Improving biohydrogen and biomethane co-production via two-stage dark fermentation and anaerobic digestion of the pretreated seaweed Laminaria digitata

The marine macro-alga Laminaria digitata is an abundant brown seaweed, which may be used as a feedstock for gaseous biofuel production via sequential dark fermentation and anaerobic digestion. Various methods, including hydrothermal pretreatment (HTP), hydrothermal dilute acid pretreatment (HTDAP), enzymolysis, and combinations thereof, were employed to depolymerize L. digitata and assess the effects on biohydrogen and biomethane yields. Scanning electron microscopic images revealed that the intact and smooth structure of the seaweed was severely damaged; some micro-pores and debris were generated after HTP (140 °C for 20 min), whilst the undegraded components remained as filamentous structures. The complex carbohydrate polymers in L. digitata constrained the catalytic effects of glucoamylase, leading to limited increase in the yield of carbohydrate monomers. With the aid of H₂SO₄ (1 v/v%) in HTP, depolymerization of biomass and its further conversion to carbohydrate monomers were significantly improved. The yield of total carbohydrate monomers after HTDAP (0.564 g/gVS) was 3.5-fold that in raw biomass; this led to an increase of 60.8% in biohydrogen yield (57.4 mL/gVS) in the first-stage dark fermentation. However, the generation of byproducts such as hydroxymethylfurfural under such harsh conditions impaired the second-stage anaerobic digestion of hydrogenogenic effluent, resulting in a 25.9% decrease in biomethane yield. HTP was considered the optimum pretreatment improving energy conversion efficiency from seaweed to gaseous biofuels by 26.7% as compared to that of the unpretreated L. digitata.