A B S T R A C T The feasibility of the hydrothermal oxidation of a wastewater derived from H 2 S scavenging operations, carried out in a topside offshore oil and gas installation, was demonstrated. The feed characterization showed the presence of mainly unreacted triazine (unspent scavenger), monoethanolamine and dithiazine (spent scavengers). The spent and unspent scavengers (SUS) were subjected to hydrothermal oxidation in a batch reactor at approximately 200 • C (low temperature, LT) and 350

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... • C (high temperature, HT), using oxygen in excess. The experiments were performed for six reaction times at each temperature, in the range 3 to 360 min at LT and 1 to 120 min at HT. Diluted SUS (COD around 30 g/kg) was used as reactor feed. The three main compounds of the SUS were not found in any of the oxidation products, which means they are fast converted. COD reductions up to 84% and 98% were obtained at LT and HT, respectively. A rate equation of second order with respect to the COD was found suitable to represent the rate of disappearance of COD, with the rate constant at HT approximately 70 times higher than at LT. The extensive analysis carried out on the reaction products showed the presence of C1-C4 carboxylic acids, pyridines and pyrazines as intermediate oxidation products. A clear trend towards complete mineralization of organic nitrogen to ammonium, except for small amounts of nitrate at LT, and of organic sulfur towards sulfate, was observed.