydraulic jump that will release H2S.

My question is, what is the maximum negative change in grade (steep to flat) that is recommended? I suspect 8% to 0.6% is too much and i want to spread it across a few manholes (eg 8 - 5 - 3- 1) but will need to justify it to the client as the pi[pe depths will obviously increase to acheive this. I have read the recommended allowable change somewhere but for the life of me cannot find it now and was hoping someone would know and be able to point me in the right direction

 

[johnpery]

For 4-inch PVC piping and a building sewer less than 50 feet long, the minimum slope is 1 inch in 8 feet, or 1/8-inch per foot, and the maximum is 1/4-inch per foot. For sewers



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