Olfactometry is a great tool to translate into a quantitative and objective value the complexity of odor perception (see blog: Are odors additives in terms of the intensity?)
Dynamic dilution olfactometer
However, in some circumstances, real-time values are required to provide the dimension of odor fluctuations of a process that olfactometry can hardly provide. Indeed, olfactometric measurements require sampling and lab analysis that practically or financially limit the amount of values available to understand the odor fluctuations out of an odor source.
Let's take the example of a biofilter exit treating composting odors. In this case, the process generates pulsed air flow for windrow forced aeration.
What do we see here?
First, using today’s standard ‘manual’ odor sampling and measurement method (blue dots), we measure three separate odor concentrations at different points in time, then connect the dots to obtain rectilinear segments.
In this case, looking at a biofilter designed to limit odor output to 400 odor units, the ‘manual’ method (olfactometry) would indicate that the biofilter is working fine and the neighbors would perceive no odor. However, there are still odor complaints; no one knows why.
Then we install an electronic nose OdoWatch (magenta curve); this gives us continuous measurement; and we now see that there are odor peaks up to 750 odor units.
The mystery is solved. Now we can act to deal with the problem. Measuring daily average odor concentration may doesn’t work. The human nose senses peak values not average values. Thus the monitoring system needs to measure odor concentration at very short intervals. OdoWatch measures odors every second.
COMPOSTING PLANT DESCRIPTION AND ODOR SOURCES
The Toulouse-Ginestous composting site (in France) treats up to 60 tons per day of 30% solids dehydrated WWTP sludge. The sludge is amended with 25 to 50%v bark and screening residues (non degraded bark, recovered from the mature compost). All composting operations are performed in a 10 000 m2 building.
This mixture is composted as 6 m wide windrows with a height of 3 m. Time-sequenced negative aeration is used over a 6 week residence time. The material is then transferred to another indoor platform where it undergoes an 8 week maturation period. The material is then screened to yield equal quantities of mature compost and screening residues.
Figure 1 Composting plan
ELECTRONIC NOSE ODOR MONITORING SYSTEM
The abatement equipment implemented at this facility was properly designed to provide effective reduction of anticipated odor emissions (combined scrubber/biofilters system). However, some processing upsets or operating conditions may trigger effectiveness reductions that can go unnoticed by the plant operator. Unfortunately, some processes can have low odor emissions most of the times and, under certain weather conditions or trigger events, become major nuisance sources.
In the summer of 2005, the plant acquired an electronic nose system in order to further their proactive approach towards odor management: in other words, to detect the potential of odor impacts before they actually affect the population.
Figure 2 View of the biofilter (a), weather station (b) and e-nose (c)
The system configuration was established to ensure monitoring of each key composting process and sources, not only those emitting to atmosphere: 1 e-nose monitors the co-products area, 1 e-nose analyses the air aspiration duct of the composting platform, 1 e-nose is located in the maturation area and 1 e-nose tracks the exhaust of the biofilter.
CONTINUOUS MONITORING SYSTEM RESULTS
The composting plant does not operate in a steady state mode. The composition of the air sent to the odor treatment system will vary continuously depending on the operations, the overall composting situation (number of windrows, stage, etc.) and with the opening of doors. The following figure presents a log of the odor concentrations for the maturation and composting areas, the co-products building stack and the outlet of the biofilter for the first week of June 2006.
Figure 3 – E-nose process and biofilter monitoring
The monitored values indicate significant odor fluctuations for maturation, fermentation and the biofilter. The odor spikes in the maturation area (+) are typical of the patterns induced by frequent door opening and closing during the day. This was expected since the e-nose tracking the maturation area is located near the door used for removal of the finished compost from the building. The information supports significant odor emissions through the opening of doors.
The composting area e-nose (×) is located in the negative aeration duct. The air is then sent to the abatement system, where the exhaust is tracked by another e-nose (○). The behavior of the composting e-nose and the abatement system e-nose present a strong correlation, with a sinusoidal pattern.
Around 7AM each morning, the plant receives up to 60 tons of sludge from the WWTP. This peak reception period creates an immediately measurable rise in odor concentration in the composting area, as evidenced by the daily rising slopes on the blue (×) curve above. The composting odor output (negative aeration system) is in effect the input stream to the abatement system. Small oscillations (of about 20% with respect to the mean value) of the composting output trigger large, lagging oscillations at the exhaust of the biofilters (of about 100% with respect to the mean value). The delay is of about 2-3 hours. This could be indicative of some adsorption by the biofilter media as the odor input to the biofilters rises, followed by a saturation and a delayed reduction of biofilter effectiveness. This type of information is extremely useful for optimized biofilter sizing for odor reduction and assessment of true odor abatement effectiveness.
The co-product odor (-) is relatively constant due to limited activity in the building.
These results illustrate that significant fluctuations exist and are not limited to the odor sources but are also present in the abatement systems, such as biofiltration. Continuous monitoring represents a valuable tool for odor management and will provide better knowledge of odor source behavior compared to discrete samples. Discrete odor sampling cannot ensure representative quantification of the possible range of emissions required for thorough odor impact assessment studies.