51
Getting To Know
The Odor Compounds
Nora Goldstein
HE STATE of knowledge about the various
compounds that can lead to odor troubles
continues to be refined. In the biosolids
world, for example, it is well known that sulfur
compounds are a key cause of odor. What is being
studied now is the impact of wastewater treatment
processes, such as dewatering technologies and the
addition of polymers, on the actual formation
and/or suppression of odorous compounds.
Different stages of the composting process have
different odor characteristics. “Our neighbors
know the difference between sludge, garbage, the
composting aeration floor odor and the biofilter,”
says Bob Spencer, who manages the Bedminster
Marlborough cocomposting facility in Massachusetts. “One day, a truck tore a hole in a door
of the composting building and we had to put up
a tarp temporarily to help contain the building
air,” he adds. “One neighbor said to me, I think I’m
smelling compost piles today, what is going on? On
another day, that same neighbor noticed a musty
odor and suggested that it was the biofilter. We
checked and found a couple of dry spots where the
air was short-circuiting through it and not getting
completely treated.”
T
Odor Characteristics
In a presentation at the BioCycle West Coast Conference in San Francisco in March 2002, Paul
Rosenfeld of Komex H20 Science Inc. in Los Angeles talked about the use of wood ash to control
odors in yard trimmings composting. Rosenfeld
started out the presentation with a review of odor
compounds common to composting. With each
compound, he gave a human detection limit in micrograms/cubic meters. A low value means the
compound is easily detected by humans (odorous);
the higher values mean it takes more of the compound to be detected (not so odorous). The folPrepared by
lowing summarizes odor compound information:
Sulfur Emissions: “Most of the sulfur odors –
about 90 percent – from biosolids composting are
related to dimethyl disulfide (DMDS),” says Rosenfeld. “DMDS smells like rotten cabbage and can be
detected at very, very low concentrations – 0.1
µg/m3.” Dimethyl sulfide is another odor contributor, which is detected by humans at 2.5 µg/m3. Notes
Rosenfeld: “If you are going to spend money on lab
analysis of odor compounds when composting
biosolids, focus on DMDS. That is the bad guy.”
Reduced sulfur gases are related to the anaerobic
digestion phase of biosolids processing. Two compounds detectable at very low levels are hydrogen
sulfide (0.7 µg/m3) and methane thiol (0.04 µg/m3).
Nitrogen Emissions: “Ammonia gas represents
approximately 99 percent of all nitrogen emissions
that come off during biosolids composting,” notes
Rosenfeld. “It has a medicinal smell, although its
detection limit is high – 27 µg/m3.” Trimethyl
amine (TMA), which has a fishy odor, is another
nitrogen based compound. It can be detected at
very low concentrations (0.11 µg/m3). The challenge with TMA, he adds, is that it is difficult to
biodegrade, as microbes cannot easily break apart
the molecule. Ammonia and TMA are not unique
to biosolids composting, and can be found at any
operation processing feedstocks with high volatile
solids content.
Indole and Skatole: “These are the horrible
smells of poop,” exclaims Rosenfeld. “They are
terrible enemies, with very low human detection
limits – 0.0004 µg/m3 for indole and 0.0004 µg/m3
for skatole.”
Volatile Fatty Acids: Volatile fatty acids (VFA) are
most associated with yard trimmings composting.
“VFAs smell like vinegar and body odor, and are
by-products of anaerobic decomposition,” he
notes. “As the chain link of carbon molecules gets
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52
ENVIRONMENTAL PROTECTION - AIR, WATER AND FIRE
the most critical lines of
longer, the methyl
Sulfur compounds
defense against odors.
groups increase and the
“Our research has
odors get more intense
found that if the oxygen
and the detection limit
Dimethyl
Carbon
Dimethyl
content drops below 17
drops dramatically –
disulfide
disulfide
sulfide
percent, anaerobic
from 1019 µg/m 3 for
(0.1 g/m3)
(24 g/m3)
(2.5 g/m3)
pockets start forming,
acetic acid to 28 µg/m3
Rotten
Rotten
Rotten
cabbage
pumpkin
cabbage
which then leads to forfor propionic acid to
mation of volatile fatty
0.3 µg/m 3 for butyric
acids, ketones, aldehydes and poacid.” He adds that butyric acid,
tentially methane,” concludes
which is highly odorous, has been
Rosenfeld.
used as a weapon.
