Combined chemical and sensory techniques were used to analyse off-flavours formed during ozonation of surface waters and fulvic acids (FA) dissolved in Milli-Q water. Chromatographic sniffing and gas chromatography - mass spectrometry (GC-MS) analysis of a dichloromethane extract of an ozonated surface water revealed n-heptanal, n-octanal, n-decanal, phenyl acetaldehyde, vanillin and several other unidentified, odorous compounds. Dissolving the dichloromethane extract in water did not, however, recreate the sweet and nauseous odour components of the original ozonated water. Continued experiments showed that FA can be important precursors of off-flavours in ozonated waters and that the major off-flavour compounds in such waters are less volatile than the above mentioned aldehydes. Furthermore, adsorption on DEAE-cellulose followed by desorption with sodium hydroxide was found to be an efficient method for concentrating the off-flavours of ozonated FA solutions. During repeated ozonation of FA solutions, both the threshold odour number (TON) and the concentrations of several organic acids first increased and then decreased.

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