Storage of catch crops to produce biogas: Effect of ensiling process on methane yield

Catch crop biomass is a promising co-substrate for manure-based biogas plants in Denmark since the cultivation of catch crops is mandatory to retain nutrients in the soil, contributing to protect the aquatic environment. In general, the growth period for catch crops is from harvest of the previous crop in July-August to the end of the growing season and harvest in late October. Hence, for use of the biomass in biogas production there is a need for storage of the biomass. Storage as silage would guarantee the availability of the feedstock for biogas production during the whole year. A proper ensiling process determines the storage loss and the quality of the final silage and, thus, the possible use of it as a substrate for biogas production. Moreover, silage has been considered as a pre-treatment since it partially hydrolyses organic matter improving cellulose convertibility. Since a large amount of the organic matter is converted to easily degradable organic acids during ensiling, this process may result in a faster anaerobic degradation process. However, the effect of ensiling a crop on the final specific methane yield compared to the correspondent fresh crop is still not clear. The objective of this study was to investigate the specific methane yield during the ensiling process of three different catch crops, namely oil seed radish (OSR), Italian ryegrass (IR) and grass-clover mixture (GC). The quality of the different silages was evaluated by composition analysis and visual characterization of the biomass after ensiling. The different catch crops were harvested and wrapped for silage. Four samples, corresponding to four different times of ensiling, were taken from each ensiled catch crop. The silage fermentation products (volatile fatty acids, lactate and ethanol) were analysed by chromatography. The specific methane yield of each silage sample was determined in triplicates and an average was calculated. The results indicated that OSR and GC could be successfully preserved as silage for up to 252 days and, therefore, be used as feedstock for biogas production throughout the year. On the contrary, the poor quality of IR silage, due to its high TS content, made it inappropriate as feedstock for biogas production. A TS content of 25-35% is preferable, to obtain a proper fermentation avoid leachate run-off and growth of Clostridium sp. or mold formation. Avoiding soil particles in the bales and proper chopping of the biomass before ensiling is recommended for a better handling and to avoid sand precipitation in the biogas reactor. The methane yield analysis revealed no signs of inhibition in the anaerobic digestion process and no effect of storage time on specific methane yield was found for any of the catch crops. For correct TS and VS determination it should be noticed that the TS content analysed with oven drying at 105°C must be corrected by adding the dry-oven losses to the standard TS determination, to avoid underestimations of the TS and VS concentration and overestimations of the specific methane yield.