Odor Management Part 3: The Basics of Odor Science

Previous articles have established the importance of controlling odors at composting facilities and initiated discussion on some of the tools available for odor management. This article will discuss the basics of odor science and how it relates to odor control. Although this article focuses on composting facilities, the same principles apply to all organic recovery facilities as well as to landfills and other waste disposal facilities. For part four, go here.

The fundamental reason to quantitatively test for odor is because in order to manage odor, we must be able to measure it. Human response to odor is highly subjective and variable among individuals. Perception of whether an odor is offensive depends on many characteristics, such as odor type, concentration, intensity and quality. To determine if an odor control measure is working, quantitative measurements and statistical verification of odor reduction are essential.

Odors from the decomposition of organic materials are complex and characterized by a wide variety of components whose precise composition may be difficult to identify. Attempts to evaluate odors from analyses of specific compounds may show an inadequate correlation with odors perceived by people downwind of a source. Rather than individual gas components, the use of olfactometry for measurement of total odor concentration is the most typical method to quantify odors.

An advantage of olfactometry is the direct correlation with odor and its use of people’s sense of smell. Another advantage of olfactometry is that it analyzes the complete gas mixture so the contribution of each compound in the sample is included in the analysis. Standardized methods of dynamic olfactometry include ASTM E679-04 and BS EN 13725 which rely on the human olfactory sense by utilizing a panel of trained individuals. Field olfactometers are also used to assess odor levels. By employing several assessors to provide multiple measurements as well as correlating data collected in the field with laboratory results, the value of field olfactometry data can be improved.

The obvious problem with relying on the human nose is that there can be quite a bit of variability in responses. Greater validity of the data can be achieved by applying the appropriate statistical analysis to the results to determine whether control methods result in a significant reduction in odor (p<0.05). Understanding variability in data, calculation of odor concentration and statistical analysis has important implications for regulatory agencies in determining allowable odor concentrations at a facility’s property line or other locations.

Odor concentration is defined as the number of dilutions of a sample with odor-free air needed to reach a threshold at which only a specific percent (typically 50 percent) of trained individuals are able to detect the odor. The detection threshold is the concentration at which 50 percent of individuals can perceive the presence of an odor without being able to identify it. The recognition threshold is the concentration at which 50 percent of individuals can identify the odor, (e.g., rotten eggs, vinegar, tobacco, ammonia), and is typically one to five times higher than the detection threshold.

In literature on odor science and dynamic olfactometry testing, the number of required dilutions to reach a threshold odor concentration is typically referred to as dilutions to threshold (D/T). In regulatory literature and other publications, odor concentration may be referred to as odor units (OU). Both terms are interchangeable and numerically equivalent. Other measurements of odor such as character (what it smells like) and Hedonic Tone (its degree of pleasantness/unpleasantness) are subjective but may be useful in describing the range of perceptions about an odor.

An example of how data can be used to significantly reduce composting odors associated with grass clippings can be seen in studies on odor control during grass composting (Buckner, 1995, 2002). Odor concentrations were quantified and compared among treatments in a turned windrow yard waste composting system. Experiments focused on identification of windrow treatments and process conditions that were associated with low odor levels during grass composting. Treatments consisted of variations in both turning frequency and feedstock mixture ratios of grass with leaves or woodchips.

The composting process was evaluated primarily by monitoring temperature, oxygen concentration, and decomposition rate. Rapid dissipation of initially high odor levels and maintenance of low odor concentrations were observed for a number of yard waste mixtures and turning frequencies. Process conditions associated with maintaining low odor levels were identified.

Results showed that, even within the limitations of an open air turned windrow system, a level of control over process conditions through the manipulation of specific factors can be achieved which is compatible with maintenance of low odor levels. Application of the results permit the development of site-specific operational procedures to minimize odor generation. 

This is part four in a four-part series. For additional reading, go to part one, part two and part four.

Stuart Buckner, Ph.D., is president of Buckner Environmental Associates LLC, a consulting firm specializing in organics management.