A team of environmental engineers at Pennsylvania State University 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.
“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,” said 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. “We think it may be applicable, in general, where there is a chemical odor issue.”
Striebig explained that the team has combined off-the-shelf instrumentation, including a gas chromatograph, with a database of information on about 20 different odorants to produce the odor index. Basically, the odor index relates the concentrate 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,” Striebig said. “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 1,000 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 of 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. But changes in value, a change from 10,000 to 100,000, for example, mirror significant changes in human perception.
In a paper presented at the Water Environment Federation Odors and Toxic Air Emission 2002 meeting in late April, the use of the odor index at the first Autothermal Thermophilic Aerobic Digestion (ATAD) facility built in Pennsylvania was described. 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 biofiltration of the exhaust air – resulted in greater than 99 percent reduction in the odor index value.
“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,” stated Striebig. “As the sludge undergoes seasonal changes, continued monitoring will help plant managers to keep potential odor problems under control.”
Newsline - August 2002 Render