Reducing the environmental impact of offshore H₂S scavenging wastewater via hydrothermal oxidation

The discharge of H₂S scavenging wastewaters, containing spent and unspent scavengers (SUS), into the marine environment is a large contributor to the environmental impact of offshore oil and gas production. Hydrothermal oxidation (HTO) can be a viable method for on-site treatment of the SUS before discharge, but the effect of the process on the ecotoxicity of the effluent has not been investigated so far. The aim of this study was to investigate the potential of the HTO technology in reducing the environmental impact by linking the chemical process design with ecotoxicity reduction. For this, we combined HTO experiments on a SUS sample from an oil and gas platform in the North Sea with whole effluent ecotoxicity evaluation before and after the treatment. The HTO process was carried out under excess of oxygen, for temperatures and pressures in the range 199 to 350°C and 83 to 228 bar, respectively, and for reaction times of 5 to 360 min. Initially, the SUS sample exhibited very high ecotoxicity, which was drastically reduced by the HTO process. More specifically, the ecotoxicity towards bacteria was reduced more than 90% for all HTO conditions, while the reduction in algal toxicity was in the range 48% to 66%, 59% to 86% and 60% to 82% at reaction temperatures of 199°C, 279°C, and 350°C, respectively. Furthermore, this work shows how typical wastewater chemical analyses, such as COD and TOC, and ecotoxicity tests towards different organisms provide complementary information, which should be used in combination to optimize operating conditions of the HTO process.