Anaerobic digestion of grass: Effect of drying and ensiling on biogas yield

The valorization of by-products for the production of renewable energy can give a significant contribution to a sustainable development. Grass from landscape management or from agricultural practices is currently destined mainly for composting, with the production of a valuable product; however, the process demands energy. Alternatively, anaerobic digestion represents an energy positive process that results in the production of fuel, biogas, and a fertilizer, namely digestate. Previous tests for the evaluation of biogas yield from freshly harvested grass gave promising results. However, for a practical exploitation of this resource appropriate conservation is necessary, in order to enable the daily load of digesters, while reducing the loss of organic matter. The present work is focused on the evaluation of biogas and methane yield from grass collected from riverbanks (Veneto, Northern Italy), after drying and ensiling in order to assess eventual biogas potential losses in comparison to biogas yields from fresh grass. Tests were performed in batch, lab scale digesters. Dry and ensiled grass showed a good potential for the exploitation in the anaerobic digestion process, showing a biogas yield of 565.9 Nl/kgVS and 573.4 Nl/kgVS respectively, compared to 639.7 Nl/kgVS of fresh grass. The conservation determined a yield reduction of 11.5% and 10.4% for dried and ensiled grass, respectively. Dry grass showed a methane yield of 316.2 Nl/kgVS, compared to 332.3 Nl/kgVS of fresh grass (4.8% reduction); presumably the higher concentration of organic acids in ensiled grass determined a higher methane content in biogas and the consequently lower reduction of methane yield, resulting of 330.8 Nl/kgVS (0.5% decrease).