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

The concept of a sustainable development is strictly related to the view of a circular economy. In this optic, the valorization of wastes or by-products for the production of renewable energy is a valid solution, also for the primary sector. Grass, derived 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. The present work was focused on the evaluation of the potential, in terms of biogas and methane yield, of grass collected from riverbanks (Veneto, 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 in the two harvesting periods. The tests were performed on both samples of grass, immediately after the collection. Tests were performed in batch, lab scale digesters, 4l volume each, in mesophilic conditions (38°C), for 40 days for each 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 VS reduction. 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.