Characterization via chemical analysis and olfactometric characterization (sensory) both have advantages and drawbacks. Despite the known benefits of the conventional chemical analysis characterization (accuracy, reproducibility, familiarity, performed by numerous analysis labs, etc.), olfactometric measurements are generally to be preferred due to the subtleties of "real" odors (multicomponents):
- Odors are normally complex blends of numerous compounds with very low concentrations (perception thresholds often bordering detection limits of established methods);
- There are interactions between mixed odorus compounds, unfortunately there are no models to predict this synergy on the perceived odor which can be positive (neutralization) or problematic (co-amplification)... 1 + 1 IS NOT 2 here!;
- Although perceived as less expensive initially, chemical analysis for multiple compounds can become costly if many different analysis are required and especially if multiple on-site interventions are required (unknown levels often leads to under/over shooting the analysis' range).
The main advantage of olfactometry for odor measurement is that it actually derives a value from the direct correlation between the odor and the sensitivity of the detector being used (and the subject of the odor management efforts), i.e. the human nose. However, there are limitations to the method which can be mitigated by proper sampling approaches (sensitivity to humidity in the sampled stream, variability, degradation of the sample, etc.). There are established sampling protocols and analysis methods (i.e. EN 13725, ASTM E679-91, CUM, Probit) that guide the approach.
Olfactometer and jury in action
Through the use of a dynamic dilution olfactometer, an odor sample can be analyzed for its:
- odor concentration;
- hedonic tone;
- odor intensity;
- perception thresholds.
There are situations where a combined approach (chemical and olfactometry) may be interesting (i.e. known predominant compound) or required (i.e. odor abatement equipment sizing/design).
One might think he knows his plant emits odor. He doesn’t need a study to tell him this.
Head in the sand – Do nothing
No diagnostic device can cure the patient all by itself. A thermometer does not end your fever. With odor assessment, you will know if and when you have a problem, how big the problem is, its origin – in short, what needs to be done, exactly. There are a lot of solutions (medicines) out there, but which one to take, when, and how much of it – that is the question.
Nowadays, real-time odor monitoring systems using electronic noses can help plant manager to know better which sources are contributing the most and but their money where they get the best bang for the buck when it is time to invest on odor control.
OdoWatch Continious odor emission and impact monitoring
Benefits of using Real time odor monitoring with enoses:
- Manage and prevent odor complaints
- Positively impact community relationships
- Accurately diagnose and treat causes of odor – quickly
- Saving plants money on labor and associated investigating costs
- Prioritize odor control projects
- Provides precise data to justify capital projects
- Right-size plant & equipment
- Decreasing plant investment costs
The perception of odorant molecules in the human nose is achieved thru an array of sensory cells called the olfactory epithelium. These cells act as biological sensors that react to the presence of odorant molecules (for example aldehydes, ammonia, butanol, dimethyl sulfide, toluene, + 10000 more!) by generating electrical signals. These signals are sent to the brain thru the olfactory cortex and travel along the olfactory nerve.
The olfactory epithelium: a network of sensors
Our biological odor sensors are not specific to single chemicals but are specialized for groups of chemical compounds. The cells therefore react strongly to the chemicals for which they are specialized but also to others chemicals, especially if present at high concentrations.
For example, let’s say we are exposed to an odor sample with hydrogen sulfide (H2S -> rotten egg odor) at very low concentration. The blue cells respond strongly enough to generate a conscious detection of this odor when the H2S concentration exceeds 0.4 ppb (the H2S perception threshold). A higher concentration of H2S would also trigger the red cells, which are specialized for amines, to send a signal to the brain.
Typically, when perceived in ambient air (for example close to a composting site, sewage treatment plant or landfill site), a mixture of numerous odorous compounds are responsible for the recognized odor. The brain creates odor images composed by odor fingerprints which are specific to each grouping of odors. The odor fingerprint is the result of all the electric signals generated by the olfactory epithelium cells.
The electronic nose mimics the same process. 16 non specific gas sensors are exposed to a sample of air and, just like our own cells, are specialized for different families of chemical compounds. After appropriate calibration, the e-nose quantifies (continuously) the odor concentration in o.u./m3 (odour unit per cubic meter).
Example of an e-nose used to monitor WWTP odors.
So you’ve taken the first step by performing an odor diagnostic and now know where your odor emissions originate from… you now want to address these odor sources to reduce or prevent off site odor complaints. Off course, you want to address them in the most efficient way, which means addressing the sources which have the largest off site impact most of the times, and those are not necessarily the sources with the largest emissions. This is where a detailed odor impact assessment comes into play.
The odor impact assessment will use the information from the site’s odor emissions, their variability, location of the sources and potentially impacted neighbors, local meteorology, topography, landuse, etc. to assess the odor impact of a site’s odor sources by using a approved model (Aermod, Calpuff, etc.). The odor emission rates (Odor unit per seconds : o.u./s) are entered with the other inputs and result in modeled odor concentrations (Odor unit per m3 : o.u./m3) at specific points on a specific averaging period. A number of parameters can be calculated from the data: maximum odor levels, average odor levels, number of odor exceedances of a specific odor level (odor criteria, odor regulation or odor target), percentiles, etc. Outputs are often presented as odor contours or odor plots which provide a visual presentation of the impact. This data can be used to establish an odor management plan or to communicate your site’s odor reduction performance after modifications. The odor impact study can also be geared toward assessing the need or supporting the strategy behind the implementation of an odor monitoring system. As for the diagnosis, the results may surprise you: your odor source ranking may indicate a totally different odor footprint than that anticipated. More information can be found on Odotech’s website
If you are facing odor issues or you are putting the final touches to your concept for a process with known odor risks… you are likely to wonder how tall your stack should be. Many parameters come into play such as the exhaust odor emission rate, gas temperature, stack diameter, building influence, topography, vertical velocity and of course location of the potentially impacted neighbors. You may have consulted the US-EPA’s GEP guidelines which present an approach to estimate the height so building effect is negligible… however, that height may not necessarily be enough to prevent odor issues or may be overly conservative from some situations.
The off site odor impact of the stack’s odorous emissions can be assessed using a dispersion model such as Aermod. The odor emission rate can be measured (source sampling and olfactometry) or estimated through the use of adjusted odor concentrations from similar sources. The data output from the model can be used to establish the optimal height by considering odor impacts, their frequency and of course the incremental costs for the proposed heights (multiple scenarios). The model can also provide valuable information on the odor risks, especially considering that while the odor plume may be lower in concentration it can also hit a more densely populated area than the previous setup. A rational and reflected approach is prescribed since there are other aspects that most also be factored into the decision, such as : local regulations which may dictate stack characteristics, potential impact on the process (throughput, contamination), bearing capacity limitations of your infrastructures and potential to generate new issues (noise, airborne particulates when converting from a capped stack). More information can be found on odor impact studies on Odotech’s website. Realtime odor impact monitoring tools can also represent an option to evaluate odor dispersion (how far odors are travelling) and support stack design modifications according to off-site odor levels.