This work elucidates the effects of pretreatment of secondary sludge by microwave irradiation on anaerobic digestion. The soluble chemical oxygen demand (COD) concentration increased up to 22% as microwave irradiation time increased, which indicated the sludge particles disintegrated. Three identical automated bioreactors with working volume of 5 l were used as anaerobic digesters at mesophilic temperature (35°C). The reactors were separately fed with sludge with microwave pretreated- and controlsludge at different hydraulic retention times (HRT). The volatile solid (VS) reduction in the control operation was approximately 23.2 ± 1.3%, while it was 25.7 ± 0.8% for the reactors with the pretreated sludge. The average biogas production rate with the pretreated sludge at 8, 10, 12, and 15 days HRTs was 240 ± 11, 183 ± 9, 147 ± 8, and 117 ± 7 ml/l/d respectively, while those with the control sludge were134 ± 12 and 94 ± 7 ml/l/d at 10 and 15 days HRTs. Maximum rates of COD removal and methane production with the pretreated sludge were 64% and 79% higher than those of the control system, respectively.

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