Development of a low-cost cultivation medium for simultaneous production of biodiesel and bio-crude from the chlorophycean microalga Tetradesmus obliquus: A renewable energy prospective

Microalgal biodiesel production experiences hurdles in every step, beginning from cultivation of microalgal strains for biomass generation followed by harvesting, drying, oil extraction, and transesterification, leading to higher cost of production. In this study, an approach was adopted to minimize the cost of biomass production by utilizing the locally available agricultural fertilizers, viz. urea, NPK 10 26 26, diammonium phosphate, potash, superphosphate, etc. for cultivation of the green microalga Tetradesmusobliquus that can be subsequently used for biodiesel and bio-crude production. For this, an optimized urea medium (OUr) was developed, which reduced the cost of production by 47-fold as compared to the commonly used N 11 medium, formulated with the analytical grade inorganic salts. This optimized medium further enhanced the biomass yield by 32% while reducing the requirements of the major nutrients like potash, magnesium sulfate and superphosphate by 29, 25 and 25%, respectively. Urea requirement was, however, increased by 20%. Raceway pond cultivation of T. obliquus in the newly formulated medium depicted a biomass productivity of 12.6 g m⁻² day⁻¹ during winter season followed by 10. 9 g m⁻² day⁻¹ in summer and 4.8 g m⁻² day⁻¹ in rainy season at a culture depth of 30 cm, thus leading to an estimated biomass productivity of 30.04 ton hectare⁻¹ year⁻¹,and a biodiesel productivity of 2.07 ton hectare⁻¹ year⁻¹. The fuel characteristics of the biodiesel produced showed comparable characteristics with petro-diesel, and were also within the limits specified by the national and international standards. Bio-crude productivity of 10 ton hectare⁻¹ year⁻¹ was projected from the de-fatted biomass. This study was thus successful in utilizing >40% of the biomass for simultaneous production of biodiesel and bio-crude from the same biomass, eventually paving a path for the development of a microalgal refinery by exploring the spent biomass for further applications.