Treatment of landfill leachate under supercritical conditions of water with Ni/Al2O3 catalyst

Supercritical water oxidation (SCWO) is a process, which decomposes organic compounds in wastewaters without the formation of harmful by-products. Therefore, it seems to be an alternative technique for the disposal of dangerous aqueous wastes. Supercritical water oxidation is performed by using an oxidant above the temperature of 374.8°C and the pressure of 22.12 MPa, which are the critical temperature and pressure of water. Water behaves as a nonpolar solvent in conditions above the critical point of water; owning unique properties and having lower values of density, dielectric constant and ionic product than those of ambient water. In this study the treatment of landfill leachate, obtained from Istanbul Environmental Protection and Waste Material Treatment Industry and Commerce Inc., has been carried out under supercritical conditions of water by using Ni/Al2O3 catalyst. The experiments were made in a continuous flow reactor, using H2O2 as the oxygen source. The influence of reaction temperature and pressure, residence time and H2O2 concentration were investigated and the optimum conditions for the maximum treatment were determined. During the experiments, the treatment of the landfill leachate that had a chemical oxygen demand (COD) of 4750 mg/L and a total organic carbon (TOC) of 2525 mg/L has been carried out at different reaction conditions. For reaction times varying from 30 to 150 s, and with the employment of H2O2 as the oxygen source, a maximum treatment efficiency of 97.05 % was achieved. When the experimental results were investigated, it was seen that higher treatment efficiencies were obtained for greater reaction times, temperatures and O2 concentrations. In addition, as the reaction pressure increased, a decrease in the treatment efficiency was observed. By using the obtained experimental data, a kinetic model of the oxidation reaction has been developed with respect to the reaction temperature and time, waste-water and oxidant concentrations. Accordingly, the activation energy, Ea, the reaction orders for the wastewater (based on TOC) and the oxidant have been determined as 3.63 kJ mol-1 0.67 and 1.33, respectively.