FREQUENTLY ASKED QUESTIONS A summary of common questions relating to odour measurement and assessment are listed below. How do you measure odour? The techniques available to measure odours can be broadly divided into sensory and chemical techniques. Sensory techniques utilise human assessors to assess odour. The most commonly applied sensory technique is olfactometry, which is used to measure the concentration of an odour in terms of European Odour Units (ouE/m3). Techniques also exist for assessment of odour character (e.g. triangular testing and odour mapping), intensity and/ or relative pleasantness/ unpleasantness (e.g. hedonic tone analysis). Methods classified as chemical utilise conventional analytical techniques to measure the concentration of specific odorous compounds within the sample gas. This can be achieved by Gas Chromatography/ Mass Spectroscopy analysis (GC/MS); the use of specific chemical analysers (e.g. chemical cells for hydrogen sulphide analysis); wet chemical techniques (mercaptans); indicator tubes and electronic noses. Which method should I use, sensory or chemical? . The choice of measurement technique is dependant upon the objective of the measurement exercise. The advantage of sensory methods is that they provide a direct link to how odours are perceived by humans. This is particularly useful for studies that involve assessment of annoyance or nuisance, or indeed how effective odour control techniques are at mitigating such issues. The disadvantage of the approach is that it is non-specific, and does not identify specific chemical(s) responsible for the odour. I.e. the approach measures the total odour of the sample in terms of odour units (ouE/m3). Sensory techniques such as olfactometry are commonly used for: - Quantifying odours in terms that can be used to assess the impact of the odour on human subjects (e.g. annoyance or nuisance assessments). - Assessing the efficiency of odour abatement systems in terms of total odour. The key advantage of chemical measurement techniques are ease of use, and the fact that the techniques are capable of identifying and quantifying specific chemical odorant compounds. The disadvantage of this approach is that it does not provide any insight into the intensity or offensiveness of odours in human terms (the commonly applied assumption that the contribution of specific odorants can be simply added to assess the odour concentration of the mixture is rarely applicable). Hence, chemical techniques are not suitable for measuring odours for impact assessment purposes. These techniques are commonly used for: - Defining design criteria for odour abatement plant. - Evaluating compliance to specific chemical emission limits (e.g. conditioned in License authorisations). - Measurement of surrogate compounds that can be used to predict odour impact (where the relationship between the target compound and odour is known). What is olfactometry? Olfactometry is aimed at characterising environmental odours, relevant to human beings. As no methods exist at present that simulate and predict the responses of our sense of smell satisfactorily, the human nose is the most suitable ‘sensor’. Objective methods have been developed to establish odour concentration, using human assessors. A European standard applies to odour concentration measurement: - CEN EN 13725:2003, Air quality - Determination of odour concentration by dynamic olfactometry. Olfactometry employs a panel of human noses as sensors. A human nose can detect odour at concentrations well below the sensitivity levels of chemical analytical methods. In the olfactometry testing procedure, a diluted odorous mixture and an odour-free gas (as a reference) are presented separately from two sniffing ports to a group of six panellists simultaneously. In comparing the gases emitted from each port, the panellists are asked to report the presence of odour together with a confidence level such as guessing, inkling, or certainty. The gas-diluting ratio is then decreased by a factor of two (i.e. chemical concentration is increased by a factor of two). The panellists are asked to repeat their judgment. This continues for a number of dilution levels. The responses of the panellists over a range of dilution settings are used to calculate the concentration of the odour in terms of European Odour Units (ouE/m3). In this way, the concentration of different odours can be measured in directly comparable terms. How smelly is too smelly? Once an odour is released from an odour source such as a chimney or a compost heap, it is dispersed and diluted in the atmosphere. The concentration of the odour on release, the magnitude of emission, and the degree of dispersion and dilution that the odour undergoes in its journey from the source to the receptor (e.g. a neighbouring house) are the key factors which influence whether or not the odour is perceptible at this location. Odour impact is defined in terms of annoyance. The fact that an odour is perceptible at a given location is not necessarily mean that the odour will cause annoyance. Annoyance is a symptom that is dependant upon the following factors: - Concentration of the odour - Offensiveness of the odour (a mixture of odour character and hedonic tone) - Duration of exposure to the odour - Frequency of the odour occurrence - Tolerance and expectation of the receptor How do you assess odour impact? The most commonly applied approach to assess impact is to use mathematical dispersion models to predict the downwind odour concentrations on the basis of odour emission rates, topography and meteorological data. The results can be checked against odour impact criteria to derive an odour impact area. Within this area, it may be expected that residents or other receptors will experience some degree of odour annoyance. Odour impact criteria are typical expressed in terms of a concentration (e.g. 5 ouE/m3) and an averaging time and duration of exposure (98th percentile of hourly average concentrations in one year), for odours generated by the facility in question only. Odour dispersion modelling provides a benchmark/yardstick for the