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Energy production from yard wastes via one-stage and two-stage anaerobic digestion and investigation of pretreatment effect

Kalaycıoğlu, Nihan Nur
Energy production from lignocellulosic biomass is receiving ever-increasing interest. This thesis study investigated the energy production from yard wastes, which is an abundant lignocellulosic substrate, via one-stage anaerobic digestion (OSAD) and two-stage anaerobic digestion (TSAD) of yard waste. The effect of pretreatment options (alkaline (AP), thermal (TP) and ultrasound (UP)) and initial solids concentration (2%, 5% and 8% total solids, TS) on hydrogen and methane production yields in OSAD and TSAD were researched. Solubility increase was obtained via AP, TP and UP by 43%, 51% and 17%, respectively. OSAD batch experiments showed that the highest CH4 yield was obtained with AP-yard waste at 5% TS as 313 mL CH4/g VS. AP was the best pretreatment option among others for all three TS% options studied in terms of CH4 yields. Considering the TSAD experiments, the highest H2 yield was obtained with AP-yard waste at 2% TS ( 30.5 mL H2/g VS) in dark fermentation stage. AP was found again as the best pretreatment option for all TS contents studied in terms of H2 yields. In methanogenesis stage of TSAD, the obtained yields were very low. The highest methane yield was obtained with TP-yard waste at 2% TS (40 mL CH4/g VS). TP-yard waste gained the highest methane yield in all three TS contents. OSAD of yard waste provided 11 MJ/kg VS energy recovery. Yet only 2 MJ/kg VS could be achieved via TSAD of yard waste. It can be concluded in this study that it is possible to achieve energy from yard wastes whether it is pretreated or not when initial TS content is 2%. For higher TS contents such as 5%, AP improves the energy yield. TSAD is not sufficient to apply for yard wastes even if it is pretreated with AP, TP and UP. This was mainly attributed to the potential inhibitory by-products produced after either pretreatments applied and/or dark fermentation.