Ketones: Acetone and methyl
Changing Odor
ethyl ketone can always be found
Hydrogen
Methane
Characteristics
at composting facilities, but the
sulfide
thiol
(0.7 g/m3)
(0.04 g/m3)
As noted above, different phashuman detection limits are very
Rotten
Pungent
es of the composting process
high. “It takes a lot to smell them,”
egg
sulfur
bring along different odor charRosenfeld explains, “and you can
acteristics. Jan Allen of CH2M
waste a lot of money trying to
Hill in Seattle, managed the Cedar Grove Commeasure the compost.”
posting facility in Washington State for a number
Aldehydes: This is a very odorous family of comof years. The site, which started out processing
pounds that are very prevalent at waste handling
yard trimmings in static piles, eventually switched
facilities. Acetylaldehyde, which can be detected at
over to aerated static piles and biofiltration in or2 µg/m3, is the smell you get when you take the lid
der to control odors, emanating in large part from
off of a garbage can. Propylaldehyde, with a sweet
loads of grass clippings. The facility also processes
solvent smell, has a detection limit of 22 µg/m3.
some food residuals. “The odor compounds that
Alcohols: Methyl and ethyl alcohol smell like alcocause the most problems at a site like Cedar Grove
hol and are “not a big deal” at composting facilities,
are amines and VFAs like acetic, proprionic, busays Rosenfeld. Methanol has a human detection
tyric and valeric acids,” says Allen. “The probabillimit of 13,000 µg/m3, while ethanol is 342 µg/m3.
ity of having an oversupply of these compounds is
Terpenes: Mostly associated with yard trimgreatest during the first two or three weeks. As the
mings composting, terpenes are semivolatile comprocess progresses, the amines and fatty acids depounds with “a pleasant smell,” he explains. Comcrease and ammonia increases.”
posting-related terpenes include menthol,
Conditions at the composting site also can be a
limonene and alpha-pinene. While fine on their
source of odors, he adds. “Sulfides can occur very
own, when mixed with aldehydes, DMDS and amreadily in leachate, condensate or contaminated
monia, terpenes add to the overall negative charstorm water runoff regardless of the feedstocks. If
acter of the odors, he adds.
open impoundments of water with high nutrient
Other odorous compounds include guaiacol,
loads are present, sulfides are a real challenge.”
which is a wood smoke odor from burning wood,
Sites processing biosolids and manure may see inand pyrans and furans, also related to burning
creased sulfides in the process and/or building air
wood. These are characteristic of the wood ash
stream as well.
that is used to control odorous emissions. Wood
At the Bedminster Marlborough plant, the use of
ash has been used successfully at a variety of
rotating digesters as the first stage
composting sites processing,
Nitrogen compounds
of composting accelerates the
yard trimmings, biosolids and
degradation of odor compounds
MSW. (See “Controlling Odors
such as DMDS and the aldehyUsing High Carbon Wood Ash,”
des. “If we hit the correct C:N raBioCycle, March 2002.)
tio when combining the MSW
While various feedstocks conand biosolids, have the right
tain these various compounds, in
moisture content and deliver the
many cases, they will not be reTrimeth
Ammonia
yl amine
correct amount of air, we are able
leased as long as composting con(27 g/m3)
(0.11
to convert two putrid odor
ditions are optimized. MaintainMedicinal
g/m3)
Fishy
sources into a different odorous
ing aerobic conditions is one of
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GETTING TO KNOW THE ODOR COMPOUNDS
53
biofilter out of finely
mix with far less ofVolatile fatty acids
textured compost. It
fensive qualities,” says
isn’t porous enough
Spencer. “The exhaust
and has too much nigases are very potent.”
trogen.”
