Batch- and Semi-continuous Biogas Production from Different Grass Species

The importance of forage as a feed supply for dairy and beef cattle stocks is decreasing. Therefore, interest is rising in alternative use of grasslands. An ecologically sound option is the anaerobic digestion of the biomass as co-substrates in biogas plants. Three fresh and ensiled grass species were investigated in lab-scale batch experiments at 35° C to determine their maximum biogas production potential. The volatile solid-based biogas and methane yield were observed to be in the range of 0.65-0.86 and 0.31-0.36 m3/(kg VS), respectively. Semi-continuous experiments were conducted to examine biomethanation of grass and cattle slurry at two organic loading rates (OLR) of 0.7 and 1.4 kg VS/(m3 d). The anaerobic digestion was carried out in completely stirred tank reactors at 35° C with a mixture of three fresh grass species as mono-substrate, cattle slurry and a mixture of both as co-digestion. The biogas yield observed from the grass as mono-substrate at OLR of 0.7 and 1.4 kg VS/(m3 d) was 0.61 and 0.56 m3/(kg VS), respectively. However, for both, co-digestion and cattle slurry digestion, the impact of OLR in the range of 0.7 and 1.4 kg VS/(m3 d) on biogas yield was small. At this range of OLR, the averaged biogas yield obtained to 0.5 and 0.38 m3/(kg VS), respectively. The biogas yield in case of co-digestion was proportional to the amount of VS from grass in a mixture with cattle slurry. Moreover, the methane content decreased from 59-63% to 53-59% with the increasing proportion of grass.