If good fences make good neighbors, then effective odor control is the nasal equivalent of a good fence. Without it, those aromatic escapees can trample your company's reputation, good will, and ability to effectively conduct business.

By Lynn Merrill

We've all heard the story. When the materials recovery facility (MRF) or composting facility was first built, the only neighbors were the cows. But as urban sprawl has moved farther outward, suddenly what used to be the boonies are now tony places to live with such romantic names as Wind Ridge or Pleasant Meadows. And the first day Mr. and Mrs. Neighbor sniff the air and wonder about that smell is the moment your odor control program will become the topic of public hearings, newspaper articles, and visits from your friendly regulators.

Odors Happen
Our world is full of smells. Unless you've got a genetic deficiency that has rendered your olfactory glands inoperative, you experience continuously a variety of aromas that can range from pleasant to stinky. Odors are gases that are emitted from a variety of sources and reactions. These can include the release of gas from the combination of two chemicals, as a byproduct of fermentation, or from digestion of an organic material by bacteria.

Some odors can be pleasant, such as the smell of baking bread or the scent of a favorite perfume. But most odors created in the solid waste industry are the result of something decomposing—food rotting, grass fermenting, or various constituents combining to create the familiar "garbage juice" brew leaking off the back of the trash truck as it dumps at the disposal facility.

The source of an odor is molecules that have a certain chemical configuration and that are light enough to float from the origin points to a receptor. Odor molecules can float at different rates, based on what chemical elements make up the weight of the molecule. Some molecules are heavy and hug the ground, while others are lighter and will float upward. Think about the movement of a soap bubble and you get a basic idea of how an odor molecule moves.

Two factors determine whether the odor becomes a problem. The first is the concentration of the odor molecules in the air stream, often measured in parts per million or parts per billion. Some odors must have a high concentration in order to be perceived by a receptor, while for other odors a small concentration can render an immediate reaction.

The second factor is the sensitivity of the receptor. For some sensitive individuals, the slightest concentration of an odor can trigger a severe reaction that can include nausea, gagging, or even difficulty breathing. For others, the same concentration may be unnoticeable. Also contributing to the issue of odor response is whether the receptor actually finds the odor obnoxious or offensive. Repeat exposures to the same odors can either increase or decrease the sensitivity of the receptor. A refuse truck driver's first day on the job may seem pretty intense the first trip to the transfer station, but by the end of the week, he will become desensitized to the odors.

The intensity of the reaction to the odor can be as much psychological as physiological. In other words, the reaction to odor can be intensified by the emotional state of being of the receptor. Homeowners downwind of that "horrible" MRF react more intensely to each repeated exposure to the same odors, due to their knowledge of the facility's proximity to their home. Unaddressed complaints increase the frustration level and generate an even lower tolerance for the odors in question. Often this level of frustration will generate a negative response even when the concentration of odor molecules is low or non-existent, or when the source of the odors in question is not a waste-handling facility. Once your facility has been "sniffed out," everything you do is under scrutiny.

Building an Odor Control Strategy
The waste industry smells, and recognizing and addressing this as part of the design process for any new facility is a first step in addressing odor issues. Since odor molecules are more concentrated closer to the generation source, maximizing the distance between the source and any potential receptors is a critical design element. Recognizing and understanding the location of both current and future neighbors can be a challenge, especially if the facility is surrounded by vacant land that has the potential for development. Even so, recognizing prevailing wind velocities and directions can help. If the frequencies of winds are from the northwest, the bulk of the odor movement will occur off the southeast property line. Increasing the buffer zone between the facility and the property line to maximize this distance will permit a dispersion of odor prior to reaching potential receptors. The use of landscaped berms along the perimeter can also aid in forcing odor molecules upward into the atmosphere while simultaneously providing a visual barrier.

Elevation of the facility in reference to adjacent properties can also play a role in whether odors remain concentrated or dispersed. While it's often desirable to literally maintain a low profile in order to reduce the visual intrusion of the facility into the community, building a facility in a depression can actually concentrate odors, especially under certain atmospheric conditions such as inversion layers or high humidity. These literally become pools of odor that can float upward toward receptors in sufficient concentrations to generate complaints.

