A few weeks ago on the podcast we asked George what an “Environmental Engineer” does… well it turned out that there is more than one type. To cut a long story short George specializes in the assessment of odour, more commonly known as “whats that smell”. Well that might be a bit simplistic, he specializes in air quality, emissions, dust and gases from businesses and such that potentially spew forth crap so that we can get our iPods.

Now we talked about this on the show this week and we had a few more questions for George, such as how do you measure smell… turns out it was a cracker of a question and could not be quickly answered. But George came through with a great answer via email. So good in fact that I have reworded (at his request) and gained permission to post it here.

Okay first things first, lets take a look at odour:

1. Odour is a mixture of compounds. Some odours contain over 300 different compounds, think odour soup.
2. Most odours do not consist of a single compound. Although, there are a few exceptions such as hydrogen sulphide from wastewater treatment plants. Because of this, measuring one compound never really tells you much.
3. The nature of the odour can be a defining factor - is it a good or bad odour? People would more likely be able to accept higher concentrations of a “nice” odour than say a waste processing plant. Think sewerage…
4. Odour licensing and assessments are never based on no odour. The aim is to try and limit the number of times the odour may exceed a limit. The limit normally is the concentration of odour that above which a person may complain. For example there might be occasions where there is some odour, but 98+% of the year there will be very little odour, if any at all.

What this means is that a “good” smell can still be a bad thing if it is excessive. For example; try living next to a perfume factory! Sure “insert favorite fragrance here” might be nice on some busty blond you are dating, but not permeating your house 24/7! Of course it is the really bad smells, such as Tim’s farts that get all the attention, no one wants to live next door to some factory that is emitting excessive levels of compounds that stink. Moving along…

Typically there are two types of odour assessments that can be done (George does both of these):

1. New facility assessments: Making sure it won’t annoy the neighbours
2. Existing facilities that has had odour complaints: are they exceeding their license limits and if so what can be done about it?

This is how these assessments are typically performed:

1. Meteorological modeling: running computer modeling software to predict the weather at the site. The reason being that a lot of sites don’t have site specific meteorological data. Models such as TAPM, CALMET and MM5 for this in Australia. For us geeks, the minimum computer specs are at least 4GB of RAM, dual or quad core processors and we can fill HDD’s no worries. A month of data is typically around 2GB. 12 months is usually required. Run time is from days to weeks.
2. Emission estimation - measure the odour emitted, or find out how much is emitted by examining the literature - how much and how is it released. Basically where can it be emitted from, is there any mitigation or need for it, how much of it is released and how high, how hot etc
3. Dispersion modeling - when it is emitted, where does it go and how much does it get diluted. Probably the best model for odour assessments is CALPUFF. Run time is from overnight to a few days depending on the number of sources.

If you want more information on these modeling systems check out Wikipedia’s article: Compilation of atmospheric dispersion models. We are not about to cover them here as that is heavy duty stuff that would take a thesis… glad you understand. But that should get you started.

Surprisingly, the tool of choice for measuring odour is called olfactometry. Basically this process involves a super accurate dilution device is used to accurately dilute an odour sample. The sample is collected in special odour free sample bag, made of a material so close to being oven bags that some of them are oven bags (think Tedlar, Teflon, Nalphan, Melinex). The sample is then passed to specially trained panellists who have a known sense of smell ( that is: can detect n-butanol at between 20 and 80 ppb). These smell experts then give you an answer based on a guess, uncertain or certain that they can detect the odour at different dilutions. From these answers you can calculate odour units (or as in America “Detection Thresholds” (DT’s)). Think of them as the number of times the odour has to be diluted to make it just detectable.

In Australia modelling is the preferred approach. However, you can do field validations or surveys for either new, existing or expanding facilities.

Another approach is to do field odour surveys, in other words get out there and sniff. Again people run around in the field and sit in different spots and see what odours they can detect using standardized methodology. Apparently the Germans have some great methods for these type of assessments.

Yes, electronic noses are available. “Enoses” (that’s tech speak for an electronic nose) probably require more development before they can used to assess all types of odours. Enoses use a combination of six or so sensors which provide a different responses to odorants (such as sulphur based compounds). If you analyze enough samples you can demonstrate a relationship between the sensor response and odour concentration. It is also worth mentioning that the cost of analyzing samples in this way is very expensive.

Going on to Tims fart, there are a number of sensitive hydrogen sulphide analyzers that can “pick up” farts. According to George; this is a favorite pass time for some bored Environmental Engineers and scientists… “For some unknown reason someone always tries to measure them”.

Image Credit to Sean Carpenter

Thanks very much George for a great answer and for allowing us to rewrite it into a post here on the the GGP blog.

Please note that this information is for the benefit of listeners of The Global Geek Podcast for curiosity and general interest only. It is not intended for use as a legal document or for use in any legal proceedings what so ever. If you do use it; you are an idiot.

Dave

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