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Old 12-17-2009, 04:26 PM   #1
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Foul smell from abandon septic tank in house


Ok, we get a foul smell in the house, mainly two air vents that are both connected to the "far" end of our central air duct work. usually only in the winter. smell like dead mice/sulfur smell, horrible

the backgound. We have a fairly new septic tank and all seems to work well there. There is an abandon septic (old tank) that the covered up under our poured slab for our dining room. It had been working when the covered it, but failed afterwards. So the sewer pipe in the basement part of our older section of house was disconnected and routed to the new septic. the outlet of the new tank ties into the old outlet of the abandoned tank 20' outside the house. there is also a floor drain in the basement that ties into the outlet of the old septic outside the house.

All vent stacks are open and all drains with traps have water in them. Occasionally, mainly in the winter, we get the foul smell coming out 2 HVAC ducts. The worst one runs right over the abandoned septic tank just below the slab there.

After 5 years or so, would there still be a foul smell that would occasionally come out of an abandoned septic? what else could it be. i need some ideas and thoughts on what it might be, what to check, and maybe how to remedy?

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Old 12-17-2009, 05:34 PM   #2
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Foul smell from abandon septic tank in house


Standard procedure when abandoning a septic tank is to pump it dry, wash it down and pump out the wash water, then fill it with sand or grout. If this procedure was followed, it is unlikely that you would get any residual smell from it. If this procedure was not followed, it is possible that the tank remained partially full of sewage, and it could smell for a very long time. If you do not know what procedure was followed to abandon your tank, you are going to need to hire a pumping company to inspect the tank, and determine its current condition.

If the tank is clean and filled, the next logical place to look would be the abandoned sewer pipe, which also should have been filled and capped. If that is not the problem, there may be issues with your septic field. Septic field problems need to be evaluated by a company with expertise in septic inspection, probably not the pumping company.

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Old 12-17-2009, 08:44 PM   #3
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Foul smell from abandon septic tank in house


yep, i know the tank was not pumped or filled with sand or grout. the outlet failed years after it was built over. it was very stupid for it to be built upon, but they did it. when it failed, the inlet was sealed off, but not the outlet, nor was it pump-only access was through the inlet pipe.

so would one recommend trying to pump it through the inlet pipe as good as it could be? would one then recommend trying to excavate to try and find the outlet and plug it? That would be complicated as the dining room slab is poured over the tank and first 10-15 ft of outlet, then there is a concrete patio over the next 15', then some landscaping.

but it is feasible to think that somehow the smell in the tank gets disturbed and penetrates the ductwork which runs just above it?
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Old 12-17-2009, 09:53 PM   #4
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Foul smell from abandon septic tank in house


The abandoned tank under the slab etc sounds like a real problem that will be very hard to resolve at this point in time but I question if that would make your HVAC ducts smell; I would be suspicious of the floor drain you mentioned if it connects to the old septic Does it have a trap? Is this just a drain in the cellar to take away any stray water from the basement floor? If it is I cannot think of any good reason it would go into the septic
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Old 12-18-2009, 10:21 AM   #5
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Foul smell from abandon septic tank in house


yes, i can't think of any practical way to resolve the abandoned tank...

so the basement floor drain is for stray water. there is definitely water in the trap and no odor coming from that. This drain actually bypasses the septic tank, but joins up with the outlet of the tank, thus routing all the effluent in one line away to leach.

i am pretty confident i've ruled out any existing sewer or septic issues. at first we thought it was a dead animal, but as it comes back each year, etc, etc, i am pretty confident it has to do with the abandoned tank. either that, or a horse crawls up in there each year and dies!

Tonight i plan to disconnect the duct that runs over the top of this old septic to isolate it as it is just a single run. Although that does not solve the problem, i should be able to plug the duct then on both ends so no odor comes into the house. if that works, i will be completely sure it is the old septic tank, but still no clue how to remedy it....

Maybe try pumping it by uncapping the inlet pipe, but will be hard to get to each compartment of the tank. Can the sewage be neutralized? guessing it's 1000 - 1500 gallons and full up to the inlet pipe.
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Old 12-18-2009, 02:56 PM   #6
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Foul smell from abandon septic tank in house


I'm not a septic tank expert, other than I have one and it's working just fine now. But maybe a suggestion: Contact one of the company's that clean out septic tanks and describe your problem with them. I'm wondering if there may be some type of enzyme they could add to the old tank to help digest what is left, or maybe add enough water to flush out the remaining offensive material. Good Luck, David
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Old 12-18-2009, 03:13 PM   #7
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Foul smell from abandon septic tank in house


Filling an old tank is not as difficult as you may think. There are contractors who specialize in pumping in sand/water slurry. This deposits the sand in the tank, and the water runs out. Of course, to allow this to work, you need an outlet that goes somewhere, but from your description there is an outlet, and possibly it could be extended to daylight without too much trouble.

