Pumping Down Hill
Pumping Down Hill
(OP)
I'm designing a small pavilion site with a parking lot and bathroom in a remote part of my town. The site is located on top of a hill. At the bottom of the hill there is a stream and on the other side of the stream is the sewer manhole I will be tying into. I was planning on using a grinder pump at the pavilion to pump the sewage downhill, under the stream and back up a few feet to tie into the manhole.
I have designed pump stations before but they were always at a low point and pumped against gravity. What considerations do I need to consider when pumping downhill? The distance from the pavilion to the stream is approx 1300LF with an elevation difference of 80'. Attached is a rough sketch of the project.
Over the past few years I have done limited utility work and am feeling a bit rusty on this one. Thanks in advanvce for any words of wisdom.
I have designed pump stations before but they were always at a low point and pumped against gravity. What considerations do I need to consider when pumping downhill? The distance from the pavilion to the stream is approx 1300LF with an elevation difference of 80'. Attached is a rough sketch of the project.
Over the past few years I have done limited utility work and am feeling a bit rusty on this one. Thanks in advanvce for any words of wisdom.





RE: Pumping Down Hill
RE: Pumping Down Hill
RE: Pumping Down Hill
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Nice picture, but no elevations and no profile drawing, so its hard to say what's possible or not from here. Is an inverted siphon impossible? Why?
RE: Pumping Down Hill
Pump will be small considering you have essentially no suction head and a negative gravity head. Essentially your only head loss will be pipe and fittings.
RE: Pumping Down Hill
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Pumping Down Hill
Ron. Good point. 2" is pretty damned small for anything with solids in it.
RE: Pumping Down Hill
There have been numerous posters on this forum inquiring about problems with force main. If the velocity is inadequate to push the air downstream, you will have trapped air in the force main. The trapped air will limit the capacity.
You need a velocity at least 4 ft/sec to force air down the force main. Since you are pumping downhill, the air will be attempting to flow backward.
Here are a couple of examples:
thread161-237329: sewer forced main problems: sewer forced main problems
http://www.eng-tips.com/viewthread.cfm?qid=270920
RE: Pumping Down Hill
I'd go gravity to avoid the future maintenance issues, personally.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Pumping Down Hill
RE: Pumping Down Hill
It seems that you will have to pump it under the stream whether you have a gravity system or not. So the choice would be between putting a lift station at the bottom of the hill or the top. It would be easier to service the pump if it was installed at the building.
This looks like a good application for one of the low pressure grinder pumps systems. These systems use a 2" discharge force main.
RE: Pumping Down Hill
I'm thinking the gravity to a mini pumpstation would be best. Do any of you have any suggestions for a mini-pumpstation?
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RE: Pumping Down Hill
Depends on the invert of the receiving junction box, and whether an aerial stream crossing is allowed.
Again, there's no topo on the attached file, and there's no way to tell if a gravity sewer will work without a profile of the proposed alignment. He might be stuck with a force main no matter what.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Pumping Down Hill
Do you really think that someone would construct an aerial sewer crossing for a 2" force main considering that the crossing will be located adjacent to a park?
RE: Pumping Down Hill
Of course I don't. If you're going to pump the whole way anyway, then there's no point in doing an aerial crossing. Go ahead and bury the thing.
In fact, if *I* were going with a "2 inch force main," I certainly wouldn't jack and bore under the road to catch that manhole, I'd turn under the road through the culvert and out the other side, bolting it to the underside of the culvert. Then run along the top face of the culvert and follow the northern right of way the rest of the stretch. Easier maintenance, much cheaper to construct, etc.
But that presumes I was going to go with a "2 inch force main" at all. My recommendation was and is to figure out what the grades are like, hoping that a (larger than 2 inch) gravity system is an option. Then make a more informed decision at the pricing phase, by pricing both options out and comparing life cycle costs for your client. You wouldn't need detailed topo, just maybe 3 spots and a site visit.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Pumping Down Hill
RE: Pumping Down Hill
The situation you describe concerning pumping downhill is a typical situation is variable grade pressure sewers. There are a number of manufacturers who provide prefabricated pump stations that handle this situation. E-one is one of the most widely known.
There has been a lot of discussion concerning the diameter of the force main. The 2" size is more than adequate when using a a small grinder pump. Using a 4" line will require larger pumps to achieve a flow rate with the appropriate scour velocity. This type of pumping arrangement is very inefficient for the size of your facility and will be an energy hog, be expensive to install because of the large pumps and its associated hardware, large wet well requirements, etc.
Before you make up your mind about the pumps, check out the E-one website, Orenco, or, the Small flows clearing house to get further information about variable grade pressure sewers. Your first instincts about grinder pump and small diameter force main seem to be right on target.
RE: Pumping Down Hill
Install at the pavilion a watertight 1,500 gallon 2-compartment septic tank with a filtered pump vault and 1/2 HP, 10 GPM pump (make sure to include an anti-siphon valve). Pump the filtered effluent via a 1-1/2" force main to your gravity manhole.
If you can convince the governing bodies that it would work, you could even omit the pump and just run a straight STEG variable grade sewer. There are effluent filters that offer 1/16" filtration and drawing from the clear zone of properly sized septic tank would prevent clogging issues. I would install a cleanout at the belly to allow for rodding or pressure cleaning of the sewer for peace of mind though.
If you have problems convincing a governing body that a STEG system would work in the this situation, you can let them know that it is a simple matter to convert a STEG system to a STEP system if problems arise. Just pull the effluent filter and drop in an filtered pump vault and controls.
Orenco Systems Inc. is a great source of reliable, well designed equipment for these sorts of projects.
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