Odour
Odour control is a specialist area of air pollution control, made complex by its direct impact on the public at large.
It is unique as in issue amid the variety of air pollution applications that exist, in that complete removal of the offending emissions may not actually remove the complaints.
The concept of "solved" is much more involved with odour issues than that of chemical removal. This is because of the difference in odour perception experienced from one person to the next.
Olfactometry has attempted to put numbers to the problem by creating the Odour Unit.
The odour threshold of a gas sample is the lowest concentration of that gas sample (as diluted by laboratory cleaned air) whereby 50% of a panel of specially screened odour candidates successfully distinguish the diluted sample from a sample of pure air.
The number of times the gas sample had to be diluted to reach the odour threshold, is then the number of odour units associated with that sample. e.g. A sample of odorous air is diluted 2000 fold to reach the odour threshold in a panel test. This sample is then said to have 2000 OUe/m3
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Odour units and odour modelling form part of the picture that helps us estimate public impact. Olfactometry allows a public impact concept to be applied as a target or consent for emission compliance. Any criteria to define success must however take into consideration that other odours not emanating from the sources in question, are causing localised nuisance. The only way of doing this is to "back model" from the process boundary or nearest receptor to yield an allowable (and measurable) odour level at the emission point itself.
Note that Olfactometry cannot, as is often assumed, provide a design basis for odour control systems.
As design engineers we are utterly aware of the concept of "No substitute for information". We need to know the chemistry, variability, frequency, humidity, temperature, etc relating to the emission and also quantify the risk of not knowing more about the emission before embarking on the design.
Don't Do's
Do not guarantee or accept a guarantee of Zero odour at any point of any emission criteria. It is not possible to achieve.
Do not guarantee or accept guarantees of zero or even reduced complaints. The level of complaints may be entirely out of the control of the project (other sources , obsessive complainants etc).
Do not use the removal efficiency of an odour control system as performance criteria. It is irrelevant due to the nature of odour and the contaminants that create odour.
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Do's
Do fully characterise and quantify the emissions and invest in accurately defining the problem.
Do involve the authorities, operations and designers in the development of the targets to be met.
Do consider the impact of maintenance and operational cost as well as capital cost of projects.
Do understand that PR and "community marketing" can greatly assist in resolving odour issues - if you have a shiny new box to control odour, show it to people.
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