Odour impact criteria

 

To use dispersion models for odour emission two different approaches are under discussion. The first is the use of a conventional dispersion model, which delivers mean values for an integration period of 30 to 60 minutes. Most of these models are used for regulatory purposes. The disadvantage of such models is the underestimation of the odour concentration at a receptor point, depending on the stability of the atmosphere and the travel time (distance) of the emission. This underestimation has to be compensated by the applied odour impact criteria which define the proportion of time which can be exceeded by a certain odour concentration. The second approach is the assessment of the odour concentration over the duration of one single breath (about 4 to 5 s) by using a so-called peak-to-mean ratio. Such an assessment of the maximum expected odour concentration mimics the odour sensation of a human nose in a more realistic way. In this paper we used the Austrian Odour Dispersion Model (Schauberger et al., 2000 and 2002a) which is based on the regulatory Gaussian dispersion model and a module which takes into account turbulent mixing, to calculate the peak-to-mean ratio. The peak-to-mean ratio is reduced with increasing distance from the source using the wind velocity and the stability of the atmosphere. In Germany a constant peak-to-mean ratio of 10 is used, leading to an overestimation of the momentary odour concentration especially for larger distances.

The definitions of the various national odour impact criteria differ to quite an extent. Miedema and Ham (1988) and Miedema et al. (2000) found a strong relationship between the 98 percentile of the odour concentration and the percentage of the highly annoyed neighbours. They used an ambient odour concentration for an integration time of 1 hour, calculated by a dispersion model without consideration of the peak-to mean ratio. In Germany, the odour impact criterion is defined by an exceedance probability of 10% for a threshold of 1 OU/m³. To apply this odour impact criterion, the calculated odour concentration (one hour mean value of the regulatory dispersion model) is multiplied by a constant factor of 10.

This demonstrates that the representative time scale for the ambient odour concentration (between 60 minutes and some seconds), calculated by the dispersion model and the odour impact criteria are not independent. An underestimation of the perceived odour concentration by neglecting the peak to mean ratio is compensated by a strong criterion (Miedema et al, 2000), whereas an overestimation of the perceived odour concentration e. g. by a constant factor 10 (Germany) is compensated by weaker criteria.  The source type (area or point; height), the distance between source and receptor, and the stability of the atmosphere are the main variables of the peak to mean ratio (Schauberger et al., 2000a and 2002), causing its decrease with distance, static stability, and wind speed. The Environment Agency, UK (2002), however, suggests to just use the 1 hour mean value as in Miedema et al. (2000), without correction by a peak to mean factor.

Apart from the exceedance probability, the odour concentration threshold of the impact criterion is of importance. The odour is measured by the human nose as a sensor by comparing a diluted odour sample with odour free air. This means that the detection threshold of 1 OU/m³ can only be perceived in an odour free environment (laboratory). Therefore the perceived odour concentration in the field must be higher than 1 OU/m³ to be distinguished against the background concentration. Field experiments must be designed such that an odour source can be distinguished against the background odour. Nicell (1994) assumes an odour concentration of 3 OU/m³ to allow for a discrimination, and one of 5 OU/m³ for unmistakable perception (also defined as a complaint level). 

Further on, the perception of the odour intensity goes with the logarithm of the odour concentration according to the Weber-Fechner law (e. g., Misselbrook et al., 1993). Based upon laboratory-based experiments on perceived intensity, the Environment Agency, UK (2002), defines: 1 OU/m³ is the point of detection, 5 OU/m³ is a faint odour, and 10 OU/m³ is a distinct odour. The discrepancy between the definition of 1 OU/m³ in the laboratory by using odour free air and the situation in the field was solved by introducing the sniffing unit (van Langenhove and van Broeck, 2001; Defoer and van Langenhove, 2003).

All these aspects of the odour impact criteria show the difficulties to compare various methods used in different countries.

