Valorisation of Banana Pseudostem Waste-Based Hydrochar for Sustainable Biodiesel Production from Microalgae

Microalgae-based biodiesel production has gained momentum over petroleum-based fuels due to its carbon neutrality and environment friendly properties. However, issues like high cost of cultivation and usage of inorganic fertilizers possess hindrance towards its commercial production and warrant new approaches of its eco-friendly cultivation techniques. In the present study, effect of hydrochar derived from hydrothermal carbonization (HTCb) of banana pseudostem as a supplement was investigated on the growth performance of microalgae, Graesiella emersonii MN877773, for biodiesel production. The study revealed that hydrochar supplementation (250 ppm) to the 10 days old microalgae culture improved ( p < 0.05) biomass productivity (2 times higher; 0.018 g/L/day). There was also 27% increase ( p < 0.05) in lipid content (16%) and improved lipid productivity (i.e. 0.003 g/L/day) noticed due to the reduction of culture solution pH from highly alkaline to slightly alkaline through hydrochar addition. Fatty acid profiling revealed presence of more amount of methyl palmitate (56% higher) with the high saturated/unsaturated fatty acid ratio (1.3). The vehicular properties of the biodiesel produced from the hydrochar treated microalgae were also found in compliance with the national and international standards with the high heating value of 40.2 MJ/kg. Microalgae grown in hydrochar-supplemented treatment also settled at the bottom within 1 h of resting period with the flocculation efficiency of 50%. This leads to the reduction (62%) in energy expenditure (5.5 kWh/g) and cost (0.4 US$/g) required for harvesting the microalgae. The present study accentuates that hydrochar supplementation at certain dosage to the pre-grown microalgae stimulates more production of quality lipids in the test microalgae with possibility to thrust sustainable biodiesel production.