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re: how to naturally process pet manure
21 mar 2000
elaine gallant proclaims:
> composting does not have a bad odor. if you can smell anything, it requires
>more carbon- yard clippings, hay, straw.
the practical handbook of compost engineering by roger t. haug, pe,
(lewis, 1993, 717pp, $99.95) has 153 pages on odor management,
including these
theorems on odor management
theorem 1--most composting substrates smell
no argument here, i hope. all of the substrates which enter a
composting process must be viewed as potential sources of odorous
molecules. this includes sludge, sawdust, yard wastes, wood chips,
refuse, food wastes, and all the other substrates we may throw
into the starting mix.
somewhere in the history of composting we got the idea that, if
left alone, mother nature would be odor free. i don't know how this
idea arose, but i assure you that it is not true, particularly with
most composting substrates.
theorem 2--mother nature never claimed to be odor free
on many occasions i have heard speakers state with great pomp,
"if composting is conducted properly there will be no odors."
these speakers are never plant operators! such statements have been
implicated as a leading cause of anxiety complex among operators.
let me reassure the operators reading this book that i have never
seen data to support such a claim. the starting substrates contain
odorous compounds and more are formed as intermediates during the
breakdown of complex substrates. yes, this includes aerobic
metabolism.
boiling points and vapor pressures for some of mother natures's
favorite odor compounds were presented in table 16.1. ammonia and
h2s boil at very low temperatures and will never stay in solution
unless converted by ph to an ionized form... acetic acid, a favorite
intermediate of aerobic microbes, has a vapor pressure of 100 mm hg
at 63 c. no wonder vinegar smells even at room temperature!...
theorem 3--thermophilic composting acts like a heat distillation process
if you want proof for theorem 3, i offer the observation that hot
sludge or garbage smells more than cold sludge or garbage... vapor
phase concentration of a compound is proportional to its vapor
pressure and the latter increases with temperature... therefore...
theorem 4--some release of odorous compounds is inevitable
during composting
theorem 5--what smells ok to you is probably an odor to someone else
theorem 6--mother nature doesn't much care what compost system you have
theorem 7--it's not nice to fool mother nature with a bad design or
bad operation
some odors will be produced even with good design and
proper operation...
theorem 8--you really should know something about your substrates
...despite the best efforts of design engineers and the claims of
equipment vendors you should always remember the following:
theorem 9--odor treatment is never 100%
the only possible exception to theorem 9 is thermal oxidation
which is capable of near complete odor destruction...
there has been a subtle but persistent tendency for the design
community to ignore theorem 9. this leads to the following:
theorem 10--many past designs didn't recognize theorem 9
scrubbers and biofilters are often designed with no attention to
dispersion of the treated gases. it is common to see scrubbers with
short stub stacks, low outlet velocities, scrubbers located near
large buildings with their plumes caught in the building downwash,
rain caps on top of discharge stacks, and other examples of poor
dispersion design. it's as though the designer assumed 100%
deodorization. this should never be assumed...
theorem 11--the worst odor is never when you're there
theorem 12--mother nature always disperses odor, but sometimes
she can use a little help
...even if you don't "enhance" the natural dispersion, the
following is always true:
theorem 13: know your micromet conditions
all major facilities should consider building an on-site micromet
station to warn of worst-case atmospheric conditions. this will
allow the operators to take corrective actions and, hopefully,
avoid an odor incident... it is important to always remember the
following:
theorem 14--there must be a receptor to have an odor
theorem 15--most receptors are at ground level
for point sources, such as exhaust stacks from scrubbers, dispersion
can be enhanced by (1) increasing stack height, (2) increasing stack
velocity to increase momentum rise, (3) providing reheat to increase
thermal bouyancy, and (4) providing forced dilution with ambient air...
theorem 16--you can stop all of the odor some of the time, but
you can't stop all of the odor all of the time
...engineers may want to hide theorem 16 from their politicians...
theorem 17--don't despair, odors can be managed
despite odor problems at some facilities, the future for composting
is optimistic. the industry generally recognizes that odor compounds
are likely to be released. recognition of the problem is the first
step toward its solution...
nick
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