Connections: Stink

Sally Brown

Sally Brown
BioCycle February 2019, Vol. 60 No. 2, p. 31

There is an upscale wine and cheese shop in Tacoma with a sense of humor. It is called Stink, potentially out of a fondness for some of the riper cheeses it stocks. I love going to Stink as they have good salads and sandwiches. The name makes me smile and is distinctive enough to help me remember it. Likely the same could not be said for a composting facility. I can just picture the ad: Stink Compost — come load all you want and bring our stench home to your garden. I would wager that approach wouldn’t move much material. Also it likely wouldn’t make the neighbors happy.

Odor at composting facilities has been a major issue, responsible for multiple lawsuits and expensive facility closures. While we have a general idea of what causes odors, we are not always as effective as possible at reducing or eliminating them. A number of factors make controlling odors problematic. One is measuring them. The same way that a ripe gorgonzola is delicious to one person and repulsive to another, different people have different sensitivities and reactions to odors.

Odors also vary greatly with time and location. Wind speed and temperature are critical variables as well. And if you work at a composting facility, you are likely somewhat less sensitive to the odors than a new neighbor might be.

Traditional Odor Measurement

The current standard practices for measuring odor involve capturing a sample in a bag and bringing it to another facility for analysis using an expert panel (trained noses) or a machine called a gas chromatograph. That bag can contain air passed through it for minutes or over the course of days or hours. The answers you get from either method will represent the average across whatever time period was sampled. Peaks and valleys will be lost in these methods.

Both odor panels or chromatography analysis are used in regulatory standards or approved protocols. Panelists quantify odors based on serial dilutions of known standards (i.e., known quantities of particular compounds are put in the bag to smell so the analyst knows exactly what the odor panel is smelling). This is a qualitative approach and not the best for understanding mixtures of smells. It gives results like “a lot of blue cheese” or “much less blue cheese” but has a hard time distinguishing between a mixture of gorgonzola and limburger.

Gas chromatography (GC) provides a quantitative mixture. The odors in the gas are often captured on an absorbent fiber. The fiber is placed into a heating chamber that makes the chemicals responsible for the odors volatile again. They are carried through to a separation phase that sorts them by their different properties. These separate gasses are then heated to show their own chroma. The size and shape of the individual peaks are then identified based on known standards of individual compounds. This approach gives you a hard answer that includes taking apart different mixtures and giving you the precise recipe used to make them. Think of it as a deconstructed cheese fondue. It also costs as much as many cheese fondues at very fancy restaurants. Thousands of dollars per sample is what you can expect. Using a GC is not for amateurs. You need to identify an appropriate lab, make sure a sample is collected appropriately and that the lab follows rigorous standards for quality assurance. There is often a significant turnaround time between sample submission and having the fondue delivered to your table.

Because of the high cost and time delay in getting results, most facilities don’t send out samples for GC analysis on a regular basis. Time and temperature measures are one thing — the meat and potatoes of composting — but the fondues are left for special occasions or regulatory requirements.

Electronic Noses

Another option is on the table — one that would let you have fondue a lot more often. Electronic noses have been around for several decades. They started out as a hard to find Velveeta equivalent. As we get better at technology and how to analyze large amounts of data, they are evolving to something much closer to a fine gorgonzola. An electronic nose consists of different sensors that are made of different materials and often have specific coatings to make them sensitive to particular volatile compounds. When the particular compound hits the sensor it reacts and sends a signal to a computer. Just pretty much the same way your nose reacts and sends a signal to your brain.

To use one of these sniffers at a composting facility, it has to be placed inside some type of container. Air is fed through the container. The electronic noses work best if the humidity and temperature inside the container is controlled. After that, the nose will sample as often as programmed. There are several articles in the peer review literature on training electronic noses to detect and quantify odorous and other volatile compounds from composting operations. An early study quantified odors with an electronic nose and compared them to results from an odor panel (Sironi et al., 2007). The nose and the odor panel had essentially the same results.

Another study compared the ability of electronic noses, set up within enclosed composting units, to measure volatile organic compounds and compared the results to measures made with a conventional gas chromatograph (Delgado- Rodríguez et al., 2012). It turns out that the nose knew almost everything. Results from the two were indistinguishable except for compounds that the GC detected at very low concentrations. Finally, researchers set up noses at a composting facility and measured VOCs every 30 seconds for weeks (Shen et al., 2012). They saw how VOC emissions changed when aeration was turned off and how time reduced emissions. If you could do that without breaking the bank you might be able to develop process controls to really make a difference.

These noses can be trained. You can place them in different parts of your composting operation. If properly calibrated and properly set up, these noses offer the potential to give real time information that can be used to fine-tune your process. Ideally this can eliminate any hint of a stench in your operation and leave you with the aroma of fresh soil. Think “earthy” and “woody” rather than “putrid” or “fishy.” This is a technology with great promise — absolutely nothing cheesy about it.

Sally Brown is a Research Associate Professor at the University of Washington in Seattle (slb@uw.edu) and a member of BioCycle’s Editorial Advisory Board.

References

Delgado- Rodríguez, M., M. Ruiz- Montoya, I. Giraldez, R. López, E. Madejón, M.J. Díaz. 2012. Use of electronic nose and GC-MS in detection and monitoring some VOC. Atmospheric Environment. 51:278-285.

Shen, Y., T-B. Chen, D. Gao, G. Zheng, H. Liu, Q. Yang. 2012. Online monitoring of volatile organic compound production and emission during sewage sludge composting. Bioresource Tech. 123:463-470.

Sironi, S., Capelli, L., Céntola, P., Del Rosso, R. 2007. Development of a system for the continuous monitoring of odours from a composting plant: Focus on training, data processing and results validation methods. Sensors and Actuators B. 124:336-346.

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