Manipulation of the Anaerobic Fermentation Process to Produce Carboxylates and Biogas

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Energy in the form of biogas can be obtained via anaerobic degradation of organic wastes. This process is carried out by a mixture of microorganisms that work synergistically. However, when applying the appropriate operational conditions, this bioprocess can be altered and the activity of these microorganisms can be directed towards the generation of other alternative products. An example of this is the carboxylates. These compounds are intermediate products of anaerobic digestion and are currently considered value-added products in the chemical industry. By tuning the process, an imbalance in the synergistic activity of microorganisms can be caused and carboxylates can accumulate instead of reaching biogas as a final product. When applying mild conditions (35 °C and neutral pH), the residue will be transformed mainly into biogas. However, under more restrictive conditions (25 °C and acid pH), the activity of biogas-producing microorganisms can be minimized, achieving the accumulation of a greater quantity of carboxylates. The residue generated after the production of carboxylates can still be valorized in the form of biogas.

Aiming at generating multiple bioproducts from a single residue, this work implemented an innovative strategy by splitting the anaerobic degradation process into two steps: i) in a first reactor, restrictive conditions for methanogenic microorganisms were applied and thus, a carboxylate rich effluent was generated and ii) in a second reactor, the solid residue from the first reactor could still be transformed into biogas. In this way, organic residues can be revalorized for the production of bioproducts of great interest to the chemical industry while generating an energy source such as biogas.

The Biotechnological Processes Unit at IMDEA Energy has recently published these results in the Journal “Bioresource Technology” (*). This scientific article proved the efficient multi-product generation (carboxylates and biogas) by applying two sequential bioprocesses. Additionally, operational conditions imposed in each reactor mediated a microbial specialization that allowed efficient products output.

This work was conducted in the framework of the projects ENE 2017-86864-C2-2-R and RYC-2014-16823 funded by the Spanish Ministry of Economy and Competitiveness, and the financial support from Comunidad de Madrid provided through the project ALGATEC-CM (P2018/BAA-4532), co-financed by the European Social Fund and the European Regional Development Fund.

(*) Llamas, M., Greses, S., Tomás-Pejó, E., & González-Fernández, C. (2021). Tuning Microbial Community in Non-Conventional Two-Stage Anaerobic Bioprocess for Microalgae Biomass Valorization into Targeted Bioproducts. Bioresource Technology, 125387.

More information: Cristina González-Fernández, Senior Researcher and Head of the Biotechnological Processes Unit (
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PRODIGIO is an EU-funded project that plan to establish a base of knowledge for the development of system failure prediction technologies that increase the performance of microalgae production and anaerobic digestion systems and advance towards more favourable techno-economic, environmental and social performance to achieve more sustainable microalgae biogas. The project aims to decode the triggers, identify early-warnings, define threshold values, and calculate warning times for critical state transitions in bioreactors. The technology behind the project could contribute to increasing resource and energy efficiencies >50% throughout the production chain, which would translate into OPEX savings and GHG emissions reduction. The results of the project will pave the way for moving the entire microalgae biogas production chain efficiently towards its theoretical maximum, enabling the development of a fully integrated and truly sustainable microalgae biogas production industry and contributing to strengthening the EU's leadership in renewable fuel technologies. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101007006. Visit for more information.

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