remote sewer service application

[EG]

Hi All,

Here is the scenario: I am designing 2,400 LF of gravity sewer for a small on-mountain restaurant for a ski area. I have performed a hydraulic analysis and the flows/velocities, etc. suggest a 4-in diameter pipe is ample. Peak flows (with peaking factor of 4) are only 0.02-cfs (+/-8 gpm), velocities are less than 10 fps, and the max. depth is less than 1/2-inch. There are NO regulatory requirements (local or State) that dictate the pipe size/materials (eg. min. 8-in pvc w/ manholes).

We are tentatively recommending a 4-inch HDPE pipe system with clean-outs every 250-ft or so (based on local jetting limitations) using direct burial with screened material (the one-way trucking trip for imported gravel is over 1.5 hours). The heartburn I have is related to everyone's expectations for a traditional main-manhole system. Does anyone have any experience with using a 4-inch diameter pipe for long stretches for a hard-to-get-to location private application?

All comments are welcome. Thanks!

EG

 

[MiketheEngineer]

Use 6''

 

[EG]

Mike,

Can you please provide some background? Seeing your emphasis is structural (per your name), do you oversize beams regardless of your calcs? That is how I am feeling about this pipe...

EG

 

[cvg]

I went through this recently, client unwilling to pay for manholes or take the risk of clogging due to low flows. In my case the velocity and slope was also fairly low. going to larger pipe does not solve the problem. Problem was in getting sufficient flow to suspend solids all the way to the sewer main and regardless of pipe size, the flushing flows were just not adequate. A small grinder pump with force main (2 inch or smaller) was a recommended alternative. May be ultimately cheaper both for initial and ongoing maintenance.

 

[bimr]

In addtion to cvg's comments, one would think constructability and durability would also be issues.

 

[civilman72]

A few thoughts:

- An 8" pipe will not tend to freeze as quickly as a 4" pipe. If you're at a ski area, this may be something to consider, particularly with your low flows.

- You may find there is not a significant change in the overall increase in costs to go from a 4" to an 8" pipe. In my area, sanitary sewer mains are designed with 7'-9' of cover, and often the pipe material costs are a small part of the overall construction costs (i.e. material/construction specs of bedding/backfill and excavation depths have a bigger impact on overall construction costs).

- Without manholes, it will be more difficult to identify the location of the line to repair breaks/failures.

- Have you considered whether this system can easily be slip-lined (in the future) without manholes?

- I've designed service lines (4") that are a few hundred feet long, but they have cleanouts every 90'-100'. 250' seems to be pushing it.

- The idea of a grinder pump is definitely a feasible option, and may be less expensive if you do not have to bury it as deep (for frost-protection).

 

[77JQX]

I believe the Mt. Ashland ski area in Oregon has a long HDPE sewer line. You may wish to contact them about their experience.

 

[SteveWag]

This application lends itself to a grinder style pump with 1 ½ or 2 inch pipe. If rock is not a problem, use directional drilling with little excavation. We just re-bid a $8,000,000 job that was conventional, changed to small diameter pipe and the bid was under $6,000,000. Hard for me to get used to after 35+ years of gravity sewer experience! Some other jobs we have done involve high class water front homes around a recreation reservoir. Pressure sewer/directional drilling was selected because of low environmental impact.
Steve

 

[EG]

Thanks All. I did consider a grinder system but this will be a shallower install (about 4-ft due to bedrock conditions) I have been hesitant to use a grinder system when we are not completely below frost depths (about 7 or 8-ft in this area) and that is its own thread. I was thinking that clogging will likely be a minimal risk due to having the small restaurant use an external septic tank for a grease trap (vs. internal) to lower temperatures and increase holding capacities. Also, the first couple hundred feet of sewer will have a 6" pipe and manhole for upstream cleaning. For the pipe run itself, the significant head 'opportunities' should help flush any minor buildup...in theory.

Also, I just returned from a meeting with the local sewer maintenance company. They said the main difference between the 4" and 6" pipes are related to the type of equipment and cameras they use for maintenance and inspection. For a 6" pipe, the camera is tractor mounted and they can go about 300-ft (600-ft between manholes in theory) while you have to push the 4" pipe camera by hand and the range is about 100-ft. Also, for flushing/jetting, the equipment is far more mobile for a 4" than a 6" and they could transport the equipment for the 4" via snowmobile (using sleds) and snow cat much easier than the heavier equipment. Access for the 6" camera requires a manhole or enlarged cleanout with wide sweeps...

It seems that if you used the grinder pump system, you would have challenges related to the maintenance and your QC would not include visual, just air leakage testing...no?

77JXQ - Thank you for the connection! That is very helpful.

EG

 

[MiketheEngineer]

Just my opinion - 6'' will accomadate more "crap" then 4'' and make clean outs easier. You bring up some details - like snowmobiles, etc that I know nothing about!! And yes - I do generally size my I-beams one size bigger than required. Like sewers - it is a rather cheap upgrdae and any future changes can usually be handled. If you size things right at max - you will get burned down the road.... What if they build another resturant?? You might easily overload the 4''

 

[civilman72]

So you're looking at a cleanout every 200' or a manhole every 600' (in theory).

I still say you price out an 8" main with 5 manholes compared to a 4" pipe with 14 cleanouts. I think you will find the price increase to be fairly low. And I'll guess that both of these systems will be more expensive than a small force main with a grinder pump.

If there is bedrock at 4' depth, this will be a bigger issue for a gravity system. Unless I'm missing something...

 

[BenJohnson]

I like 77JQX post listed above. Talk to someone who has dealt with a situation like this before. Low flow, low slope, possible freezing, cost effectiveness of manholes. This type of project is prone to problems that are difficult to predict in advance. This is like a very long lateral.

If you don't like that idea, the grinder pump/force main. You might consider a way to introduce fresh water into the wet well, just to keep the solids moving from one end to the other in a day. Water should be plentiful around a ski resort.

 

[MiketheEngineer]

It's called snow and doesn't flow too well

 

[civilman72]

"Water should be plentiful around a ski resort."

Ben-While I agree with adding water to the system to prevent buildup...

Because of water rights, most ski resorts (here in Colorado) are limited in the amount of surface water they can capture and use.

 

[dicksewerrat]

Move the facility to the bottom of the hill? Truck out the effluent? This line will freeze. The pipe even empty will have water vapor traveling up the pipe and it will freeze to the walls. Each time water goes down the pipe it will eat at the frost inside but may not remove it. Then it forms an ice coat. Are they going to have a dishwashing machine? Maybe that will heat the water to 180 degrees. But if they have water restricions, that may not have enough flow either.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com