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.