Letting Your Malodors Fly Away

Water treatment plant malodors can be moved by the wind to populated communities where odors can become an unwelcome neighbor, but effective techniques can mitigate odor impacts

One pleasant June morning, I was relaxing on my patio, when I observed a robin hovering momentarily above the grass in the backyard. Its fluttering wings were forcing the air beneath to wave the lush green grass. The air in motion (the simplest definition of wind) generated by the robin was in the range of what meteorologists call the microscale. The air movement (or lack of it) on this scale is critical to the dispersal of obnoxious odors. Of course, water-treatment plant owners and operators don't depend on avian aerobics to disperse their objectionable fumes; but air-in-motion in the smallest, medium (mesoscale), or even the largest (macroscale) range can send malodor molecules to undesirable destinations.

Malodor Molecular Motion
Before we look at malodor movement in ranges we can readily see, let's explore what happens to such particles on the molecular level.

The diffusion of air contaminants (like odor molecules) is a crucial area of study in the air-pollution field. The term diffusion is defined by the Glossary of Meteorology as the "transport of matter solely by the random motions of individual molecules not moving together in coherent groups. Diffusion is a consequence of concentration gradients..." (Glickman, 2000). But, what actually happens when diffusion of malodors occurs?

All gas particles have mass and take up space and are in constant motion, going in random directions. Particles will collide and recoil, like billiard balls, with each other and the physical boundaries that confine them. Over time, particles tend to distribute evenly through the available space.

Upon release from the confines of a water-treatment unit, malodor particles move outward as they continue to collide and recoil with each other, as well as now colliding and recoiling with the additional outside-the-container (ambient) particles. Over time, if the odor particles have characteristics similar to the ambient particles (such as having the same particle density and temperature), the odor particles will mix rather evenly with the other particles throughout the available volume. In other words, they will achieve equilibrium.

Particle motion is influenced by temperature. Cooler particles move more slowly than warmer particles. The difference in motion between particles of different temperature affects particle diffusion.

As stated above, individual particles have mass and occupy volume. This means an individual particle has density. A group of individual odor particles with the same properties, like density and temperature, enclosed in a container will exert pressure on each other and the container as the particles randomly move about. Particle velocity is dependent upon temperature, with faster moving (higher energy) particles exhibiting higher temperatures. The total amount of particles in the volume of the container represents the density of the group of particles.

Upon opening the container, the odor particles will scatter (diffuse) as they continue to bump into each other and now bump into the particles in the ambient space. If the individual odor particles are the same density and same temperature as the ambient particles, after a brief duration (e.g., one minute) the random movement of the released particles will most likely cause them to diffuse more-or-less evenly throughout the collection of ambient particles.

If the individual odor particles are more dense (like many sulfur-containing compounds) and/or colder (i.e., moving more slowly) than the ambient particles, the released group of particles will most likely diffuse to the lower portion of the collection of ambient particles. Heavier or slower moving odor particles sink within a brief duration after release because as a group the more massive or slow-moving particles remain closer together, occupying less volume (which means they are more dense as a group) than the ambient particles.

If the individual odor particles are less dense and/or hotter (i.e., moving more quickly) than the ambient particles, the released group of particles will most likely diffuse to the upper portion of the collection of ambient particles. (hot-particle diffusion). Faster-moving odor particles rise within a brief duration after release because as a group the fast-moving particles spread farther apart, occupying more volume (which means they are less dense as a group) than the ambient particles.

Circulating the Odor Problem
Now back to the more visible manifestations of malodor movement. Different weather phenomena occurs within different air-circulation ranges. For example, within the microscale can be found turbulence, which is the slowest wind speeds that occur over the shortest distances (like the fluttering of a bird's wing). Sea and land breezes that form diurnally are examples of wind circulations in the mesoscale range. And within the macroscale (including the synoptic scale), are jet streams, the rivers of wind traveling at up to hundreds of miles per hour at altitudes around 30,000 feet. This upper-level jet, as meteorologists like to call it, provides a steering current for lower-level weather patterns. (Thus, with a typical westerly jet (meaning the high-altitude winds are coming from the west), if a low-pressure system is centered over Cincinnati producing rain showers, there's a good chance that the system will eventually dump its rain on Pittsburgh, some 350 miles to the east.)

