Effect of limited air exposure and comparative performance between thermophilic and mesophilic solid-state anaerobic digestion of switchgrass

Switchgrass is an attractive feedstock for biogas production via anaerobic digestion (AD). Many studies have used switchgrass for liquid anaerobic digestion (L-AD), but few have used switchgrass for solid-state anaerobic digestion (SS-AD). Limited air exposure to the reactor headspace has been adopted in commercial scale anaerobic digesters for different applications. However, little research has examined the effect of limited air exposure on biogas production during SS-AD. In this study, the effects of air exposure and total solids (TS) content on SS-AD performance were evaluated under mesophilic (36 ± 1 °C) and thermophilic (55 ± 0.3 °C) conditions. Limited air exposure did not significantly influence the methane yield during SS-AD. Thermophilic SS-AD had greater methane yields (102-145 L CH4 kg−1 VSadded) than mesophilic SS-AD (88-113 L CH4 kg−1 VSadded). Both mesophilic SS-AD (73-136 GJ) and thermophilic SS-AD (2-95 GJ) produced positive net energy based on a theoretical 'garage-type' SS-AD digester operating in a temperate climate.