Odours: how to understand, control and eliminate them

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Dr Ruth Fisher has shared some of her findings from her work in the UNSW Odour Laboratory for those controlling bad smells.

A lecturer in the UNSW School of Civil and Environmental Engineering, Dr Fisher and her team have been conducting research aimed at understanding what certain smells are, how they’re formed and how it is possible to vary the processing of biosolids to make them less smelly – and how to best communicate with communities who live near treatment plants.

“Biosolids are the solids remaining after the wastewater treatment process,” says Fisher. “They’re pretty smelly, but they have lower microbial activity and after processing aren’t hazardous. [They] are a really good source of organic matter and nutrients so, in Australia, we apply a lot of biosolids to land.”

The same principles of odour control apply in industrial and residential settings. “We want to understand the state of the odour through its different phases, because that helps us to design process changes and informs the best treatments to get rid of,” she says.

The four stages of stench

Fisher says the key to minimising odour is knowing what stage of the transmission process you are targeting. Each stage can be addressed to minimise odours and their effects on the population. The four stages of odour development are:

  1. Formation. How the odour is being produced – the easiest way to stop an odour is to prevent it from being formed.
  2. Emission. Controlling this can be as simple as closing doors in an industrial setting when odorous products, like garbage, are being unloaded.
  3. Transmission or dilution. Fisher uses sewer pipes as an example of how controlling the way that odours transmit is a big part of managing them: “In industrial settings… you can vent odours through very tall stacks – this is done with sewer pipes in urban areas, because when the odour reaches the ground it’s been diluted and its concentration is low so you’re less likely to smell it.” Buffer zones around industrial sites like landfills, so people cannot reside too close, is another example.
  4. Perception. The fourth and final stage is minimising how people perceive odour. This involves masking the smell with agents like floral or citrus scents. “This method is commonly used in industry,” says Fisher. “Big sites like landfills or wastewater treatment plants have nozzles along their boundaries, which spray masking agents… However, these masking agents don’t always work; sometimes they result in strange, more unpleasant, combinations of odours, or the masking agent itself can annoy people – if there’s a particularly cloying floral scent, for example.”

Odour and perception

Fisher says odour is a person’s sensorial response when breathing in a volatile chemical compound. “Something in the air binds to receptors in our nose and the odour is the signal that is sent to the brain. Odours are made up of lots of different compounds, which can produce slightly different signals. Typically, a compound that elicits a response is called an odorant.”

There’s a great variation in the way different people detect and perceive odours. There are also different ways to describe them:

  • character: chocolate or pine are examples
  • hedonic tone: the lower the tone, the more unpleasant the odour
  • intensity: the strength of the odour, and
  • concentration: the more diluted, the more difficult it is to detect.

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