Containing odors within a facility by enclosing the operation is appropriate and allows for better odor management. Placement of vehicle access doors into a building so their orientation is not in direct line with prevailing winds assists in containment. The worst design is to place entry points on either side of the facility that are in direct line with the prevailing or prominent wind stream. It's the equivalent of creating a concentrated river of malodors that is sure to find an unwilling receptor.

Finally, designing the facility to be easily cleaned reduces the opportunity for odors to concentrate by eliminating the sources of decomposition. Too many nooks and crannies can create housekeeping nightmares where waste materials can build up and putrefy. Creating floor plans where automated sweepers can operate easily will allow for more timely cleanup and reduce odors before they become a nuisance.

Odor Management Systems
In addition to designing new waste and recycling facilities to incorporate appropriate housekeeping features that minimize odor transit offsite, state-of-the-art systems are also available to aggressively manage odors before they leave the facility. These systems also have the inherent advantage of being retrofittable into existing older facilities that may suddenly find themselves the target of unwelcome attention from the new neighbors.

Because odors are airborne, systems that can attack the odor in air and neutralize it have gained prominence in the last decade. Fog or mist systems have the added benefits of providing dust suppression as well as cooling within waste and recycling facilities.

Most fog systems are designed to atomize liquids into droplets of sizes ranging down to 10 microns. The size of the droplet creates varying environments for odor control, with the smallest droplets creating a light, fine, or dry fog and proceeding through heavy or wet fog, fine mist, or heavy mist to a spray. According to Tom Latta, president of Fogmaster Corporation (Deerfield Beach, FL), atomizing liquids provide two approaches to odor management. It means either "atomizing an odor control chemical into the air to travel with escaping vapors or atomizing a chemical into a mist to settle on an odor source."

"Odors are gases and gases are pretty mobile," states Latta. "The way of dealing with these gases is to make your odor control chemical be gaslike so it travels with the odors. That typically requires small droplets. The technology for making droplets is pretty mature and well-established. The differences are in the droplet size. You can have a hose and you can have a spray and you can have a mist. As droplets get smaller and smaller, you get into the fog range. Droplets can be so small they're not even visible. If you have a water-based solution of an odor control substance, then as you atomize your water or your water solution, you're carrying the active ingredients into the air in these droplets."

Fogging systems are placed over an area of odor concern, whether it's the live floor on a transfer station or sorting stations on a MRF. For a building, the odor neutralization system's design is based on the square footage of the building, the distance from the ceiling to the floor, and the year-round temperature/humidity extremes of the climate where the facility is located. "First what you have to do is identify what is producing the odor," says Bill Falkenstein, president of Cool Zone USA, (Las Vegas, NV). "If it's an open area like a landfill, you have to determine what the prevailing wind directions are for the area and where population centers are located that could provide odor complaints so that the odor neutralization system will operate, automatically, when the winds are in the direction of the population centers."

The systems can be designed to operate automatically when odors are the worst and in all climatic extremes, including dry arid deserts, highly humid environments, and/or extreme cold. "Many factors have to be taken into consideration for an effective odor neutralization system," says Falkenstein. "Certain times of the year are a factor. We did a transfer station in northern Nevada. They don't have a problem in the winter. But in the summer, they do have a problem, especially with all the greenwaste, especially on Mondays, when the waste is more fermented from sitting over the weekend." Automatic controls shut down the fogging system during freezing weather. "The systems can be designed to have automatic air purging to prevent water-freeze damage to the fog atomization lines. We can design an odor control system for anywhere in the world, in any building, or around any landfill and make it work."

As part of the water fogging system, acting as the carrier of the odor neutralizer chemical, injection of the odor chemicals is accomplished by a metering pump. "The odor neutralizer is injected into the water at a ratio anywhere from 1,000 parts of water to one part odor neutralizer, to 200 parts of water to one part odor neutralizer based on the chemical used and on the intensity of the odor. The metering pump allows you to accurately adjust the dosage of injection proportionately to the amount of water going through the system."

Solid impurities that have the potential for clogging the nozzles are filtered out using dual 10- and 5-micron water filters. High calcium content can be addressed using water softeners. "We ask clients either to send us a water analysis or send us a sample of water, which we'll have analyzed," states Falkenstein. Good maintenance is key to keeping the system operating. "I'll give you an example of one of the transfer stations using our system on extremely hard water. Their water softener broke down and they didn't repair it for a year. Their system had constant nozzle clogging and it was determined that calcium particles had accumulated inside the stainless steel pipes and when they got the water softener back online, the soft water was flaking the calcium loose, which was then clogging the nozzles within a day. They had to install a new fog atomization line in order to correct the nozzle clogging problem."

