ChemicalCharacterisation and Olfactometric Measurement
ofOdours from Sewage Treatment Process Units
ReportNo WSAA 75
Aninvestigation was undertaken to identify and quantify the chemical constituentswhich characterise odour emissions from specific unit processes at two sewagetreatment facilities. Air in contact with each of the unit processes wassampled. The processing units investigated included: inlet works, primarysedimentation tanks, aeration tanks, return activated sludge pumping stationsand sludge de-watering/handling.
Theair samples were collected in special atmospheric sampling bags and transportedto off-site laboratories at the Australian Nuclear Science and TechnologyOrganisation and at the University of New South Wales for testing. Aliquotportions of each sample were transported to these laboratories respectively forchemical and olfactometric analysis. Techniques were developed to transfer thechemical constituents, including odourous species, to a solid adsorbent forsubsequent qualitative and quantitative analysis by gas chromatography and massspectometry (GC-MS). The analytical techniques focused particularly on thesulfur containing compounds. The larger aliquot portion of each sample wastested by dynamic olfactometry utilising a panel of human observers. Thesamples were diluted with specially prepared odour-free air to determine thenumber of dilutions required to reach the average threshold of perception forthe panel. The average number of dilutions defines odour concentrations inOdour Dilution Units (ODU).
Olfactometrysimulates natural atmospheric dispersion processes which occur down wind ofsewage treatment facilities. The number of atmospheric dilutions which occur atnearby residences resulting from such natural dispersion processes are sitespecific and are determined, interalia, by distance to the nearest residence,prevailing wind direction and speed, atmospheric stability, and topography.
Potentiallyodourous chemical constituents were ranked both according to their absoluteconcentrations and to their significance as odorants. The significance of eachodorant was determined as the quotient of the concentration of the odorantdivided by the accepted published threshold of perception concentration forthat odorant. This quotient is analogous to odour concentration and isexpressed as “Chemical Odour Units”. Total Chemical Odour Units for each samplewere recorded as the sum of Chemical Odour Units for each Chemical speciesidentified. Neither olfactometric odour concentration in ODU’s nor ChemicalOdour Units describe the perceived odour strength of a gaseous mixture ofodorants. Rather, they describe the number of times that the mixture must bediluted to just reach the threshold of perception. The determination ofChemical Odour Units is complicated even in the case of pure odorants asexisting threshold of perception concentration data for many compounds are notdefinitive. Correlations between olfactometric Odour Dilution Units andChemical Odour Units for each sample tested were investigated.
It isconcluded that the sample collection and GC-MS techniques developed arereliable and permit the simultaneous analysis of hydrogen sulfide and organicsulfur compounds. However, the direct measurement of odour concentrations byolfactometry is generally more useful in assessing the significance of odouremission sources at sewage treatment plants.
Sulfurcompounds are important odorants for all of the unit processes investigated andin some instances Chemical Odour Units derived from their concentratescorrelate well with olfactometric Odour Dilution Units. The most significantsulfur compounds are hydrogen sulfide, methylmercaptan and dimethyl sulfide.Consequently, odour abatement technologies which remove sulfur compounds shouldhave significant applications within the sewage treatment industry.
Odoursfrom the specific process units tested at both sewage treatment plants couldnot be uniquely identified by the presence of specific characteristic sulfurcompounds or the relative concentration of the chemical species identified.Consequently, chemical characterisation of ambient odour constituents will notresult in the identification of specific unit processes. Therefore, theformulation of odour abatement strategies requires the acquisition of emissiondata for specific unit processes to be utilised together with atmosphericdispersion modelling.
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