Olfactometric sampling of odorous emissions: instruments and methods

UNI EN 13725:2022 is the reference for technical aspects and procedures relating to olfactometric sampling and analysis in dynamic olfactometry. 

The standard defines procedures for sampling and measurement of odour concentration, as well as dilution procedures and presentation of results.

It is on this normative basis that we carry out odour measurements and monitoring in our laboratory.

In this article, we focus on how to conduct olfactometric sampling according to the type of source (point, areal or volumetric).

Odorigenic emissions, as defined by Legislative Decree 152/2006 and subsequent amendments and additions, are ducted or diffuse emissions having effects of an odorous nature. In particular, as indicated by SNPA Resolution 38/2018, the term 'odour' refers to the sensation generated by the interaction of certain chemical compounds, present in a gaseous mixture and characterised by sufficient volatility, with receptors in the olfactory system.

The monitoring of odorous emissions can be defined as the set of activities that are necessary to fully define the main aspects related to the presence of odour in a living environment and/or to acquire the fundamental information to be able to set up and propose an adequate resolution response to this emission problem.

As indicated by Resolution SNPA38/2018, sampling is an element of fundamental importance: the quality of the analytical data and the evaluation of the resulting results depend, in fact, on the representativeness of the sample taken. The main purpose of sampling is fundamentally to obtain volumetric fractions of gaseous samples that are representative of the context that one wishes to characterise (typically the source or ambient air) and of the emission conditions of which one wishes to have specific feedback (for example, information on maximum and minimum levels rather than on the average emission level, which presupposes sampling on different time bases, which may extend from a few minutes to an hour and even more).

Odour sources may have several characteristics, including:

• time course of the emission, including emission peaks;
• mode of transfer of odorous substances from the source to the atmosphere (which may or may not be conventionally measurable, depending on whether the source is defined or diffuse)
• geometric configuration of the source, which may be point-like if present at a precise point, areal if present over a wider area or volumetric if it has a complex shape.

The conditions under which samples are collected, the duration and number, must be chosen so that they accurately reflect the odour source. 

The sampling phase requires a number of preliminary activities that are essential to ensure effective olfactometric monitoring.

For this reason, our technicians carry out the following activities before sampling:

• analysis of the territorial context in which odour problems occur and identification of the main odour sources;
• identification of sampling points and assessment of their characteristics;
• assessment of the constancy or variability of emissions over time in relation to the production cycle.

To ensure a safe and suitable environment during sampling, it is essential that the sampling location is easily accessible to the operator, allowing him to perform his task under the prescribed safe conditions. The sampling point must be appropriate to allow sampling and any other measurements required.

The selection of materials for olfactometric sampling is crucial to ensure the accuracy and reliability of results, following the specifications dictated by UNI EN 13725.

These specifications include:

• inertness: materials must be selected to minimise any possible interaction with the aeriform sample, ensuring its integrity during the sampling process
• smooth surface: the materials used must have a smooth surface to prevent the formation of traps or accumulation areas that could influence the composition of the sample
• odour-neutrality: materials must not have any inherent odour that could influence the olfactory perception of the sample.
• tightness: it is essential that materials exhibit low porosity and a low diffusion coefficient to prevent sample loss or entry of outside air during the sampling process.

Sampling equipment must be clean to avoid contamination of samples.

Samples, according to UNI EN 13725, must not exceed a storage time of 30 hours in order to minimise the possibility of sample alteration. 

The sampling strategy varies according to the type of source, distinguishing between sources

1. point sources
2. areal
3. volumetric.

The fundamental parameter to consider is the odour emission rate (OER), obtained as the product of the odour concentration by the gas flow rate. 

In the case of a point source, odour emission occurs from a single point, usually through a stack. In this case, the sampling consists of taking a fraction of the ducted aeriform.

If the aeriform to be sampled is under pressure, sampling can be direct by inserting a Teflon tube into the sampling bag in the duct. Otherwise, a vacuum must be created and a pump used to suck the air into the bag without it coming into contact with the pump or other materials that could alter its characteristics. 

The sampling period varies depending on the source and fluctuations in the odour flow rate. In general, three situations can be distinguished

• Non-constant emissions: it is recommended to sample several gaseous portions, each representative of a different emission condition, possibly during the most critical time of odorous emissions. It is important to verify the variability of emissions directly in the field with continuous instrumentation.
• Constant emissions: it is advisable to carry out at least three individual samplings in a representative time interval of at least 30 minutes.
• Variable emissions: a single sampling can be carried out to identify the most critical emission moment, or several samplings to assess the variability of the emission in different emission levels. These samplings must be analysed individually.

For emissions with well-defined production phases, the sampling methods must be adapted to each phase. If it is desired to highlight only the maximum emission levels of a specific activity, sampling may be concentrated in the phase considered most critical, after verification of its reliability.

In the case of areal sources, there are typically emissions from rather large solid or liquid surfaces. These surfaces may be active (with induced flow) or passive (without induced flow).

