Wind movement in the atmosphere
carries away odorous gases emitted from a source. Odour annoyance
occurs when a person exposed to an odour perceives the odour as unwanted.
Significant odour annoyance may trigger a complaint to a regulatory
authority. The major factors relevant to perceived odour annoyances
are:
- Offensiveness (a mixture of odour concentration, odour intensity, odour character and hedonic tone),
- Duration of exposure to the odour,
- Frequency of the odour occurrence,
- Tolerance and expectation of the receptor.
Using an air dispersion
model, such as Ausplume or Calpuff, it is possible 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 dispersion modeling
provides a benchmark/yardstick for the prediction of odour impact
from odour sources. It is best to be 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 validate the results of
odour dispersion modeling be validated using an odour community survey
or an odour complaint history.
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.
In
essence, odour Impact assessment uses inputs of source odour concentration,
ventilation rate and emission strength (odour emission rates), topography
information together with meteorological data (one-year data), and
an air dispersion model (eg Ausplume or Auspuff) to model odour dispersion
about the source. Odour impact areas can be defined by plotting isopleths
of odour concentration corresponding to selected values for odour
impact criteria. The approach can be illustrated in the following
flow chart:
Odour Impact assessment flow chart
Odour impact criteria are parameters derived from experimental results and
scientific evidence. Using odour dispersion modeling together with
odour impact criteria, odour impact areas can
be defined. Within an odour impact area, typical receptors (e.g. residents)
may be expected to experience a
certain degree of
odour nuisance . Odour impact criteria are not ambient
air quality standards but rather provide a scientifically derived
benchmark for the making of informed decisions in planning, design,
environmental management and regulation.
A wide range of odour impact criteria has been reported and the question
could be asked as to why there is so much variation. One reason is
that in recent years, modern performance based forced choice dynamic
olfactometry has greatly improved the sensitivity of odour measurement
but as yet not all criteria values are based on such measurement.
For instance, the butanol threshold measured using a three port IITRI
(Illinios Industrial Triangle Research Institute) olfactometer, ranged
from 80 - 200 ppb while modern dynamic olfactometry is capable of
measuring butanol threshold levels from 20 to 80 ppb. Assuming that
the same sensitivity applies to environmental odour samples, comparable
odour impact thresholds could be 3 - 20 times lower. Correspondingly,
a nuisance threshold determined as 1 ou/m3 using the less sensitive
earlier equipment could be rated at 3 - 20 ou/m3 using modern equipment.
In summary, the use of advanced olfactometer based methods could result
in nominally much higher odour concentration limits being specified
in odour impact criteria.
