Sustainable biofuel from microalgae cultivated with waste streams from a biogas plant

Microalgae are considered a promising feedstock for biofuel production due to their lipid content and ability to assimilate CO₂. This study investigates the valorization of pig slurry through anaerobic digestion and the subsequent reuse of 40% of the biogas-derived CO₂, along with digestate water rich in nutrients, to stimulate native microalgae growth in open ponds at a biogas facility in Soria, Spain. Dominant microalgae genera identified include Chlorella sp., with minor amounts of Scenedesmus and Ankistrodesmus. After harvesting and drying, the biomass was subject to Soxhlet extraction using two solvents: dichloromethane–methanol mixture (leading to oil-DM) and petroleum ether (oil-E), yielding 18.9 and 2.3 g oil/100 g dry algae, respectively. Oil-DM and oil-E were esterified and transesterified, producing fatty acid methyl esters (FAME-DM and FAME-E), with ester contents of 63.91% and 11.19%, respectively. FAME-DM was further purified by chromatographic separation to isolate FAME-DMP, with a final ester content of 93.3% w/w. The different extracts and esters were characterized using spectroscopy and chromatography techniques, including FT-IR, UV-vis, GC-FID, and GC-MS. Solvent polarity played a key role in oil extraction efficiency and composition. Although biomass presented low heating value and high ash content, discarding its use as a solid biofuel, it showed strong potential as a feedstock for biodiesel production. Integrating microalgae cultivation with waste streams from a biogas plant, such as CO₂ and digestate water, demonstrates a feasible circular approach for biofuel generation. Estimated oil production from 3.75 m³/day slurry could exceed half a ton per year, supporting sustainable, small-scale biodiesel applications.