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Question 10
: How is odour impact assessment
carried out?
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 Auspuff, 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.
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