Processing of marine microalgae biomass via hydrothermal liquefaction for bio-oil production: study on algae cultivation, harvesting, and process parameters

Bio-oil production from microalgae is still not practicable due to the high demand for water and nutrients, low biomass yields, and lack of efficient harvesting and biomass conversion techniques. More realistic approaches are required to improve the scope of microalgae-derived bio-oils. This study cultivated two marine strains Chlorella salina and Spirulina subsalsa utilizing municipal wastewater, which reduced the need for scarce fresh water resources and nutrients. Furthermore, enhanced biomass yields were accomplished through optimization of environmental factors such as light intensity, photoperiod, wavelength, aeration, and acclimatization of marine cultures to wastewater. Growth optimization achieved maximum biomass yields of 2.57 g L−1 in respect of C. salina at light intensity of 150 µmol m−2 s−1, 16:08-h photoperiod, aeration rate of 0.2 vvm under blue wavelength. Likewise, S. subsalsa gave a maximum biomass of 2.85 g L−1 at intensity of 100 µmol m−2 s−1, 16:08-h photoperiod, and 0.2-vvm-aeration rate under red light. Acclimatization of marine cultures to municipal wastewater further improved the biomass yields to 3.02 and 2.78 g L−1 in respect of S. subsalsa and C. salina demonstrating the viability of utilizing 40 to 60% wastewater blended media as nutritional source. Besides, the prospects of utilizing natural flocculants Strychnos potatorum and Moringa oleifera for harvesting marine biomass were also established achieving higher biomass recovery efficiency of 87–90%. Furthermore, hydrothermal liquefaction of wastewater-acclimated marine biomass performed at optimized biomass loading and reaction temperature produced higher bio-oil yield of 32.18 wt% and 28.37 wt% in respect of C. salina and S. subsalsa.