The best time to install an odor control system is after the building is constructed but before operations begin. "It's actually easier to retrofit it once the building's completed but not in use," states Falkenstein. "Once the transfer station is in operation, it is difficult to use man lifts and other tools to efficiently install the fog system without disrupting the daily operation of the transfer station or MRF."

Permanently installed fogging systems can be a less-efficient means for controlling odors in certain applications, says Latta. "Because the equipment produces small droplets, it is often used for vapor phase contacting. This is most efficient when the odorous gases are conveyed in some enclosure [chimney, plenum, ductwork, vent system, etc.] because the chemical travels with, and can mix thoroughly with, the odor before it is discharged. In an open-air situation—say, odors from a solid waste transfer station or a landfill—fogging uses chemicals with less efficiency. You end up treating the perimeter because you don't know the specific location of the odor source. Plus, it will probably move around from day to day, depending on operations." Latta suggests that some applications warrant the use of mobile units in those situations that allow the operator to move the system to the location needed.

Punching Out the Odors
The fogging system is only one part of a two-part punch for odor control. While the fogging system can deliver the right-size droplet for capturing the odor, it's the selection of the chemical that actually takes out the odor.

Odor control chemicals fall into two classes. The first class is masking agents. This family of products introduces a pleasant but powerful additional odor into the air volume to cover up the malodor. The odor is still present in the atmosphere but is simply overwhelmed by the masking agent to the point that the receptor smells the masking agent. The second class of products is neutralizing compounds. These products attack the odor molecule and through a chemical reaction cause the odor to literally stop being an odor.

"An odor neutralizer makes the odor ‘go away,' if you will," says Charles Timcik, technical director for OMI Industries (Barrington, IL). "It works primarily through a number of mechanisms primarily through adsorption and then absorption of the gas molecules of the odor compounds, into the atomized product." (Note: Adsorption involves the adhesion or sticking of a molecule of gas, liquid, or dissolved substances to a surface—in this case the surface of the odor molecule. Absorption involves drinking in the odor molecule by the odor counteractant.) "At that point, the neutralization process has begun and there are a number of mechanisms that kick in. We've identified that the product enhances the solubility of these gases such that we can take up more into the aqueous solution than would occur with just water. It also enhances the adsorption phenomenon in that when the product is atomized the active ingredients tend to become attracting, causing the product to be attracting and wanting to draw the gas molecules onto and then into the airborne atomized product."

The challenge for proper odor control, states Timcik, is to ensure that the neutralizer comes into contact with the malodor in appropriate concentrations and applications. "The biggest challenges in designing and implementing an odor control system is we have to cause contact between the malodor and the atomized product," he says. "If we don't, the product doesn't work. If we can't absorb the malodor into the atomized product or if it's a topical application and it's not being laid down uniformly, then it's not going to work. That tends to be the challenge across the board in most of our industries. In transfer stations, it's a little less challenging. One needs to identify the sources or the locations such as the tipping areas and the load-out ports."

Understanding the chemistry of the odor molecule is also important to ensuring a successful odor control strategy, says Ian Howard, executive vice president of Ecolo Odor Control Systems Worldwide (Toronto, Canada). "Our experience has allowed us to create a certain base of blends that we use in various applications," he says. "We also will sometimes do some custom blending. For instance, we have a solution that is there to eliminate hydrogen sulfide, and depending on the facility and what's happening, we may choose to add or not add that into the mixture or add it in, in the appropriate percentage. We have about a half-dozen basic blends, depending on what we find on the site, that we would start with."

While the odor control industry has matured relating to the delivery systems, ongoing research continues to address the science of the odor compounds. "It may be a somewhat mature industry, but I think you can always move forward," states Howard. "We certainly think research and development is very important. We have an ongoing research and development team, headed by our corporate chemist, and they continue to develop new and even more efficient systems, as well as solution mixtures. It's our experience that no one solution can solve a full gamut of odor problems that are out there, and quite frankly, we need to constantly move forward."

Lynn Merrill is director of public services for the City of San Bernardino, CA.

MSW - March/April 2005