Treatment of dyehouse waste-water by supercritical water oxidation: A case study

BACKGROUND: Supercritical water oxidation (SCWO) of dyehouse waste-water containing several organic pollutants has been studied. The removal of these organic components with unknown proportions is considered in terms of total organic carbon concentration (TOC),withaninitial valueof 856.9mg L ^-1.Oxidation reactionswereperformedusingdilutedhydrogenperoxide. The reaction conditions ranged between temperatures of 400-600 °C and residence times of 8-16 s under 25MPa of pressure. RESULTS: TOC removal efficiencies using SCWO and hydrothermal decomposition were between 92.0 and 100% and 6.6 and 93.8%, respectively. An overall reaction rate, which consists of hydrothermal decomposition and the oxidation reaction, was determined for the hydrothermal decomposition of the waste-water with an activation energy of 104.12 (±2.6) kJ mol^-1 and a pre-exponential factor of 1.59(±0.5) × 105 s^-1. The oxidation reaction rate orders for the TOC and the oxidant were 1.169 (±0.3) and 0.075 (±0.04) with activation energies of 18.194 (±1.09) kJ mol^-1, and pre-exponential factor of 5.181 (±1.3) L^0.244 mmol ^-0.244 s^-1 at the 95% confidence level. CONCLUSION: Results demonstrate that the SCWO process decreased TOC content by up to 100% in residence times between 8 and 16 s under various reaction conditions. The treatment efficiency increased remarkably with increasing temperature and the presence of excess oxygen in the reaction medium. Color of the waste-water was removed completely at temperatures of 450 °C and above.