Force Main Design - Down Hill Then Uphill

[Arcadian6]

I am designing a 400-ft long, 2-inch PVC sewer focemain for a single family home in Massachusetts. Due to wetland setback constrainsts, pump chamber is situated such that I am pumping downhill then uphill to my leaching area. There will always be effluent in the line. Concerned about freezing and odors should residents leave for an extended period. Any thoughts?

 

[bimr]

I would recommend that if you bury the force main 4-5 feet deep and keep the velocity above 4 ft/sec that you should not have any problems with it.

 

[rconner]

You may wish to check all your governmental and regulatory requirements (and/or get appropriate approvals for what you are wanting to do here, particularly with regard to sizing if this is not a grinder pump effluent, and also that it sounds like this new sewer "forcemain" is apparently to run under some sort of "Wetlands") [While it may not be the or the only governing body or document, I have noticed the verbiage on page 57 at

http://www.mass.gov/dep/water/laws/smwwtpgl.pdf).

]

 

[Spartan5]

I am designing a 400-ft long, 2-inch PVC sewer focemain for a single family home in Massachusetts. Due to wetland setback constrainsts, pump chamber is situated such that I am pumping downhill then uphill to my leaching area. There will always be effluent in the line. Concerned about freezing and odors should residents leave for an extended period. Any thoughts?

Regarding freezing, bury the pipe 4-5' deep as suggested by bimr.

Regarding odor, there should be no problems. In any event, a standard installation is vented through the vent stack on the building the system is serving.

I do not know the size of your pump chamber, but your leach field will function better and last longer with smaller doses of wastewater spaced out over the course of the day. It will take a flow of over 40 GPM to hit 4 ft/sec in a 2" main. That would be a one or two daily slugs to your leach field over the course of 5 minutes. I would suggest an 1-1/2" force main, and a target velocity of 2 ft/sec (provided you are pumping filtered septic tank effluent [there's no good reason not to]). Your flow will be more reasonable at 12+ GPM and that will allow you to dose the leach field several times over the course of the day.

You may wish to check all your governmental and regulatory requirements (and/or get appropriate approvals for what you are wanting to do here, particularly with regard to sizing if this is not a grinder pump effluent, and also that it sounds like this new sewer "forcemain" is apparently to run under some sort of "Wetlands") [While it may not be the or the only governing body or document, I have noticed the verbiage on page 57 at

http://www.mass.gov/dep/water/laws/smwwtpgl.pdf).

]

The suggestion to check governmental and regulatory requirements is a good one. But that document page you cited pertains to the pumping of raw sewage in systems ranging in size from 10,000 - 150,000 GPD.

In any case, don't let inappropriate regulatory requirements supersede your own sound engineering judgement. Where I practice, this regulation is done on a local level and it is often very poorly thought out. I'd fight tooth and nail if someone was requiring me to put in a 4" force main in this instance.

 

[rconner]

I apologize for providing a link to a reference that has upfront a facility flow range that is likely higher than that required by the OP. Let me provide another reference at

http://140.194.76.129/publications/eng-manuals/em1110-3-173/c-5.pdf

that does not mention a daily flow limit, or e.g. Ten States Standards that I believe simply say,

"The minimum force main diameter for raw wastewater shall not be less than 4 inches."

While I believe I understand some fairly good arguments for very small forcemain pipes, I'm not sure I could dictate when any particular regulatory requirements are necessarily "inappropiate", in the face of all circumstances with which regulators and others have historically had to deal (I guess I would first ask the reason for the requirements, if I can get one from the respondent).

This being said, while I may not know all the reasons nor applicable guidance documents for for this requirement, the image of a grit accumulation, a wash rag, diaper, Maxi-Pad, or any other unmentionables lodged in a very small diameter pipe deep under the middle of a wetlands (that might be hard to deal with remotely from huncreds of feet away in such very small pipe) is not an attractive one to me. It may not really make a difference how same got deposited or fallen in a house drain or commode, whether it was by accident from the home owner or his children, or simply dropped there by an inconsiderate guestor baby-sitter who couldn't find some paper! For this reason, it just seems that if one insists on very small pipe this may sort of cry out for at least a grinder pump or some other special system that maybe can somewhat reasonably deal with same.

 

[Spartan5]

rconner:
The Ten States Standards apply to the "more conventional municipal wastewater collection and treatment systems." They specifically exclude "innovative approaches to collection and treatment, particularly for the very small municipal systems."

Perhaps either you or I misunderstand the OP's situation. Given that they have mentioned a single family home and a "leaching area" I am operating under the assumption that the force main is conveying septic tank effluent to a drain field or other absorption system. It is uncommon for a wastewater treatment system serving a single family residence to pump raw wastewater any distance. There is almost always a septic tank for primary treatment prior to pumping. That takes care of grit accumulation, wash rags, diapers, Maxi-Pads, and any other unmentionables.

Not that you are doing it in this instance, but attempting to apply portions of the Ten States Standards (or other similar documents) to the design of wastewater treatment systems for single family homes is the sort of "inappropriate" regulatory shenanigans of which I spoke. Usually it is smaller, local bodies of government with little if any engineering background.

 

[rconner]

Thanks, Spartan for your further and patient information/clarifications (When OP used verbiage “sanitary sewer forcemain”, for some reason a vision of an essentially clear water effluent (or sort of outfall) didn’t immediately pop into my head! I guess if the very small, relatively long pipeline under the wetlands holds up, and does not become blocked, you are probably right in your “assumption”).