A close-loop integrated approach for microalgae cultivation and efficient utilization of agar-free seaweed residues for enhanced biofuel recovery

The aim of this work was to evaluate a novel integrated biorefinery route for enhanced energy recovery from seaweeds and microalgae. Agar extraction prior to anaerobic digestion recorded the highest biogas productivity of 32.57 L kg(-1) VS d(-1). Supplementation of the microalgal growth medium with anaerobic digestate from agar-extracted biomass enhanced the microalgal growth, recording the highest dry weight of 4.57 g L-1 at 20% digestate ratio. In addition, lipid content showed the highest value of 25.8 %dw. Due to enhancement of growth and lipid content, 20% digestate ratio showed the highest lipid productivity and FAMEs recovery (65.2 mg L-1 d(-1) and 123.3 mg g(-1)dw, respectively), with enhanced biodiesel characteristics. The present study estimated annual revenue of 1252.7 US$ ton(-1) from the whole Gracilaria multipartita biomass conversion into biogas, while that through agar extraction deserved 36087.0 US$ ton(-1), with enhanced annual biodiesel yield by 69.7% over the control medium.