Chemical Characterisation and Olfactometric Measurement of Odours from Sewage Treatment Process Units
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report no. UWRAA 75
An investigation was undertaken to identify and quantify the chemical constituents which characterise odour emissions from specific unit processes at two sewage treatment facilities. Air in contact with each of the unit processes was sampled. The processing units investigated included: inlet works, primary sedimentation tanks, aeration tanks, return activated sludge pumping stations and sludge de-watering/handling.
The air samples were collected in special atmospheric sampling bags and transported to off-site laboratories at the Australian Nuclear Science and Technology Organisation and at the University of New South Wales for testing. Aliquot portions of each sample were transported to these laboratories respectively for chemical and olfactometric analysis. Techniques were developed to transfer the chemical constituents, including odorous species, to a solid adsorbent for subsequent qualitative and quantitative analysis by gas chromatography and mass spectrometry (GC-MS). The analytical techniques focused particularly on the sulphur containing compounds. The larger aliquot portion of each sample was tested by dynamic olfactometry utilising a panel of human observers. The samples were diluted with specially prepared odour-free air to determine the number of dilutions required to reach the average threshold of perception for the panel. The average number of dilutions defines odour concentrations in Odour Dilution Units (ODU).
Olfactometry simulates natural atmospheric dispersion processes which occur down wind of sewage treatment facilities. The number of atmospheric dilutions which occur at nearby residences resulting from such natural dispersion processes are site specific and are determined, interalia, by distance to the nearest residence, prevailing wind direction and speed, atmospheric stability, and topography.
Potentially odorous chemical constituents were ranked both according to their absolute concentrations and to their significance as odorants. The significance of each odorant was determined as the quotient of the concentration of the odorant divided by the accepted published threshold of perception concentration for that odorant. This quotient is analogous to odour concentration and is expressed as “Chemical Odour Units”. Total Chemical Odour Units for each sample were recorded as the sum of Chemical Odour Units for each Chemical species identified. Neither olfactometric odour concentration in ODU’s nor Chemical Odour Units describe the perceived odour strength of a gaseous mixture of odorants. Rather, they describe the number of times that the mixture must be diluted to just reach the threshold of perception. The determination of Chemical Odour Units is complicated even in the case of pure odorants as existing threshold of perception concentration data for many compounds are not definitive. Correlations between olfactometric Odour Dilution Units and Chemical Odour Units for each sample tested were investigated.
It is concluded that the sample collection and GC-MS techniques developed a re reliable and permit the simultaneous analysis of hydrogen sulphide and organic sulphur compounds. However, the direct measurement of odour concentrations by olfactometry is generally more useful in assessing the significance of odour emission sources at sewage treatment plants.
Sulphur compounds are important odorants for all of the unit processes investigated and in some instances Chemical Odour Units derived from their concentrates correlate well with olfactometric Odour Dilution Units. The most significant sulphur compounds are hydrogen sulphide, methyl mercaptan and dimethyl sulphide. Consequently, odour abatement technologies which remove sulphur compounds should have significant applications within the sewage treatment industry.
Odours from the specific process units tested at both sewage treatment plants could not be uniquely identified by the presence of specific characteristic sulphur compounds or the relative concentration of the chemical species identified. Consequently, chemical characterisation of ambient odour constituents will not result in the identification of specific unit processes. Therefore, the formulation of odour abatement strategies requires the acquisition of emission data for specific unit processes to be utilised together with atmospheric dispersion modelling.