Ensilage and anaerobic digestion of Sargassum muticum

Abstract

The brown seaweed Sargassum muticum is an invasive species to the coasts of the British Isles, mainland Europe and North America. Attempts at its eradication and control have generally not been successful, although time-consuming and costly. Commercial exploration of this biomass for fuel could encourage its harvesting and control. Anaerobic digestion (AD) has been suggested as one of the most promising methods of exploiting algae for biofuel. The harvesting of S. muticum is seasonal; thus, there will be a need to preserve and store seaweed to supply a year-round anaerobic digestion process. Ensiling is widely used in terrestrial agriculture, but there has been little research on ensiling seaweed. The aims of this research were to: a) study the effect of ensiling on the biomethane potential of S. muticum, b) effect of size reduction prior to ensilage on leachate and other losses during ensiling and c) examine the mass balance and energy losses of ensiling S. muticum. Ensiling was found to be an effective, low energy loss method of preserving seaweed with energy loss from the biomass due to ensiling <8 % of the higher heating value of seaweed feedstock. Ensiling results in losses of salt from the biomass and the virtual total loss of organic sulphur. Size reduction of seaweed prior to ensilage reduced leachate and energy loss from the biomass. Ensiling had no significant effect on methane yield. However, methane yields from S. muticum are low ≤0.11 L CH₄ g⁻¹ volatile solid (VS) at ~25 % of the theoretical maximum. Further research is needed to establish the reasons for the recalcitrance of S. muticum, but the C:N ratio of S. muticum is low (8:1), and co-digestion with a low nitrogen content substrate such as crude glycerol may be a potential method of improving methane yield.