Conversion of waste microalgae into caproic acid using anaerobic membrane bioreactors without external electron donors

Abstract

Microalgae represent an abundant and underutilised biomass resource with high potential for conversion into renewable fuels and chemicals. However, their valorisation into high-value chemicals such as medium-chain carboxylic acids has been hindered by the prohibitive cost and carbon footprint of external electron donors required for microbial chain elongation. Here, we present an anaerobic membrane bioreactor platform that overcomes these drawbacks by integrating yeast to generate electron donors in situ. Results showed that yeast addition increased ethanol availability and eventually elevated caproate production rate by 1.6-fold. Metagenomics analysis revealed that yeast restructuring of the microbial community enriched Clostridia and other functional chain elongators, while shifting metabolic potential toward reverse β-oxidation. Consequently, the system achieves high-specificity production of caproate from waste microalgae without external inputs. Techno-economic and life cycle assessments confirm the transformative potential of this strategy, yielding a minimum product selling price of $1.86 kg-1 for caproic acid (lower than current market price of $2.58–3.58 kg-1) while reducing the global warming potential by 76% compared to conventional ethanol-based chain elongation processes. By integrating in-situ electron donor generation with a membrane bioreactor, this work presents a green and carbon-efficient route for the valorisation of organic waste into drop-in biochemicals.