prediction of odour impact from odour sources. It is best used when a comprehensive study is carried out and then the same methodology is used to compare the likelihood odour impact. It is important to ensure criteria applied to assess impact is validated for the model that is used. Odour Impact Assessment can provide an effective tool for the following purposes: - Preparation of environmental management plans - Development of appropriate regional and local planning and development control instruments - Odour regulation - IPPC and planning applications Can I use olfactometry to measure odour in ambient air (e.g. to assess compliance to an annoyance criteria)? No. Olfactometry is not a suitable technique to determine compliance to an annoyance criteria by sampling ambient odours for the following reasons: - It is not possible to determine compliance to criteria that consider the contribution of odours from the site in question only by collection of ambient olfactometry samples at the boundary or off-site. This is due to the fact that odour measurement by olfactometry does not differentiate between odour sources, it merely provides a measurement of the odour concentration of the air at the point of collection. In an urban or rural environment, the ambient air is likely to have a background concentration of between 10 and 100 ouE/m3, which is due to the sum of all contributing sources of odour in the area (e.g. traffic, agricultural activities, natural odours associated with woodland and grassland, other industrial sources etc). It is therefore technically impossible to identify whether a 2 ouE/m3 process odour condition has been achieved or breached using this methodology. - The lower limit of detection of olfactometry is approximately 20 to 30 ouE/m3. Hence, even if it was possible to identify only the process contribution, the resolution of olfactometry is insufficient to allow compliance to the condition to be determined by collection of ambient samples. Olfactometry should be used to measure odours collected ‘at source’. E.g. direct from the chimney releasing the odorous air. Are different types of odour more or less likely to cause annoyance? Yes. The offensiveness of the odour perceived by a receptor is a factor that will determine the likelihood of annoyance. More offensive odours cause annoyance at lower concentrations, while less offensive odours cause annoyance at higher concentrations. The range between the annoyance criteria applied for the most offensive and least offensive odours is typically in the region of a factor 10. For example, in the Netherlands the most stringent criterion is 0.5 ouE/m3 to 1 ouE/m3 (for extremely offensive odours generated from rendering operations) and least stringent is around 8 to 10 ouE/m3 (for relatively pleasant odours generated by bakeries etc). It should be noted that even pleasant odours (bakeries, perfumeries) can cause annoyance if the concentration, frequency and duration of exposure is great enough.  How can odours be controlled? Odours can be controlled by modifications to process to prevent odour generation at source, or containment, collection and treatment of odorous emissions using end-of-pipe techniques. The appropriateness and applicability of-end-of pipe solutions will be dependant upon the characteristics of the gas stream to be treated. The following technologies are available: - Biological treatment, which treats odours bio-chemically, encouraging the proliferation of various micro-organisms, which, in turn, facilitate desirable chemical reactions to oxidise odours (e.g. biofilters and bioscrubbers). - Adsorption, which removes odorous species by capturing them onto an adsorbent solid surface. (e.g. activated carbon, charcoal). A subset of this technique is dry chemical scrubbing, which incorporates some form of oxidising chemical into the adsorption matrix to promote chemical oxidation of trapped odorant chemicals. (e.g. carbon impregnated with chlorine dioxide/permanganate). - Absorption (scrubbing), which involves the mass transfer of odorants from the gas-phase to the liquid-phase. The scrubbing liquid often contain oxidising chemicals specific to the air-stream to be treated (e.g. scrubbers for treating hydrogen sulphide typically contain caustic soda/hypo chlorite). - Incineration (thermal or catalytic), which involves oxidation of odorous compounds by the application of heat. Other ‘innovative’ techniques are also available, including the use of odour counteractants/masking agents, plasma technology, surfactant enhanced adsorption, ozone and ultraviolet techniques etc. Why Odournet? The accumulated experience of carrying out thousands of odour surveys and consultancy projects puts Odournet in a unique position, having the benefit of hindsight, to provide independent answers and solutions for odour annoyance management to its clients. The focus is always on achieving the most cost effective solution for a specific problem. Who are our clients? Odournet companies provide advice to industry, consulting engineers and government bodies. The clients of Odournet range from global multinationals to small and medium sized firms, national and regional regulatory agencies, local government and larger firms of environmental consultants buying in our specialised expertise. How do we work? Odournet brings together odour specialists from complementary scientific disciplines to provide a comprehensive suite of odour consultancy services. The key features of the Odournet approach are as follows: - The scope of each study is carefully designed to meet the specific objectives as communicated by the client. - Odournet utilises consultants with direct experience of the industry in question. This ensures that studies are performed with knowledge of the concerns and issues appropriate to each sector. - Odournet continuously monitors developments in the UK, European and International arenas to ensure, as far as possible, that the adopted approach reflects the state-of-the-art. Toute remarque ou suggestion pouvant améliorer le contenu de ce site sont les bienvenues et peuvent être adressées à webmaster@odournet.com