Grossly oversized force main

[pjohnson]

Our firm is interviewing for a contract with a local municipality that is looking for a "review" of their wastewater collection system. There have been several operational problems identified, but the main problems are due to poor engineering. It seems that the system built (1996) with very optomistic growth projections. There is approximatly eight miles of eight inch diameter force mains installed.

While I don't have much data at this point, an off the cuff estimate suggests that four inch diameter force mains would be sufficient for current flowrates, as well as reasonable growth projections. Their main complaint is odor in downstream lift stations.

The operators have tried injecting air into force mains to try to inhibit anerobic conditions, without much success, if any.

Does anyone have any suggestions as we prepare to interview for the contract to review the system and make reccomendations for improvements? How would you suggest that they deal with these oversize force mains, forever?

thanks,
pj

 

[RWF7437]

One suggestion.

You might review the level sensor settings at the lift stations. You may want to set the "on" and "off" switches closer together to make the pump cycles shorter and more frequent. Remember that domestic wastewater goes septic in two to four hours. Late at night flows may be very near zero and remain that way for several hours. If so, it might even be necessary to "augment" the flows during those hours to flush out the wetwells.

Good luck.

 

[semo]

Look at this thread or other searches.

http://www.eng-tips.com/gviewthread.cfm/pid/161/qid/89274

 

[Maury]

It sounds as if your client may have to bite the bullet and install another force main, sized correctly.

 

[JRW261]

8 miles of one forced main.. or is this several lines of shorter lengths?

If its an 8mile line....

What is the range of depths of the line in several middle sections?

What is the access like.... shear cliff.. residential.. typical woods?

Reason I ask, have it relined. If its one long line, it will need to be done in sections.... as it would be hard to steam/boil 8miles of line, it probably could be done, though very time consuming and risky.

One shot would also limit your lining choices as getting a coil of 45,000 ft of liner would be insane. CIPP could probably be done but man, thats a long inversion let alone ALOT of liner to haul to the site in one piece. Dont get me wrong, it can be done.

Several smaller shots would shorten your pumpstation down time, mean while we could haul out any flows encountered.
One long shot would require alot of down time, weeks possibly and the need for constant hauling of flow and constant monitoring.

What area is this work in need of being done, if your within the area of where we travel, maybe you would like to hear from our sales personell

 

[semo]

Be careful in reducing the pipeline size. Make sure to do your calcs and see if you have a pump that will handle the job.

100 gpm through 8 miles of 4-inch PVC pipe has a headloss of 230 feet. Depending on your route (elevations) you might be hard pressed to find a pump to handle the high heads. I've done systems where I had to oversize the pump flow and forcemain size to get a pump that will handle the heads and pass a 3-inch solid.

Just be careful and make sure of your hydraulics.

 

[BRIS]

changing the pump start and stop times does not improve anything - the retention time in the force main remains the same.

2) You can inject chemicals such as Nutriox to increase the retention time before the sewage goes septic.

3) Unless you reduce the pipe diameter your only feasible solution will be to flush daily. You will need a source of raw water to flush through the main. You will also need valving to dispose of the raw water - and may need to treat it before disposal.

I guess your answer is to reduce the pipe diameter or al least parts of it depending on access.

 

[BRIS]

PS Nutriox is a brand name of hydro

http://www.hydro.com

 

[BRIS]

http://www.water-technology.net/contractors/water_chem/hydro/

Another solution to consider is installing a HDPE liner within your existing pipe. There is plenty of technology around for no dig relining.

http://www.advantica.biz/hardware_licensing/swagelining/licensed_installers.htm