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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.
Any suggestions of additional
questions for this section are welcome and should be directed
to the webmaster@odournet.com
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