Evaluation of non-thermal effects by microwave irradiation in hydrolysis of waste-activated sludge

The activation energy (E_a) for waste-activated sludge (WAS) hydrolysis was compared between microwave irradiation (MW) and conventional heating (CH) methods to evaluate the non-thermal effect of MW. The microwave-assisted hydrolysis of WAS was assumed to follow the first-order kinetics on the basis of volatile suspended solids (VSS) conversion to soluble chemical oxygen demand (SCOD) for different initial VSS concentrations. By comparing the VSS decrement and the SCOD increment between MW and CH at different absolute temperatures of 323, 348 and 373 K, the average ratio of VSS conversion to SCOD was determined to range from 1.42 to 1.64 g SCOD/g VSS. These results corresponded to the theoretical value of 1.69 g SCOD/g VSS based on the assumption that the molecular formula of sludge was C₁₀H₁₉O₃N. Consequently, the E_a of the MW-assisted WAS hydrolysis was much lower than that of CH for the same temperature conditions. The non-thermal effect of MW in the hydrolysis of WAS could be identified with the lower E_a than that of CH.