Conversion of kitchen waste to bioenergy: Influence of microwave and thermal pretreatment

Kitchen waste (KW) is a promising substrate for anaerobic digestion (AD) due to its high carbon and moisture content; however, its complex structure limits biogas production. This study evaluated the impact of varying substrate/inoculum (S/I) ratios, between 0.5 and 4, based on total volatile solids (TVS), pretreatment methods (thermal heating (TH) and microwave irradiation (MWI)), and operating temperatures on the AD performance of KW. Under mesophilic (37 °C) and thermophilic (55 °C) conditions, biogas production varied significantly with varying S/I ratios, confirming the strong influence of feed loading. Pretreatment greatly improved biodegradability: soluble chemical oxygen demand (COD) increased by 47 % after TH at 250 °C for 30 min, and by 63 % after MWI at 1000W for 2 min. These gains resulted in higher biogas yields, with increases of 69.77 (1539.90 mL/gTVS) for TH and 54.22 (1398.92 mL/g TVS) for MWI compared to unpretreated KW (907.06 mL/g TVS). The modified Gompertz model accurately fit the data (R2 > 0.98), allowing reliable prediction of biogas yields and kinetics. These results show that optimized S/I ratios combined with effective pretreatments can greatly enhance KW valorization through AD.