Sewer Gas -
Carbon Monoxide Questions
“ I recently had occasion to sample a gas
which was emanating from an open drainage pipe in a client's
basement, from which he reported smelling "gas" and called the
natural gas distributor (us) to come in and investigate. The odour
was coming from an open drain pipe installed to take the clothes
washer effluent and run it down the basement floor drain. From time
to time, my gas analyzer showed up to 275 parts per million of
carbon monoxide.
Where can this CO be coming from, and smelling of sewer gas? I did
not think sewer gas could contain this much CO. It showed only 0.1%
L.E.L. methane, which I thought should have been higher. I have to
capability to test for hydrogen sulphide, although I suspect it is
present too, due to the characteristic "rotten egg" smell associated
with the gas”.
“I have been advised to put
carbon monoxide
detectors in my office due to a similar office having suffered many
ill people due to carbon
monoxide poisoning from sewer gas. Although I can't find
fault with carbon monoxide
detectors, my heat is electric, and I can't find anywhere that sewer
gas contains carbon
monoxide. So, the carbon
monoxide detector would not keep us safe
from hydrogen sulfides. Am I misinformed?”
“I had to move out of a one-story
(slab-built, with no basement) duplex apartment in Arizona because
I became very ill when I was inside it, and recovered whenever I
left it. Before I moved out, I tried to resolve the unidentified
problem by hiring an indoor environment testing company to help me
identify the problem. They noted an odor of sewer gas coming from
beneath
the shower floor, but all drain traps in that bathroom -- and in the
entire apartment -- were working properly, so they couldn't figure
out where the odor was coming from. (Later, the shower was replaced,
and the odor of sewer gas seemed to be coming through the bare dirt
floor that was under theshower floor.) Also, they did a test
involving breaking a sealed glass tube
that would somehow identify the presence of carbon
monoxide, and they determined that the level
of carbon monoxide
was higher inside the building than in the outdoor ambient air.
(They repeated this test on a different day, with the same result.)
They could not explain this, either, since the building was a duplex
that had one hundred percent electric appliances, and no one nearby
left their car running, etc. Their only comment was that on the days
they measured the carbon
monoxide, the levels were higher than the outdoor air, but
were not dangerous. I left the apartment many weeks later,after I
woke up extremely groggy, with the sensation that if I fell back
asleep I would never wake up again. So, I opened all of the windows
and then packed up and left. Away from that apartment, I recovered
and did not experience that symptom again. Do you have any ideas why
there was an odor of sewer gas and a slightly elevated level of
carbon monoxide in
that apartment? Thank you for your help”.
“I have a 2
story brick house in Chicago w/ an exposed soil crawl space. The
building, originally built in late 1800s, was extensively renovated
about 12 years ago. There’s a crawl space (about 18” high) just
below grade. 1st floor is only 1 step above grade. 1st floor
forced air furnace and hot water heater are on the 1st floor. 2nd
floor has a rooftop HVAC unit. In 12 years there, I never had any
water problems with the crawl space, although there was a damp smell
throughout the summer.
Last Friday
afternoon a contractor broke a section of the water main down the
street. Water overflowed as high as the sidewalk on my street, and
the city shut down water until that evening. We came home Friday
evening to a strange odor in the house 1st floor. This was not
stale water, but as if there was a chemical or gasoline leak. At
first I thought it was natural gas, so turned off the pilot light to
the hot water heater and a gas fireplace. I set both 1st & 2nd
floor furnaces on fan mode to exhaust the smell, plus opened the
windows. Early Saturday (1am) the 2nd floor CO detector went off 3
times. Saturday afternoon People's Gas came out and found a small
leak, but not the cause of the odor. Tuesday night I put water in a
floor drain in the 1st floor mechanical room, and the smell seems
better (less chemical), but not totally gone. Any insights you can
provide would be greatly appreciated”.
As can be seen from the
questions above this site has recently received a number of queries
about the potential presence of carbon monoxide (CO) in sewer gas.
The typical answer is that no carbon monoxide is likely to be found
in sewer gas since it is a product of incomplete combustion and
should therefore only be associated with combustion appliances in
some way or be drawn into a building from motor vehicles or a nearby
source where combustion emissions are taking place. In addition
sewer gas is produced under anaerobic (absence of oxygen) conditions
which indicates that CO production in sewers would not be chemically
possible. The first question above from a gentleman working for a
utility company in Canada has posed a challenge;275 PPM CO is quite
a high reading.
In the atmosphere methane (the
dominant gas in sewer gas) undergoes oxidation reactions that
eventually are responsible for about 30 % of the CO in the
atmosphere. This is a multi-step process that requires the presence
of very strong oxidizing agents such as hydroxyl radical. In the
stratosphere methane can be oxidized to CO and then to CO2
by reaction with hypochlorite. Hypochlorite down here on the
ground is the prime ingredient in chorine bleach used in cleaning
clothes. The hypochlorite is of course flushed down the drain where
it has the potential to react with whatever is present in wastewater
as well as gases that have already been produced.
In theory under
certain conditions it is possible for carbon monoxide to be produced
in wastewater lines particularly those associated with laundry
water. The question is how probable is it?
November 16,2007
