Venting of a Force Main Discharge Manhole 1

[MarshfieldTimC]

I am looking for help with venting of a forcemain discharge manhole which is located in a residential neighborhood, so nuisance odors are a big concern. The forcemain enters the existing manhole approximately 18" above the gravity sanitary sewer (outfall). The manhole is presently within the pavement of a street intersection, so venting will not be typical. I am considering installing a remote vent, with the riser located approximately 25' from the manhole, but have not done this previously. Also, tips regarding proper sizing of vent pipes would be appreciated.
Thanks,

 

[bimr]

It is undesirable to ventilate objectionable odors through untrapped connections because the odors escape to the street surface. Normally, vent stacks and unobstructed outlets provide all the ventilation necessary in sanitary sewers.

If you have objectionable odors, consider the use of an odor control system. Pull the gases from the sewer with a fan, and exhaust through an odor control system.

To size a vent, convert the force main flow to cubic feet per minute. Then size the vent for 750 feet/min velocity. That is a guideline from a plumbing handbook for sizing sump vents.

 

[alexcmmi]

Odors from FM discharge MHs are a tough problem in residential neigborhoods. An odor contol system would be great, but in most cases are usually not practical. You would have to set up another u/g vault near the dishcarge location to house such a system, and usually this is either not an option due to site constraints, power / O&M requirements, aesthetics, etc.

If you wanted to use a remote vent, you would need to use an in-line vacuum or blower in the piping. Again, power is requried, blower would be noisy, etc....Active odor control systems are usually a PIA in a residential area.

The best solution is to properly size the pump station and force main so that you have enough turnover in the FM so that it doesnt go septic. THAT IS IDEAL. Of course, its not always easy. You have your diurnal peaks, you might have phased construction w. only a few units coming on-line at a time, you might be in a vacation area where flows vary widely, etc...all those factors contribute to pump and FM sizing.

I have found ONE manufacturer who makes a passive MH odor control system, although I have never seen an installion and I have no idea how well they work. Its called a MH scrubber. Not sure how well it works, but would love to know. Let us know if you try it:

http://www.purafil.com/Products/purafil-water-wastewater-air-filters.shtml

Mole MH Scrubber...its a few items down on that page

 

[rcooper]

I've seen drawings of a solution. I don't know how well it worked.

The problem causing manhole is made air tight, including an air tight manhole cover. A second chamber is connected to the manhole and a pipe is laid from the base of the second chamber to the air void in the manhole. The pipe slopes from the second chamber to the manhole.

A GRP mesh flooring is fitted to the second chamber above the pipe. A further fine perforated layer may be needed above the GRP flooring. All fasteners and fixings are stainless steel.

Granular activated carbon is placed on the GRP flooring, depth and type advised by the supplier. The cover on the second chamber is ventilated.

There is no forced ventilation, but any air leaving the manhole has to pass through the activated carbon which will go some way towards removing the odour.