Materials remain in
At the Marlborthe digester for three
ough
facility, Spencer
days, and then are
Acetic acid
Propionic acid
Butyric acid
has tried different
screened and taken to
(1019 g/m3)
(28 g/m3)
(0.3 g/m3)
Sour
Rancid
Putrid
media mixes, mostly
a composting build(vinegar)
playing with the ratio
ing with an aeration
of chips to compost.
floor. “There is still an
Right now, they are testing tire chips in the mix. As
odor associated with the materials, but as long as we
far as identifying which mix does best with which
don’t pile too high and we deliver enough air, the
compounds, he has found that microbes acclimate
odors are managed,” he adds.
to whatever gases are being put through the biofilter.
The rotating drum system, because it provides
“If we had more sludge gases and less MSW gases,
“360°, 24/7 agitation,” constantly exposes the orthe biofilter would adjust,” he says. “If we dropped
ganic matter to microbes that consume the prosludge and just composted MSW with another niteins and sugars that are odor compound “retrogen source, the biofilter would adjust. That is the
leasers.” The ideal situation from an odor control
beauty of biofilters – they acclimate to the gases bestandpoint would be to have a longer retention
ing fed. It takes two to three weeks to acclimate, and
time in the drums, but that would be too costly,
then just improves over time.”
notes Spencer. “Basically, the digesters are a very
CH2M Hill has a portable test unit that enables a
powerful – and very expensive — way of exposing
client to compare two different medias in a side by
organic wastes to new microbes.”
side comparison. “There are two identical chambers
Odor Compound-Biofilter Media Connection
in parallel with sniffer ports and you can measure
One question that arose while doing research
the air quality,” says Allen. “This is an important
for this article is the relationship of the types of
enough issue that we decided to construct the unit.”
odor compounds generated to the types of media
Have Respect
used in the biofilter – if, for example, a certain
Chemical compounds and mole-cules aside, bamix is better for yard trimmings related comsic knowledge and instincts are a good line of depounds versus biosolids compounds. There
fense when it comes to odor management. In a nutseems to be consensus that most if not all of the
shell, woody materials and brush don’t have much
feedstock and composting process related comodor. On the other hand, grass clippings and
pounds are readily treated with biofilters.
spoiled produce are notorious, says Allen. “Feed“There are so many factors when it comes to
stocks that are more putrescible will naturally debiofilter construction, e.g. how deep it should be,
grade quickly – in fact, so darn fast that it is hard to
how much air will go through it, the residence
do it gracefully. With grass and some food waste,
time, and so forth,” says Allen. “The optimal
decomposition and odor generation already have
biofilter ends up being a combination of biology
started before the materials arrive.”
for the odor compounds plus the physical conUpon their arrival, adding amendments is a
straints of the project. Media selection should
good idea. “This fixes the C:N ratio, porosity and
consider the nitrogen and amine loads to insure
the moisture,” adds Allen. “Most of the troublethe media is appropriate for the application. Mesome materials have no pore space and are off the
dia replacement should be more frequent with
chart on moisture, so adding dry, structural carhigh fatty acids or sulfide loadings as the media
bon is always a good idea.”
deteriorates more rapidly under these low pH
Process and prepare the mix right away, and then
(acidic) conditions.
don’t touch it. “Have some respect for it because for
“In general, the best biofilters are ones that are
the first few weeks, the pile will be potentially odorstarved for the pollutant(s) you are trying to treat. So
ous,” advises Allen. “With a low tech operation, it
if you run a high ammonia or high amine pollutant
could be over a month. Basically, disturbing a pile
through rich, high nitrogen compost, the microbes
or compost batch without vessels and enclosures is
will keep eating the compost. They won’t have any
generally a bad idea until all organic acids are conaffinity for the pollutant because they have plenty to
verted to organic matter.”
eat already. The biggest mistake made is building a
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© 2008 The JG Press, Inc.