Then the occurrence of the odour sensation was analysed according to the FIDO (frequency, intensity, duration and offensiveness) factors which were suggested by Watts and Sweeten (1995) to assess odour nuisance. In New Zealand (Ministry for the Environment, 2003) and Victoria, Australia (Department of Sustainability and Environment, 2000), a fifth factor, the location, in addition is in use. This factor describes the nuisance with regard to the sensitivity of the receiving environment. The location factor can directly be compared with the factor reasonableness, suggested by Miner (1995). He defines reasonableness of odour sensation as odour causing fewer objections within a community where odour is traditionally part of the environment. For example, it is expected that rural smells will occur as part of the rural environment and industrial smells in industrial areas. Problems then often arise if incompatible activities are located near each other. For example, complaints about existing intensive farming operations often occur when land use in the vicinity is changing. Lohr (1996) found that personal knowledge of the operator of the livestock unit, long term residency, economic dependence on farming, familiarity with livestock farming and awareness of the agricultural-residential context are related to a reduced incidence of formal complaints. An assessment of this factor is often done by the land use category where the neighbours are situated. A pure residential area has a higher protection level than a rural area.

In many national guide lines, the separation distance between residential houses and livestock buildings is modified according to the reasonableness and/or location factor (e.g., Austria (Schauberger et al., 1997; Schauberger and Piringer, 1997a and 1997b), The Netherlands (Ministrie van Landbouw, 1991), Germany (VDI 3471,1986; VDI 3472, 1986), and Switzerland (Richner and Schmidlin, 1995)). For pure residential areas the protection level is higher compared to rural sites. E.g., the separation distance for rural sites, calculated by the Austrian guideline, is half of the distance compared to pure residential areas. In New South Wales, Australia (NSW Environmental Protection Authority, 2001), the odour threshold C_T depends on the population density D (inhabitants/km²) by C_T = -(log D - 4.5)/0.6 (eg, 2 OU/m³ for urban areas (D is about 2000 /km²) and 4 OU/m³ for rural sites (D is about 125 /km²)).

In a similar way this objective is included in the odour impact criteria for the evaluation of dispersion models. An adaptation of the protection level is realised by one of the two parameters of the impact criteria. In Germany the exceedance probability is growing to reduce the protection level, in Australia the adaptation of the impact criteria is done by the odour threshold. An overview of the odour impact criteria and the corresponding land use categories used in several countries, can be found in Schauberger et al. (2001).

 

Table 1      

Odour impact criteria: Limits of odour concentration and exceeding probability used in Austria (Stangl et al., 1993), Germany (Knauer, 1994; Kypke, 1994), Thüringen, Germany (Lotze and Schwinkowski, 1998), UK (Hobson, 1997, personal communication), Australia (Jiang and Sands, 1998), The Netherlands (Hagen and van Belois, 1998, Denmark, New Zealand and Massachusetts (USA) (after Jiang and Sands, 1998) aus Schauberger et al. (2001)
 

Odour impact criteria1	Land use category2	Comment
Germany
1 OU/m³ / 10%	pure residential areas and residential areas
1 OU/m³ / 15%	village-areas with predominantly agricultural utilisation
UK
10 OU/m³ / 2%		Serious annoyance expected with near certainty
5 OU/m³ / 2%		Generally acceptable for existing installations. Emissions from stacks or large area sources may be acceptable at the relaxed end of the range
1 OU/m³ / 2%		No serious annoyance expected in the large majority of cases
1 OU/m³ / .5%		Safe target value for new sources
10 OU/m³ / 0.01%		Safe target value for new sources applicable to highly intermittent sources
Austria
1 OU/m³ / 8% and 3 OU/m³ / 3%		threshold for reasonable odour sensation for medical purpose
Australia
5 OU/m³ / 0.5%	rural and urban area
2 OU/m³ / 0.5%	residential area	New South Wales
10 OU/m³ / 0.5%	residential areas	Victoria
The Netherlands
1 OU/m³ / 2%	residential areas	existing units
1 OU/m³ / 0.5%	residential areas	new installations
1 OU/m³ / 5%	residential areas outside of villages and business areas
Denmark
5 - 10 OU/m³ /.1%		plants
0.6 – 20 OU/m³ / 1%		surrounding
New Zealand
2 OU/m³ /.0.5%		property boundary
Massachusetts, USA
5 OU/m³ / 0.5%		plant boundary

 

¹    Odour concentration threshold (OU/m³) / Percentile compliance: Exceeding probability for the odour concentration threshold p (%)

²   The lend use category varies the accepted protection level

³   The labels are used in the following tables and figures

 

 

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