Exploring The Science Behind Odor Control
Understanding what odor is, how humans detect odor, and how odors are produced can be helpful in the quest to control odor. This section is designed to provide basic information on the science involved in this field of study.
What is Odor?
People cannon smell solids or liquids; we can only detect certain gases and vapors.
Odor is the quality or characteristic of
a gas or vapor that is perceivable by the
sense of smell.
Odor is not a physical object: odor is
the property of an object that causes
it to be perceived by an animal.
Mammals can't smell solid materials, such as garbage. We smell the malodors that are emitted from garbage. We can't smell liquids, such as sewage, either. We smell the malodors that are emitted from sewage. The gas or vapor that exhibits an odor quality is referred to as an odorant. Odorants that have unpleasant odor characteristics are called malodors, or malodor compounds.
The Scense of Smell
With mammals, odorants that are inhaled from the nostrils pass through the nasal cavity en route to the lungs.
The roof of the nasal cavity is coated with a thin layer of lipid rich mucous.
This mucous layer traps lipophilic molecules from inhaled air.
The mucous lipids transport odorants to thousands of fine hairs (cilia), where odor reception and sensory transduction occurs.
Malodors must travel from the odor source (i.e. garbage or sewage), through the air, to the olfactory system in our nasal cavities in order for perception to occur.
Malodors will travel with the wind following gas diffusion and pressure differential patterns with the predictability (and unpredictability) that is standard of all gases and vapors.
Each specific malodor has its own detection level. The odor from hydrogen sulfide gas, for example, can be detected by humans in concentrations less than one part per million (ppm).
Understanding how malodors migrate and diffuse is a very useful tool in odor control. By comparing gas concentration levels, wind speeds, and detection rates you can predict how far malodors will travel before natural dilution with fresh air renders them undetectable.
Biological Odor Production
Most living organisms produce odorants: some are fragrantly pleasant while others are offensive.
Flowers are good examples of biological odor sources; Roses produce volatile chemicals that evaporate into the air with a remarkably pleasing fragrance.
Conversely, the carrion flower attracts insects that feed on rotting flesh by emitting a strong odor similar to decaying meat.
Microbiological Odor Production
Most malodors are produced by microbes (tiny creatures that can only be seen under a microscope).
Microbes exist in abundance almost everywhere on the earth. There are millions of microbes on every flower: billions on every bush.
Ammonification is the microbial production of ammonia and amines from organic nitrogen.
Sulfur Reduction is the microbial production of hydrogen sulfide (a toxic malodor) from sulfur.
Sulfur reduction and ammonification are just two of the processes in which microbes produce malodors. While microbiological odor production can be categorized separately from biological production, the truth is that microbes are responsible for both. The microorganisms in our stomachs, for example, are responsible for the digestion of our food, and subsequently responsible for any odors produced as a byproduct of the digestive process.
Inorganic Odor Production
Volcanoes are a good example of an inorganic odor source.
Far below the earth's surface, volcanic gases are dissolved into molten lava.
These gases are emitted through air channels and vents in the earth's soil and through infrequent volcanic eruptions.
Volcanic emissions include sulfur dioxide and hydrogen sulfide, both with the characteristic odor of rotten eggs.
Factors in Microbiological Odor Production
The production of malodor compounds is normally a microbiological process that is affected by a number of environmental variables. Following are some general rules of thumb that apply to microbial malodor production and to odor control in general.
Aeration... Aerobic microbes (microorganisms) need oxygen in order to survive. Anaerobic microbes do not use oxygen. Facultative microbes use aerobic respiration when oxygen is present and switch to anaerobic respiration when oxygen is depleted. Generally speaking, anaerobic respiration produces far more odor than aerobic respiration.
Time... Like all creatures, microorganisms reproduce. As time passes the microbial population grows, yielding a significantly higher amount of microbes and therefore producing significantly more odor. As the microbes multiply, they tend to deplete oxygen, compounding odor problems by creating an anaerobic environment.
Hydration.. All living creatures need water in order to survive. In a wetter environment, microbes produce more odor. In a dryer environment, microbes produce less odor. Excessive water will result in a saturated environment, in which oxygen will quickly become depleted and odor producing anaerobes will thrive.
Temperature... Microorganisms are at the highest activity level between 77°F (25°C) and 104°F (40°C). Nutrient sources stored at temperatures within the optimum activity range for microorganisms will result in significantly more odor production than those stored in a cooler environment.
The Basics of Odor Control
"Operational Modifications" are the best place to begin your journey through odor control. Many odor problems can be solved by changing your methods. Take household trash, for example:
Aeration... Don't pack trash into the garbage can so densely that you seal off the air. Create an aerobic environment for the microbes.
Time... Don't leave your trash around for long periods of time. Take it to the curb every garbage day to reduce retention time.
Hydration.. Don't put liquids in your trash can. Drain containers before throwing them away in order to dehydrate the microbial colony.
Temperature... Store trash in a cool area until you can take it to the curb, to keep the microbiological activity down to a minimum.
There are several more odor control rules of thumb that can be helpful. For example, turbidity in liquid waste will enable more malodors to flare off into the atmosphere; try not to disturb odoriferous liquids.
In some cases it is best to completely seal off intense odor sources so that no air can get in or out. For instance, pet waste should be double wrapped and tied off in two plastic bags before discarding in household garbage cans.
Understanding the variables that effect odor production can be very useful in the pursuit of an effective odor control plan.
Operational Odor Control for
Solid Waste Management
The same factors that play a role in dealing with household trash apply to solid waste facilities, though on a far larger scale. Don't pack trash into hauling trailers or RR containers and leave it to sit for days under anaerobic conditions. Drain leachate out of trash on the tipping floor before loading. Don't leave trailers or dumpsters in the sun to fester.
Operational Odor Control
for Compost Facilities
At times the rules in the odor control game are a bit different. Take a biosolids or greenwaste compost facility. While it is true that you don't want to saturate a windrow of compost, you still have to keep the material hydrated enough for the microorganisms to thrive. When composting, you don't want the material to get too hot, but it can't be too cold, either.
Operational Odor Control for
Sewage Treatment Plants
Sewer treatment plants are another example with complex issues involved with the factors of odor control. Some plants use aeration to digest the solids in sewage, yet others utilize anaerobic digesters (which produces a LOT of malodors). Time is critical in sewage treatment; it is a microbiological process that needs proper retention time.
Odor control can be as simple as covering a garbage can or as complex as your imagination can make it. There are many factors at play, and many different industries and applications that govern how the factors affect malodor production. This article was intended as an introduction to the science involved in odor control, and as an introductory piece, it leaves much untouched.
Hopefully you have learned something.
Perhaps you have something to share. It could be something you've experienced that may help others in your position. We would love to hear from you.
Questions or comments? We welcome them also.
Look for the next article in this series for more information.
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