Public Release:  Odor index: A new tool to help minimize odor problems

Penn State

A Penn State-led team of environmental engineers has developed an Odor Index, a standardized, instrument-based process to rate the "smelliness" of an air sample and identify the offending chemical components in it so that they can be removed or controlled at the source of the emission.

Dr. Bradley A. Striebig, head of the environmental technology group at Penn State's Applied Research Laboratory and leader of the Odor Index development team, says, "Right now, we're using the Odor Index to monitor municipal wastewater treatment processing plants but it could also be used at pig farms, oil refineries, paper making factories, or, with some modifications, at landfills. We think it may be applicable, in general, where there is a chemical odor issue."

Seth Hepner, a master's degree candidate in environmental engineering and Striebig's student, presented a paper today (April 30) detailing the application of the Odor Index to a wastewater treatment plant in Ephrata, Pa., at the Water Environment Federation (WEF) Odors and Toxic Air Emission 2002 meeting in Albuquerque, N.M.

The paper is "Odor Generation and Control from the Autothermal Thermophilic Aerobic Digestion (ATAD) Process." Hepner's co-authors are Striebig, Dr. Raymond Regan, Penn State professor of environmental engineering, and Richard Giani, scientist, Pennsylvania Department of Environmental Protection, Division of Wastewater Management, who is assisting with the project.

Striebig explains that the team has combined off-the-shelf instrumentation, including a gas chromatograph, with a database of information about 20 different odorants to produce the Odor Index. Basically, the Odor Index relates the concentration of a particular odorant, or smelly chemical, with what a person experiences as an odor.

"We use the gas chromatograph to identify and measure the concentration of odorants in a particular air sample which we capture in a plastic-like (Tedlar) bag," he says. "Then, we compare the data on each component in the sample with the information in our database about what people experience as odors and the levels at which these odors are detected. The result is a Odor Index Value, a number that can be used in a comparative scale."

An Odor Index Value of 1000 would be just barely detectable by the human nose. A value between 1,000 and 10,000 would be where a human would begin to recognize an odor. A number in the range of 10,000 to 100,000 for an air sample would produce a fairly significant reaction in a human. A value between 100,000 and 1,000,000 would produce a strong physiological reaction.

Striebig cautions, however, that the numbers are comparative rather than objective. For example, a value of 10,000 and 10,100 would not be that different. Big changes in value, a change from 10,000 to 100,000, for example, mirror significant changes in human perception.

In the paper that Hepner is presenting at the WEF conference, he describes the use of the Odor Index at the first Autothermal Thermophilic Aerobic Digestion (ATAD) facility built in Pennsylvania. The plant treats the sludge, or semi-solid end product of its wastewater treatment process, in enclosed containers where heat accelerates the breakdown of the material into biosolids, a natural organic fertilizer.

The Odor Index analysis showed that the final biosolid product had a greater than 99 percent reduction in odors compared to the sludge upstream in the ATAD process. It also showed that the techniques the facility is using to prevent odors -- wet scrubbing and biofitration of the exhaust air -- resulted in greater than 99 percent reduction in the Odor Index Value.

Striebig says, "Air sampling and calculating Odor Indexes three times over the past year at this facility showed that it is producing a Class A biosolid end-product and that the means being used there to control odors are effective. As the sludge undergoes seasonal changes, continued monitoring will help plant managers to keep potential odor problems under control."


Odor Index development is supported by grants and contracts from the Environmental Protection Agency Regions 2 and 3, the Pennsylvania Department of Environmental Protection and the New Jersey Department of Environmental Protection.

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