Co-digestion of cattle manure and grass harvested with different technologies. Effect on methane yield, digestate composition and energy balance

The effect of co-digestion of grass with cattle manure (CM) on digestate composition, methane (CH4) yield and energetic and economic balances was investigated by using three different harvesting technologies in two continuous experiments. Both experiments performed at thermophilic temperature and a 25-day hydraulic retention time. Results showed that the addition of 5% grass in fresh matter increased the volumetric CH4 production by around 20% and the organic matter content of the digestate, decreased the protein content and did not affect the N:P:K ratio. Residual CH4 production from the digestate increased and a 6% decrease in the CH4 concentration in the biogas was observed when grass was added to reactors. The best CH4 yield was achieved when excoriated grass was added, with increments of 20% and 35% compared to mono-digestion of CM in 15-L and 30-m3 reactors, respectively. Our results showed that the extra energy required for mixing and harvesting are the two operations with the highest energy inputs when grass is used for biogas, however in spite of the extra energy used, co-digestion of CM with grass is very favorable both, from an energetic and economic point of view, with excoriation being most favorable compared to other harvesting technologies.