Active areal sources

Active areal sources (with induced flow) are characterised by a defined size and an effluent flow that can be controlled or controllable.

An example of this category are open biofilters. For sampling these sources, a static hood is used that isolates part of the emissive surface and conveys the flow into the outlet duct, where the sample is taken, following the same procedures as for point source sampling

The static fume hood consists of two bodies: the first is a truncated pyramid or hollow cone and the second, surmounting the first, is a cylindrical expulsion chimney. Openings are provided in the outlet duct of the hood to allow sampling and measurement of the physical parameters of the emission. The hood must be made of an odourless material.

During sampling, the hood is positioned over the emission surface to isolate the sampling point from the external environment and in particular to prevent the wind from diluting the emitted gas before it is sucked out of the sampling bag.

To obtain data representative of the entire source, it is necessary to take several samples at different points evenly distributed over the emissive surface. Each odorous gas sample is taken by inserting the PTFE tubing of the sampling bag into the inspection port, after sufficient time has elapsed to allow the odorous flow to internally fill the entire hood body.

Checking the uniformity of the flow across the emissive surface is also important in order to define the average emitted odour concentration, i.e. the average value which, multiplied by the effluent flow rate, gives the odour flow rate value.

Two possible cases can be distinguished

• active areal sources with homogeneous flow distribution for which the measured discharge velocities on the different partial surfaces differ by a factor of 2 at the most;
• active areal sources with non-homogeneous flow distribution.
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Passive areal sources

In passive areal sources, the flow is generated only by the transfer of matter from the surface to the surrounding air.

Landfills and tanks of sewage treatment plants are examples.

Assessing OER for these sources is complicated because it is difficult to measure a representative odour concentration and to define an accurate air flow rate.

Specific sampling methods called 'hood methods' are used, which isolate part of the emissive surface. For this type of sampling, we recommend the use of wind tunnel type hoods, which simulate the parallel flow of air without vertical mixing, collecting volatilised odour compounds for sampling.

Above the emissive surface, convective mass transfer occurs. Odorants mix with the gas stream and escape from the outlet duct from which the sample is taken.

The advantage is that the measurement can be obtained relatively easily and inexpensively.

The problem with this system is that in order to be able to correlate the experimental measurements with the actual emission capacity of the odour source, it is necessary to assess the aerodynamics of the fume hood. It is important to know the velocity profiles inside the wind tunnel in order to be able to express the emissions as a function of the average velocity over the monitored surface.

As far as the number of samples to be taken on a passive areal source is concerned, this must be sufficient to obtain data representative of the emission characteristics of the entire source. In general, it can be established that:

• for homogeneous passive areal sources (e.g. moving tanks of liquid materials), in which the emissive surface can be considered reasonably uniform due to mixing, it is considered sufficient to take a single representative sample,
• for non-homogeneous passive areal sources (e.g. landfill surfaces, waste heaps, compost or materials in general, etc.), the number of samplings shall be evaluated both in relation to the characteristics of the individual portions of the areal source, and in relation to their specific extension. 

Volumetric sources are those that emit odours from three-dimensional spaces, such as odour tanks, large industrial or commercial environments. Unlike areal sources, these occupy a three-dimensional space and it is not possible to define a specific flow.

Sampling from such sources is more complex because it requires the consideration of variables such as:

• the size of the emissive volume
• the distribution of odours within the space
• the speed and direction of the air flow.

These variables influence the dispersion of odours in the surrounding environment, so they are crucial during the sampling and analysis process. 

As indicated by the MASE Guideline, efficient sampling is essential to obtain meaningful results.

Correct sampling is essential in order to carry out the subsequent analysis phase of dynamic olfactometry and assess the efficiency of environmental odour abatement equipment.

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Bibliography 

Bliss P. J., Jiang K., Schulz T. J., 1995. The Development of a Sampling System for Determining Odor Emission Rates from Areal Surfaces: Part II. Mathematical Model. Journal of the Air & Waste Management Association 45, 989-994.

Capelli L., Sironi S., Del Rosso R., Céntola P., 2009. Design and validation of a wind tunnel system for odour sampling on liquid area sources. Water Science and Technology 59, 1611-1620.

Frechen F.B., Frey M., Wett M., Löser C., 2004. Aerodynamic performance of a low-speed wind tunnel. Water Science and Technology 50, 57-64.

Lucernoni F., Capelli L., Sironi S., 2017. Comparison of different approaches for the estimation of odour emissions from landfill surfaces. Waste Management 63, 345-353.

Delibera SNPA 38/2018, Metodologie per la valutazione delle emissioni odorigene Documento di sintesi, elaborato a cura del Gruppo di Lavoro 13 nell’ambito dei lavori del Programma Triennale 2014-2016 dell’SNPA – Maggio 2018

Decreto Direttoriale MASE 309/2023, Indirizzi per l’applicazione dell’articolo 272 – bis del D. Lgs 152/2006 in materia di emissioni odorigene di impianti e attività.