Prediction of Perceived Odour Strength and Type from Composition of Sewage Odour Mixtures

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Report no. UWRAA 53

February 1993

Synopsis

Odorous emissions from sewage treatment plants are complex and contain many different types of odorants which vary in quantity depending upon the contents and efficiency of treatment processes. Since little is known about how individual odorants in complex mixtures affect the perception of each other, it is difficult to develop mathematical models that can predict the pleasantness, strength and quality characteristics of an emission at different distances from a source. The present project investigated the interactions of the four major types of odorants emitted by treatment plants, namely, hydrogen sulphide, isovaleric acid, butanethiol and skatole, by measuring the perceived intensity of individual odorants alone and in mixtures, the overall perceived intensity of mixtures, the odour characteristics and unpleasantness of mixtures, and models for predicting odour strength. The results indicated that:

The perceived odour intensity (odour strength) of mixtures of the odorants was greater than that of any of the individual constituents, but less than the sum of their intensities. However, as the number of components in a mixture increased, the intensity of the most dominant component provided a good approximation of the intensity of the mixture.

In no instance was the intensity of one odorant enhanced by another i.e., no synergistic interactions occurred.

The greater the number of odorants in a mixture, the greater was the degree of suppression of the individual constituents.

The greater the number of constituents in a mixture the more difficult it became to identify individual constituents.

Hydrogen sulphide was the least frequently suppressed constituent, whilst isovaleric acid and skatole were the most frequently suppressed constituents in mixtures.

The unpleasantness of mixtures of hydrogen sulphide, isovaleric acid, butanethiol and skatole was greater than that of the individual constituents, indicating that models used for predicting complaint levels in communities affected by sewage odour and based on assumptions related to a single odorant e.g. hydrogen sulphide, will underestimate the number ofcomplaints. Even mixtures with low but above threshold concentrations of these odorants are likely to generate complaints.

A model was tested which satisfactorily predicted the odour intensity of mixtures containing 2,3 or 4 of the odorants investigated.

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