Study on kinetics of supercritical water oxidation of municipal sludge

The experiment of supercritical water oxidation (SCWO) of municipal sludge was conducted in a batch reactor at 400-450°C, 24-28 MPa, and reaction time 40-515 s. The experimental results indicate that the chemical oxygen demand (COD) reduction can be over 99.9%. Moreover, the volume of residual solids is only 4 percent of the dehydrated sludge, and the residual COD of reaction liquid is less than 10 mg·L^-1 at 155 s and 420°C. The global kinetics for municipal sludge COD disappearance was described with a power-law equation, which is first order in sludge COD and zero order in oxidant when having excess oxidant. The relation between the global rate constant and temperature can be satisfactorily explained by Arrhenius law. The COD reduction of municipal sludge obviously increases with reaction time and temperature and hardly changes with the excess oxidant ratio when it is over 3.25. The global rate constant increases with the increase of pressure, but its rising rate diminishes as the pressure further enhances and the activation volume is not a constant. At 24 MPa, the values of activation energy and frequency factor are (39.11 + 2.216) kJ·mol^-1 and (45.2 + 17.2) s^-1, respectively. The deviations between calculation and experimental values are within ±8%.