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.