The effort of birds not withstanding, the wind typically originates from the fact that warm air rises. This fundamental principle of meteorology begins to explain most atmospheric circulations from turbulence to jet streams. But regardless of whether the wind is thermally or mechanically produced, once generated, its path is diverted by a wide variety of obstructions. Such obstructions might include plant buildings and tanks, large offsite objects, and terrain features.

The path air takes across a landscape is similar to the path water takes across terrain. Water will flow into and along gullies and move in sheets across impervious open fields -- so too the wind. Air will generally follow the twists and turns of the land, especially when speeds are slow. And air will flow rather straightforwardly across flat, open fields.

If obstructions exist, the air can take some tortuous paths. For example, if buildings are present, the air will go around the walls and over the roof. A cavity can form in the lee of the building. Such cavities can produce high concentrations of fumes as the air becomes trapped and stagnant.

Cures for a Malodor Malady
So what does this all mean to the water-treatment plant owner and operator? Sometimes wind and water can be a bad combination. Plant operators know that under certain weather conditions their plant can become an unwelcome neighbor. If winds carry malodors from the operation to nearby residents, neighbors may perceive more than foul odors. "They could perceive their property values dropping. And more, local residents are likely to equate odorous emissions with chemicals harmful to their health. These perceptions could spell a community relations, legal, and regulatory compliance nightmare for the plant" (Sadar & Shull, 2000, p. 5).

Odor-minimization practices can reduce your facility's chance of emitting offensive odors beyond its borders. These practices include:

  • Material substitution/reformulation -- replace odorous chemicals with less odorous ones
  • Good housekeeping -- train employees in beneficial operating practices like spill and leak prevention, proper material handling, and preventive maintenance
  • Equipment redesign -- replace or renovate old, inefficient equipment that lets malodors escape

To reduce the likelihood of malodors impacting employees and surrounding neighborhoods, if possible, locate your odoriferous sources:

  • Away from building air intakes
  • At or near the center of your property
  • At a stack/vent release height sufficient to disperse emissions above building aerodynamic cavities and wakes This height is roughly "two times the lesser of the height or crosswind width of the largest nearby building" (Sadar and Shull, 2000, p. 5).

If odor controls are needed, a variety of methods can be applied, such as:

  • Thermal oxidation -- combustion or incineration of odorous organic gases
  • Absorption -- chemical removal of odors using an absorptive medium consisting of "water, acids, gases, bases, chemical oxidants, and various solvents and reducing agents" (Sadar & Shull, 2000, p. 6)
  • Adsorption -- capture of gas molecules on activated carbon or other porous surface
  • Condensation -- application of a heat exchanger to cool and condense gases from a low airflow emission source
  • Containment -- use of geodesic-type domes or other configuration covers to reduce surface evaporation of odorous compounds
  • Biofiltration -- microorganisms attached to a suitable medium can dine on otherwise objectionable matter

Chemical characteristics and concentration of the offensive molecules and budgetary and engineering constraints will help guide the decision on which method or combination of methods will meet your malodor challenge. For instance, containment, collection, and control through a biofilter may be a good choice. Besides being quite effective for some sulfurous compounds, the additional advantage of a biofilter is that the public views such filters as more natural and environmentally friendly. A disadvantage is that biofilters can require more attention from knowledgeable operators.

More quickly than a bird in flight, the winds can carry offensive odors from your water-treatment plant to your neighbors, fowling (oops...fouling) your good-neighbor image. But, by understanding the basics of malodor molecular motion, implementing odor-minimization techniques, careful siting and construction of potential odor-producing sources, and the use when necessary of odor controls, your plant can successfully minimize malodors and keep its good-neighbor status.


  • Glickman, Todd S. (Managing Ed.) (1990). Glossary of Meteorology (2nd Ed.), Boston, MA: American Meteorological Society.
  • Grenci, Lee M. & Nese, Jon M. (2001). A World of Weather: Fundamentals of Meteorology, Third Edition, Dubuque: Kendall/Hunt Publishing Company.
  • Sadar, Anthony J. & Shull, Mark D. (2000). Environmental Risk Communication: Principles and Practices for Industry. Boca Raton: CRC Press/ Lewis Publishers.

This article originally appeared in the 09/01/2004 issue of Environmental Protection.

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