By Anna Bokowa and Magdalena Bokowa
Odour assessment methods are divided into two categories: assessments at potential sources, and ambient assessments at affected areas, which are usually residential areas located close to potential odour sources. There is no standard method for odour assessment, but it can include, or be a combination of, several approaches:
Source odour and/or specific compound testing with analysis and dispersion modelling can predict off-site odour concentrations. This is the most common method for assessing odours at the source.
Ambient sampling with odour panel evaluations is a very common approach to assess odours in residential and complaint areas.
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Odour monitoring, using portable instruments, such as the Nasal Ranger or Scentroid SM100, is a very inexpensive method for measurement and can be used as a screen tool.
Ambient monitoring using the grid method is commonly used in Europe. However, it is very expensive, requiring a large data base and therefore months of observations. Most data are based on one-person observations and this depends on his or her sensitivity. The data can vary easily if persons with different sensitivities towards odour perform this type of assessment.
Continuous odour monitoring using electronic noses can be very expensive, difficult to use and is not applicable for all types of sources. Also, sensors need to be changed frequently and calibration is required before use.
Continuous specific compound or a group of compounds monitoring can be applied to source or ambient monitoring. Continuous monitors typically measure concentrations for individual odorants or a group of compounds every few seconds and record the data as a one-minute average value.
Some of these systems are easy to operate, but most are complex, and installation and operation require extensive technical expertise. There are also some limitations such as the detection limits for the instrument. Some odours may be detected by a human nose, but not detected by the instrument. Problems may occur with interferences and false responses of the instrument.
Specific compound sampling followed by specific analysis requires that for different measured compounds a different collection media is used. Sampling is inexpensive but the analytical cost may be expensive, depending on the compound. Some odours may be detected by the human nose but are below the analytical detection level.
Community odour surveys are usually performed by screened and trained independent observers downwind from the potential source. The number of observers will depend on how large is the facility, the budget for the project, the terrain and other factors. Community odour surveys can be an effective alternative or supplement to source testing for odour. This is particularly true in cases where there are a number of potential odour sources that can affect a community, where sources are difficult to sample, or when sources are expected to vary with meteorological conditions. Usually observations are made periodically over an extended time frame.
A large data set is required to determine the odour levels at specific locations and under a range of weather conditions. This requires a long period of observations and therefore it can be very expensive. If community odour surveys are performed by community members, or by staff from a facility, it may lead to adaptation to particular odours, and could therefore tend to underreporting odour occurrence. To avoid this limitation, independent outside observers should be used.
In most cases, due to unavailability or for safety reasons, observations are not performed during early morning or late-night hours when odours could be at their worst. Therefore, it is easy to underestimate the actual situation in the affected area.
Resident observations and questionnaires for residents involve logging of odour observations by an individual, such as the intensity of the odour, character, duration and pleasantness. Date and time of the odour episode is usually also recorded, as well as environmental conditions at the time of the episode. This method is not expensive and is easy to use. It just requires short training and screening for residents. It may be helpful for initiating an investigation of potential odour or potential odour sources in the area.
Analysis of testing, evaluation and dispersion modelling
The general approach for odour testing at the source is to first select the potential source at the facility, collect odour samples, perform analysis using dynamic olfactometry, and then use dispersion modelling to predict off-site odour concentrations at sensitive receptors. These predicted by model odour concentrations could be verified by actual ambient odour sampling at sensitive receptors on the days of the odour testing at the source.
Several factors should be considered before or during assessments, including:
- Careful selection of all potential odour sources in the plant, including point, area and fugitive sources.
- Determination of any odour background before testing commences. It is important to determine any other potential sources in the area.
- Methodology used for collection of samples from sources.
- Number of collected samples should be representative of the actual process of the assessed facility.
- Determine if the process is continuous or batch.
- Odour analysis techniques.
- Dispersion modelling analysis.
There are different methods for odour sampling and they vary by jurisdiction. Compliance sampling in Ontario follows the MOE Ontario Source Testing Code, Method ON-6 Determination of Odour Emissions from Stationary Sources.
Different sampling techniques will apply to different types of odour sources:
- A point source is a single, identifiable source of air pollutant or odorant emissions. Point sources are characterized as being either elevated or at ground level. Point sources will have a defined exhaust diameter. Examples include stacks and vents.
- Area sources are two-dimensional sources of diffuse air pollutant emissions. The dimensions of these sources are either known or can be estimated. These include primary or aeration tanks at wastewater treatment plants, tailing ponds, etc.
- Fugitive sources can be any open doors or windows, and trucks waiting to unload or load odorous material.