I design commercial projects where we fill pipes, tanks, and vessels routinely, and the sand slurry technique is frequently used. Alternatively, it is possible to fill the tank with grout, or lightweight concrete. This may take care of the smell, but more importantly it would provide adequate support for the structure built above it. Obviously this should have been done at the time of construction, but it is not too late to fix the problem.
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Old 12-18-2009, 05:46 PM   #8
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Foul smell from abandon septic tank in house


thanks for the thoughts. i disconnect the one duct tonight and will see if that "isolates" the smell. We live in the rural midwest so don't have ready access to contractors that might specialize in filling the old tank. I'm going to get a guy out to try and pump the tank out through the inlet and go from there. thanks again.
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Old 12-18-2009, 05:52 PM   #9
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Foul smell from abandon septic tank in house


How did they build over the tank ?
Is there a crawl space ?
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Old 12-18-2009, 06:09 PM   #10
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Foul smell from abandon septic tank in house


Oh no crawl space...of course not. They poured the slab right over the tank, although i assume there is probably a foot or so of dirt inbetween the two, this is where the duct (6" PVC) is run. Yes, the duct of course is not glued at the joints.

Years later, we assume the outlet collapsed thus rendering the tank useless and inaccessible. It gets better....in laws sold us the house....and failed to mention this, amongst many other issues where "riggage" ran rampant. Too much trust and not enough inspection on my part.
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Old 12-18-2009, 07:34 PM   #11
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Foul smell from abandon septic tank in house


Simply pouring water into the inlet and draining (or even suctioning) the outlet will not get the offensive material (sludge) out of the tank. Both the inlet and the outlet are somewhat high up on the tank sides.

1. If the sludge is churned up and stirred up "good" as water is added, the resulting output (slurry) will consist of a lot of it. This is difficult to accomplish unless the clean out hatches are still accessible and uses a lot of water.

2. The customary way of pumping a septic tank is to force the suctioning tube or wand down into the sludge so the sludge is removed first, before the liquid content.

If there is a crack in the tank or tank lid or connections, then odors can escape and filter through the soil under the house and come up through any crack in the foundation.
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Old 12-18-2009, 07:55 PM   #12
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Foul smell from abandon septic tank in house


Quote:
Originally Posted by tsduncster View Post
So the sewer pipe in the basement part of our older section of house was disconnected and routed to the new septic. the outlet of the new tank ties into the old outlet of the abandoned tank 20' outside the house. there is also a floor drain in the basement that ties into the outlet of the old septic outside the house
Also possible the field is partially blocked & forcing effluent into the old tank
Depending upon how far a drop it is where the new tank connected to the old tanks output
Or maybe I'm reading this wrong & the old tank is disconnected from the field ?
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Old 12-19-2009, 08:55 AM   #13
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Foul smell from abandon septic tank in house


right, so i disconnected the duct routed over the abandoned septic last night and sealed off both ends. smell gone. so i definitely know it is the old tank. i do believe that occasionally effluent from the new tank is getting into the outlet of the old tank which causes some major odors. So i do speculate i may also have a partial blockage in the line to the leach. ok, we'll call it a leach, but quite often in rural areas, the leach is actually just a field tile running to a ditch-that's another story. (and an issue i hope to fix correctly in about a year.)

so yes, sewage in the old tank and the outlet is still open and effluent can back up into it-I'm guessing. Understand I can't pump it correctly as there is no access except the sealed off inlet.

My immediate concern now that I've confirmed some things is are quality in the home. even though i've now sealed off the duct, I'd guess the gases are still penetrating the home and have been for quite some time. This cannot be good for us! As i have the now abandoned HVAC duct running just over the abandoned tank, i plan to rig some way to exhaust that pipe to the outside. It is 6" PVC pipe, so i plan to hook up to it in the basement with an exhaust fan and route it out the foundation.

Again, thanks for all the input, it helps to have others think this through. I know i have not remedied the problem yet, but know what i need to do for that and also for the immediate time. Friggin Riggin In-laws! Gotta luv em

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