Combined Production of Three Bioenergy Resources from Nannochloropsis sp. Microalgae

Abstract

In this study, three bioenergy resources namely lipid, sugars and biogas were produced using two treatment sequences to combine production of three bioenergy resources from microalgae Nannochloropsis sp.,. Sequance-1 started with hydrolysis hydrocarbons using cellulase enzyme obtained from Trichoderma viride fungi. Following hydrolysis, lipids were extracted and the spent algae was mixed with wastewater sludge and anaerobically digested to produce biogas. Sequence-2 started with lipids extraction, followed by hydrolysis then biogas production. In sequence-1, hydrolysis improved with increased enzyme dose and produced a maximum of approximately 103 mg/g total sugars and 88 mg/g reducing sugars. Lipids were then extracted, which resulted in approximately 0.48 g lipids/g algae. Biogas production increased as the quantity of spent algae added to sludge increased but enzymatic hydrolysis reduced the substrate value of spent algae. The maximum specific biogas (263 mL/g) was produced from the water-hydrolyzed algae and sludge mixtures, then from the enzyme-hydrolyzed algae and sludge mixtures (213 mL/g), then from sludge alone (106 mL/g). In sequence-2, lipids (0.48 g/g) were fully extracted then enzymatic hydrolysis of the residues produced a maximum of approximately 83 mg /g total sugars and 79 mg/g reducing sugars. The ultimate biogas produced from the spent algae and sludge mixtures reached 238 mL/g. The results confirmed the potential of combining production of three bioenergy resources from Nannochloropsis sp. The sugars can be used for bioethanol production, the lipids for biodiesel production and the biogas can be processed to benefit from its methane content. Furthermore, the results suggested that the production sequence influences the relative production of the three resources, with sequence-1 being the better option.