Advanced biomethane production by arundo donax under changing harvest time and nitrogen fertilization

According to the RED II (2018/2001/EU), Member States must supply a minimum of 14% of the energy consumed in road and rail transport by 2030, of which the contribution of advanced biofuels and biogas must reach at least 3.5%. Reduce biomass recalcitrance of high-yielding lignocellulosic crops by means of agronomic strategies would significantly contribute to the advanced biofuel production goal. Lignocellulose is the lowest cost raw material on earth, it is a no-food biomass and its use alone or in mix with other biomasses can strongly increase the biomass availability for advanced biomethane production. The present study evaluated the suitability of the lignocellulosic, herbaceous Arundo donax as a biomass feedstock for advanced biomethane production. Harvest time and nitrogen fertilization treatments were adopted to reduce biomass recalcitrance thereby increasing biomethane yield. Biochemical Methane Potential (BMP) was evaluated on batch anaerobic fermenters in mesophilic conditions. The BMP at 30 days of incubation was influenced by the investigated treatments, the incubation time and the interaction of these two factors. The trend showed a lag phase for the first 5 days of testing, due probably to the adaptation of the bacterial flora to the lignocellulosic matrix, followed by an exponential increase up to approximately 18 days; after that a slight increase tending to an asymptotic trend in the final phase (up to day 30) was observed. The highest BMP was reached by the combination of winter harvest (W) and 80 kg N ha-1 (123.4 Nml CH4 g-1 SV), followed by the autumn harvest (A) and 80 kg N ha-1 (118.1 Nml CH4 g-1 SV). The unfertilized treatments showed an opposite BMP, with the autumn higher than winter harvest (106.2 and 100.3 Nml CH4 g-1 SV, respectively). In terms of biomethane yield per unit land area, WN80 showed the highest (1717 ± 203 m3 CH4 ha-1) and WN0 the lowest (859 ± 93 m3 CH4 ha-1). Nitrogen fertilization looks a promising strategy to reduce biomass recalcitrance for bioconversion by anaerobic digestion mainly due to the higher content of neutral detergent soluble and protein; however, it should be proved by an energy, economic and environmental analysis to ascertain an overall sustainability.