Low Pressure Sewer with Lower Discharge

[llamallama]

I have been doing site work for awhile, but haven't had to do alot of LPSS work. I was given a project to design with a network of nine closely spaced grinder pumps and about 2,000 LF of force main. The profile has a "hump" in the middle with the discharge manhole at the same elevation or lower than the homes (i.e. the pumps are simply needed to get over the hump). My first thought is that the system will syphon and empty the line each time. Is this correct? The client does not want to install 1000 LF of gravity main. Are there any other options?

To make things simple the profile is Homes @ 500', 1,000 LF of Main to Hump @ 540', 1,000 LF of Main to discharge manhole @ 490'

 

[bimr]

Valves used in wastewater applications must be considered carefully due to the nature of the fluid. The requirement for the valve is to maintain a back pressure in a sewage force main. The valve used in this application is called a sewage back pressure sustaining valve.

The pressures that you have described are not significant. The pipe size is also not large either. For those reasons, I would try to avoid installing such a valve because the installation is marginal and you would have to maintain the valve.

If you decide to use a valve, you can get one from GA Industries or Ross Valve:

http://www.rossvalve.com/PDF/70swrbp.pdf

 

[llamallama]

Thanks,

I think that valve should create a workable solution. I would rather switch to gravity for the downhill leg, but the client is rather adamant about lowering cost.

 

[cvg]

long term O&M cost is generally lower for gravity sewers. And how much more could the gravity sewer possibly cost than the force main?

 

[francesca]

Has the owner considered a STEP system? The sewerage tanks need emptying once a month or so, but the effluent lines are 1" or 2" and low maintenance.

http://www.orenco.com/

 

[redbridge]

Install an air/vacuum valve. This will break a syphon.