Anaerobic digestion of grass: effect of the harvesting period on biogas yield

Abstract. Grass derived from landscape management or from agricultural practices, is currently destined mainly to 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. The present work was focused on the evaluation of biogas and methane yield of grass collected from riverbanks (Northern Italy) in two different periods, spring and summer. The effect of the composition of the organic matter, content of cellulose, and lignin in particular, plays a relevant role in the conversion to biogas. Biomass production per hectare and the consequent biogas production were also evaluated.

The tests were performed on both samples of grass, immediately after the collection, in batch, lab scale digesters (4l volume, in mesophilic conditions (38°C), for 40 days per cycle). Digestate from a biogas plant fed with dairy cow manure was used as inoculum. Inoculum and grass were dosed to achieve a VS ratio equal to 2. The paper presents the results in terms of biogas production, biogas quality (CH₄, CO₂, H₂S), and emissions from digestates (NH₃, CO₂ and CH₄). Input and output were subject to the determination of Total Solids (TS), Volatile Solids (VS), pH, volatile fatty acids (VFA), alkalinity, acidity vs. alkalinity ratio, fibers (cellulose, lignin), Total Kjieldal Nitrogen (TKN), Total Ammoniacal Nitrogen (TAN). A further study will consider the effect on biogas yield sorted by conservation techniques, such as drying and ensiling of grass.