Sustainable management of lignocellulosic wastes: Temperature strategies for anaerobic digestion of artichoke

Abstract

Lignocellulosic residues are a widely available energy resource, but their conversion into biogas through biomethanation may be hindered by a chemical structure with strong bonds. A strategy based on temperature-phased anaerobic digestion (TPAD) is here applied to optimize anaerobic digestion of artichoke as a representative lignocellulosic waste. The experiments consist of a first thermophilic stage conducted at 55 °C (7 and 5 days), followed by a second mesophilic stage at 35 °C. In addition, for comparison, single-stage temperature trials are carried out within the most common conditions for biomethanation (mesophilic at 35 °C, and thermophilic at 55 °C), as well as at the intermediate range (42 °C). In general, biomethanation of artichoke residues under TPAD configuration provides better performance than single-stage systems. Particularly, the best performance is observed for the TPAD arrangement using a thermophilic phase of 7 days. Thus, at this optimal configuration, volatile solids (VS) removal of 54%, and biogas and methane yields of 442 mL/g-VS and 375 mL-CH4/g-VS were reached, respectively. Moreover, a kinetic model coupling biogas generation and substrate consumption has been applied. Good agreement is found for both variables in each system tested. The calculated non-biodegradable substrate (S∞) is lower in TPAD systems (mean 49.6%) compared with that of single-stages (average 53.9%). The maximum specific growth rate of biomass (μM) is higher for TPAD digesters (mean 0.0625 d−1), although the thermophilic single-stage reactor reaches a close value (0.0615 d−1). The biogas yield (α) is similar for all conditions (0.358 L/g-VS), but with a maximum value of 0.428 L/g-VS for the TPAD digester with the 7-days thermophilic period. Therefore, temperature-phased anaerobic digestion has proved to be an efficient strategy for sustainable management of artichoke wastes.