From waste to biofuel with energy-filled microbes

Life after oil is approaching… Finding alternatives to fossil fuels is now vital. Although there has been plenty of progress in harvesting wind and solar energy, these renewable sources are essentially intermittent. Sometimes, other sources of energy need to be used to store the surplus (such as turbine pump stations) or alternatively, to meet a shortfall (if there is no wind to power a turbine). While electric and hybrid cars are equipped with batteries, their expense and environmental viability have led researchers to look at other energy carriers.

Microbial electrosynthesis could be the answer.

This process involves producing relevant organic molecules by feeding them electricity from renewable sources and reducing the available CO2 through microorganism activity. These molecules can then be used as fuel or in chemical synthesis. This is an extremely promising technology for the biorefinery of organic waste (food, garden, agricultural, etc.), a low cost and readily available raw material [1]. Waste that is rich in energy but heterogeneous and complex. How can this energy be used in microbial electrosynthesis?

A team at Irstea has taken on this challenge, and for the first time, has managed to optimize the electrochemical process while cutting the electricity consumption needed for the transformation by one third. A globally relevant innovation [2].

Chemical relationships

Microbial electrosynthesis has been studied since the early 2010s, particularly in the US. Irstea's innovation isn't so much in the process itself but rather in the procedure to implement it.

Decryption Waste contains energy rich molecules. This energy is extracted by a process of oxidation (electron emission) at the anode. The energy in these electrons is augmented using a generator fed from renewable energy. The electrons then pass to the cathode where they are used in a biological reaction: microbes connected to the electrode feed on CO2 and the electrons to create a high value molecule. This is the famous molecule that makes it possible to store the energy produced from renewable energy sources (wind turbines, solar panels, etc.).

© Irstea

American scientists have so far been using water at the anode. So why use organic waste? "Waste contains much more energy," explains Théodore Bouchez, manager of the microbial bioprocedures and biotechnologies for waste recovery team. "Because of this, we need much less electricity to run the device." In practice, this means that an anode fed by waste rather than water has a much lower electrostatic potential This lower potential is such that the process uses almost a third less electricity!

"We increased the energy efficiency of the transformation."

A really positive development! The American Department of Energy has published economic studies looking into the cost-effectiveness of microbial electrosynthesis technology. While calculating the production costs for 1 kg of manufactured molecules, it became clear that 2/3, or even 3/4, of these costs are a result of the energy (solar, in this study) invested into the system. According to Bouchez, it's obvious: "Using waste to decrease energy consumption by 1/3 increases the economic appeal of the technology, even if the process is more delicate to implement than when using water and requires real leadership." This is due to the living organisms (microbes) involved on both sides of the device!

Electrons must be transmitted from the anode to the cathode at the same intensity. If the anode delivers more electrons than can be consumed by the microbes on the cathode, there is a risk of losing electrons and creating hydrogen during the reaction. In contrast, if there are fewer electrons, the molecules won't be manufactured at the optimum speed. In addition, microbes at the cathode must not be too greedy. "Poor synchronisation can cause the device to become completely inactive! We noted this during our initial laboratory experiments, although we also managed to understand what was happening and suggest solutions to fix the problem." In this way, the patent was created.

Disruptive technology

So far, results have been promising but microbial electrosynthesis must still prove itself. Right now, the most commonly produced molecules are essentially simple organic molecules (methane, carboxyilic acids). Other, more interesting molecules (alcohols, ketones, etc.) have been produced but the process must be optimized and made more reliable.

"It is a disruptive technology, but we are still slow in creating these transformations, and this is preventing it from becoming a technology of mass production."

That is not to say they aren't aiming high. This research is part of the BIORARE project [3], which has the ultimate aim of combining this process with facilities for the anaerobic digestion of waste to produce energy: waste, electricity and CO2 on tap. One single, unique field, as optimized as possible.

For more information

[1] In France, 345 million tons of waste are produced, according to Agency for the Environment and Energy Management (Ademe – 2012 Figures)

[2] Patent application no. 14.59281: Process and device to regulate the activity of a bioelectrochemical system comprising both a bioanode and biocathode.

[3] The BIORARE project (2011-2016), financed through the Investments for the Future Program AAP Biotechnologies and Bioresources. It brings together 5 partners: Irstea (HBAN and GERE research units), INRA-LBE, CNRS-LGC and Suez-Environnement.