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

ReportNo WSAA 53

February 1993

SYNOPSIS

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

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

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

Thegreater the number of odorants in a mixture, the greater was the degreeof suppression of the individual constituents.

Thegreater the number of constituents in a mixture the more difficult it became toidentify individual constituents.

Hydrogensulphide was the least frequentlysuppressed constituent, whilst isovaleric acid and skatole were the most frequently suppressed constituentsin mixtures.

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

Amodel was tested which satisfactorily predicted the odour intensity of mixturescontaining 2,3 or 4 of the odorants investigated.

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