Synergetic pretreatment of sewage sludge by microwave irradiation in presence of H2O2 for enhanced anaerobic digestion

A microwave-enhanced advanced hydrogen peroxide oxidation process (MW/H2O2-AOP) was studied in order to investigate the synergetic effects of MW irradiation on H2O2 treated waste activated sludges (WAS) in terms of mineralization (permanent stabilization), sludge disintegration/solubilization, and subsequent anaerobic biodegradation as well as dewaterability after digestion. Thickened WAS sample pretreated with 1 g H2O2/g total solids (TS) lost 11–34% of its TS, total chemical oxygen demand (COD) and total biopolymers (humic acids, proteins and sugars) via advanced oxidation. In a temperature range of 60–120 °C, elevated MW temperatures (>80 °C) further increased the decomposition of H2O2 into OH• radicals and enhanced both oxidation of COD and solubilization of particulate COD (>0.45 micron) of WAS indicating that a synergetic effect was observed when both H2O2 and MW treatments were combined. However, at all temperatures tested, MW/H2O2 treated samples had lower first-order mesophilic (33 ± 2 °C) biodegradation rate constants and ultimate (after 32 days of digestion) methane yields (mL per gram sample) compared to control and MW irradiated WAS samples, indicating that synergistically (MW/H2O2-AOP) generated soluble organics were slower to biodegrade or more refractory than those generated during MW irradiation.