I am currenty working on a project for a City (City A), to run a force main to an existing industrial park. The project consist of a lift station and and about 9,000 ft of force main, that will run from the industrial park to the sewer plant. This project will also pick up a lift station at the 1/2 way point on this force main. The adjacent City has plans to run a force main, which will carry all their sewage, and connect to the lift station at the 1/2 way point.

I recommended to City A, that they have the adjacent City tie into the force main instead of the lift station to prevent odor problems and to not have the expense of pumping the sewage. The way the hydraulics work out the adjacent City will not need much more of a pump to tie into the force main. The engineer for the adjacent City is against connecting force mains together. I have tied force mains together many times. What would you all do?

You have not presented enough information to develop an opinion.

Allowing two separate pump stations to tie into a common sanitary forcemain may have potential problems.

Forcemains should be designed to have flows from 3.5 to 5 feet per second. The minimum flow is necessary to resuspend solids that settle after the force main stops flowing. The maximum flow is an economical pumping rate.

Centrifugal pumps are not positive displacement machines. The flow from the centrifugal pumps varies with the back pressure that the centrifugal pump is pumping against. When two different mismatched are pumping against each other, the stronger pump will be the one that passes more flow.

There may be an additional issue having to do with peak flows from storm water.

How would you propose to maintain flow rates in the force main when you have independent pump stations turning on and off?

These are some of the issues that need to be addressed.

Thanks Bimr

The pumps will pump flushing velocities when they pump independently. When the pumps are running at the same time they will all pump but not enough to flush the lines. There will be times during the day that each pump will run by itself. This is what is required by the regulatory agency.

The force main from the adjacent city is about 6 miles long so I am worried about odor problems at the lift station.

Are connecting forcemains like this common? I have done this many times before and never had a problem.

There could also be a commercial issue. Perhaps one of the authorities may want to privatise their facilities in the future and combined systems always present a difficulty.

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It is a simple fix to install a nitrate-based chemical injection system to remedy the odor issue.

http://www.webs1.uidaho.edu/ce432/H2S-NEWEA.pdf