High-solids anaerobic mono-digestion of riverbank grass under thermophilic conditions

The purpose of this study was to investigate the potential of high-solids anaerobic mono-digestion of riverbank grass under thermophilic conditions, focusing on the effects of the strength and the amount of inoculum. Ensiled grass was inoculated with three different inocula; inoculum from liquid anaerobic digester(LI), inoculum from dry anaerobic digester(DI), and mixture of LI and DI(MI), at feedstock-to-inoculum ratio(FIR) of 1, 2 and 4. The ensiling process of riverbank grass reduced moisture content(p > 0.05), while the hemicellulose content was significantly increased from 30.88% to 35.15%(p < 0.05), on dry matter basis. The highest methane production was at an FIR of 2 with MI(167 L/kg VSadded),which was significantly higher(p < 0.05) than with DI, but not significant compared to LI(p > 0.05). At an FIR of 4, digesters inoculated with LI and DI failed to produce methane,whereas 135 LCH4/kg VSaddedwas obtained with MI. The kinetic studies showed that at an FIR of 1 with LI and MI, the inoculum had less of effects on the hydrolysis rate constant(0.269 day-1and 0.245 day-1) and methane production(135 versus 149 L/kg VSadded); rather,it affected the lag phase. In a thermophilic HS-AD of riverbank grass, the mixture of inoculum with low and high total solids content(TS) helps increase the TS of inoculum and digestion process. An FIR of 2 was deducted to be the limit for a better startup time and higher volumetric productivity of methane.