The bio-methane potential of whole plant and solid residues of two species of red seaweeds: Gracilaria manilaensis and Gracilariopsis persica

Abstract

Macroalgae, known commonly as seaweed, is extensively used in industry for extraction of its valuable products such as agar and carrageenan. This extraction process generates a large amount of waste in the form of biomass residues. In this study, bio-methane potential of two red seaweeds, Gracilaria manilaensis and Gracilariopsis persica, both as whole plant and their industrial residues, were compared after a mild acid pre-treatment at pH 2, 100 °C and 1 h in batch assays. The results showed that the pre-treated residual biomass of both G. manilaensis and G. persica had higher bio-methane potentials at 70% and 62% of the theoretical yield, respectively, compared to the whole plant biomass which achieved only 48% and 46% of the theoretical yield, respectively. The pre-treatment step increased the initial reaction rate of the residues by 47% and 77% compared to the untreated samples. On the other hand, the pre-treatment step only enhanced the initial reaction rates by 25% and 39%, respectively, with the whole plant sample. The results confirm that the seaweed residues are more suitable as a feedstock for anaerobic digestion compared to whole plant biomass.