At a point source during testing, odour samples are usually dynamically diluted with nitrogen. For this, a dynamic dilution sampler is usually used to collect samples. After collection of samples at the source, they are evaluated for odour detection threshold values (ODTV), which, together with volumetric flow rates measured at the source, result in the determination of odour emission rates from the source.
For some point sources where expected odour is low, a lung sampling method may be used for collection of the samples. Lung samplers contain a pump which creates a vacuum inside a sealed container (a vacuum chamber). This draws a source sample into the sample bag.
When estimating odour emission rates from area or fugitive sources, it becomes more complicated. There are some challenges when it comes to assessing odours from any area, or fugitive sources, mostly because of the difficulty in accurately measuring emissions from these potential odour sources. Therefore, careful selection of the method is important for proper assessment.
There are five methods commonly used to predict odour emissions from area sources. They are the flux chamber method, portable wind tunnel method, a back calculation with an air dispersion model method, static hood method, and the mass transfer method.
For fugitive sources, a back-up calculation method is commonly used for estimation of emissions from these sources. When estimating fugitive emissions, the following steps are usually required:
- Collect ambient samples within the cavity of the building or structure attached to the fugitive source.
- Evaluate the collected samples using dynamic olfactometry.
- Calculate cavity concentration.
- Calculate the dilution factor.
- Calculate odour emission rates using the formula:
Odour Emission Rate (ou / s) = Dispersion Factor (m3 / s) x Ambient Odour Concentration (ou / m3).
In order to predict off-site odour concentrations at selected sensitive receptors, the measured or estimated emission rates are used in the dispersion modelling analysis. Odour models can be classified according to their working principles. In Ontario, only two models are approved for regulatory purposes: AERMOD and CAPUFF. In order to run dispersion models for odour assessments, several inputs are necessary, such as emission and source parameters, including nearby buildings, meteorological data, terrain data and land use characteristics.
Ambient odour assessments
When it comes to ambient odour assessments there are different techniques and they may vary from jurisdiction to jurisdiction. In Ontario, the most popular method is ambient odour testing and dynamic olfactometry analysis.
Ambient sampling for odour assessment is typically conducted using the lung sampling technique, with the sample collection done in ambient air rather than within a source such as a stack.
This is one of the most common methods to assess ambient odour, provided the concentration is sufficient to give odour panelists’ responses at the lowest dilution levels of the olfactometer. If the odour concentration is too low for a sample to be evaluated by olfactometry, then the panelists can evaluate the sample directly from the sample bag.
The lung sampling procedures for ambient odour monitoring are similar to sampling at the source using the lung method, except that the sampling probe is located about 1.5 m above ground level. However, specific heights may be selected based on the nature of the monitoring program. Sampling periods depend on the jurisdiction (e.g., 10 minutes in Ontario) but can vary depending on the nature of the upwind source and meteorology.
Ambient odour monitoring
A field olfactometer, such as the Nasal Ranger or Scentroid SM100, directly estimates odour concentration in the ambient air without the requirement to collect a sample in a container.
The field olfactometer, which is used by one person at a time, draws ambient air into the instrument. The diluted sample is presented to the odour observer via a face mask and the observer indicates whether an odour can be detected at each dilution. The results from the Nasal Ranger are used to calculate the detection to threshold (D/T) which is the number of dilutions needed to make the odour ambient air non detectable.
All field olfactometers are based on individual one person readings for one-minute maximums, whereas ambient sampling for laboratory olfactometry generally occurs for a longer time. Due to this short time frame, accurate results cannot be guaranteed; therefore, it should be considered a screening tool only. As one observer operates the instrument, results depend on their sensitivity. In addition, the observer likely breathes odorant before using the field olfactometer, increasing the opportunity for odour fatigue.
There are different methods for assessing odours; however, they will depend on several factors.
Careful selection of the methodology to be used should be made before any assessments are performed.
Consideration should be given to assessing the location’s jurisdiction; the appropriate standards that need to be met; the amount of time available for assessment, whether it be days, months or years; and the financial budget allowed for the project. Careful thought should also be given to the type of sources tested (point, fugitive and area sources) and the difficulties in being able to assess them.
Lastly, any investigation should attempt to yield the most accurate results and therefore a variety of assessment methods can and should be used so that underestimation does not occur.
Anna H. Bokowa and Magdalena A. Bokowa are with Environmental Odour Consulting Corporation. Email: firstname.lastname@example.org, or email@example.com. (References available upon request)
Read the full article in ES&E Magazine’s June/July issue below.