Stack gases from the sewage treatment plant at Bondi, Sydney, were analysed for organic compounds and for hydrogen sulphide, as part of a project to evaluate a number of emission control systems at the pilot plant stage of development. Analytical methodology was custom-designed for the analysis of sewage gases and optimised at each stage.

For the analysis of organic compounds, gas was drawn (1 L/min for 5 min; personal air sampler) via a teflon sampling line through a glass tube (115 mm x 4 mm ID) containing beds of Graphtrap GB and activated charcoal in series between plugs of deactivated glass wool. Sample tubes were sealed in special glass/teflon containers and analysed within 2 - 12 hours of collection. Internal standards (benzene and biphenyl; 200 ng each) were adsorbed on the Graphtrap GB and charcoal beds immediately prior to analysis, using a Dynatherm 890 thermal desorption unit (Supelco) in the preparative mode (250°C, 5 min). Compounds were desorbed at 330°C for 5 min in the same instrument and passed, via a heated transfer line (deactivated silica tubing, 0.32 mm ID; 220°C), to a Hewlett-Packard 5890 gas chromatograph attached to a VG Trio 2000 mass spectrometer. In the thermal desorption unit, helium was used as the carrier gas, with flow rates of 7 mL/min (preparative mode) and 20 mL/min (desorption mode). The split ratio in the desorption mode was adjusted to approximately 3:1, which ensured the maximum flow of carrier gas through the analytical column which the vacuum system of the mass spectrometer could accommodate. During desorption, the GC oven was at 30°C, resulting in condensation ("focusing") of the desorbed material at the beginning of the analytical column. The gas chromatograph was programmed from 30°C (5 min) at 2°C/min to 150°C (5 min) and an AT 624 column (0.32 mm ID, 1.5 µm phase thickness, 30 m long; Alltech and Associates) was used. The mass spectrometer was operated in electron-impact mode (70 eV; source temperature 180°C; 1 second scans from 35 to 200 amu) with a 3 minute solvent delay. Mass spectral data were acquired and processed using LabBase software and standard mass spectral libraries.

The organic compounds in the stack gases were dominated by hydrocarbons with 7-14 carbon atoms, comprising large numbers of both aliphatic and aromatic compounds, indicative of petrol or organic solvent mixtures. The most abundant hydrocarbons were in the C₁₀-C₁₁ range, but often a prominent series of peaks dominated by toluene and xylenes occurred. Several chlorinated compounds were detected. The most abundant was tetrachloroethylene, followed by chloroform and methylchloroform; an origin from industrial solvents is likely. Small amounts of dichlorobenzenes (predominantly the 1,4 isomer) were present, probably arising from toilet deodorants. Several terpenoid compounds related to pinenes were also detected. Acetic acid was the only fatty acid found in the samples and no organic sulphur compounds were detected.

The analytical technique allows emission control systems to be evaluated for organic compound removal and offers a reliable method for locating sites which are discharging undesirable materials to the sewers.