Sustainability drivers of microalgal biodiesel production using a hybrid stainless steel–glass photobioreactor: Integrated assessment of cultivation productivity, fuel quality, techno-economic performance and life-cycle impacts

Microalgae-based biodiesel offers a renewable alternative to fossil diesel, but its sustainability depends on biological productivity, fuel quality, process economics, and environmental performance. This study evaluated Chlamydomonas sp. and Euglena sp. cultivated in a hybrid stainless steel–glass photobioreactor by integrating cultivation performance, biodiesel characterization, fuel-property prediction, techno-economic assessment (TEA), and life-cycle assessment (LCA). At day 15, Chlamydomonas sp. attained a biomass concentration of 1.84 ± 0.04 g L⁻¹ , lipid concentration of 0.66 ± 0.02 g L⁻¹ , lipid content of 35.63 ± 0.06%, and volumetric lipid productivity of 43.94 ± 2.07 mg L⁻¹ d⁻¹ , compared with 0.96 ± 0.02 g L⁻¹ , 0.21 ± 0.01 g L⁻¹ , 21.53 ± 0.31%, and 13.64 ± 1.19 mg L⁻¹ d⁻¹ , respectively, for Euglena sp. Experimentally measured methyl-ester yields were 65.48 ± 0.48% and 75.46 ± 0.64%, respectively. Fatty acid methyl ester composition was experimentally determined by GC-FID, while selected fuel properties were estimated using established empirical correlations and benchmarked against ASTM D6751–24 and EN 14214:2012 + A2:2019. The predicted properties indicated favorable fuel characteristics; however, formal compliance with all EN 14214 requirements was not claimed because the properties were estimated rather than directly measured by standardized methods, and predicted oxidative stability remained below the current 8 h EN criterion. The TEA identified biomass and lipid productivity, conversion efficiency, and energy demand as key economic drivers, with Chlamydomonas sp. providing the stronger production basis. The cradle-to-gate LCA, conducted according to ISO 14040/14044 using 1 kg biodiesel as the functional unit, identified productivity-dependent energy, nutrient, water, and downstream-processing intensities as important environmental burden drivers. Allocated petrol intensity was approximately 520.2 and 1420.2 L kg⁻¹ biodiesel for Chlamydomonas sp. and Euglena sp., respectively. Overall, the findings demonstrate that improving biodiesel productivity and reducing fossil-energy dependence are central to improving the economic and environmental performance of microalgal biodiesel production.