For whom the smell tolls
The application of advanced control technologies to mitigate odorous emissions has been limited, largely because odour has not been regarded as a major environmental hazard or health risk. However, as the public easily identifies odours, many perceive a risk to exist. Martin Key and Mohit Uberoi of Megtec, an air pollution specialist exhibiting at this year's ET2001 exhibition, review the nature and causes of odours and offer some control and management options.Assessing the effect of odour emissions is more complex than for the more traditional air pollutants. Direct measurement is difficult because odours are usually caused by a 'cocktail' of chemical species. It is possible to measure known chemical components by mass spectrometry, for example, but the correlation between individual component concentrations, possible synergistic effects and subjective odour perception is almost impossible to predict.
In addition, it is difficult to synthesise the olfactory response of the nose - the olfactory sense is extremely sensitive and varies with the receptor. It is possible to measure the strength of an odour by dynamic dilution olfactometry, whereby a sample is passed into an olfactometer (an instrument which mixes the odorous gas with clean air to provide a wide range of dilutions of the odour for assessment by a panel of pre-screened persons). The olfactometer determines the odour strength by using the subjective response of the panel to identify how many times the odour must be diluted to reach its threshold of detection. Whilst it measures strength, and not 'offensiveness', it is widely used for comparison of different odour sources and for design and verification of any odour control equipment.
Further, odour perception is dependent upon meteorological conditions, and odour generation within a process varies with raw material and process.
A familiar emission control hierarchy can, however, be applied:
The first phase of an investigation into a potential odour problem is to carry out an odour audit of the facility. This will include a detailed process assessment and odour analysis by simple sensory means, and if the odour profile is very complex (e.g. where there are multiple sources) it may be necessary to carry out olfactometric monitoring to obtain relative odour quantification.
In the final analysis, it is necessary to determine the balance between the costs and the environmental benefits of compliance.
|Equipment Type||Sensitivity to various operating conditions||Typical odour destruction efficiency|
|sulphur||temperature||high loads||changes in odour types|