Bacteria batteries turn sewage into clean energy

Purple bacteria could turn wastewater treatment plants into zero-carbon fuel generators, says Frontiers in Energy Research. The new findings are the first reported use of photosynthetic microbes in a battery-like bioelectrochemical system.

Organic compounds found in household sewage and industrial wastewater are a rich potential source of energy, bioplastics and proteins for animal feed. Yet with no efficient extraction method, treatment plants discard them as contaminants.

Now, researchers at Frontiers in Energy Research have found an environmentally-friendly and cost-effective solution, showing for the first time that purple phototrophic bacteria can recover almost 100 per cent of carbon from any type of organic waste while generating hydrogen gas for electricity production.

“One of the most important problems of current wastewater treatment plants is high carbon emissions,” explained study co-author Dr Daniel Puyol in a statement. “Our light-based biorefinery process could provide a means to harvest green energy from wastewater, with zero carbon footprint.”

Purple phototrophic bacteria capture energy from sunlight using a variety of pigments and can use organic molecules and nitrogen gas to provide carbon, electrons and nitrogen for photosynthesis. This means they grow faster than alternative phototrophic bacteria and algae, and can generate hydrogen gas, proteins or a type of biodegradable polyester as byproducts of metabolism.

“What is unique about our approach is the use of an external electric current to optimize the productive output of purple bacteria,” said Puyol.

Known as a bioelectrochemical system, the concept works because the metabolic pathways in purple bacteria are connected by electrons. A supply of electrons is required for capturing light energy, while turning nitrogen into ammonia releases excess electrons, which must be dissipated. By optimizing electron flow within the bacteria, an electric current can maximize the rate of synthesis.

“Recordings from our bioelectrochemical system showed a clear interaction between the purple bacteria and the electrodes: negative polarization of the electrode caused a detectable consumption of electrons, associated with a reduction in carbon dioxide production,” added Puyol.

Capturing excess CO2 produced by purple bacteria could be useful not only for reducing carbon emissions, but also for refining biogas from organic waste for use as fuel, according to the statement.

Photo credit: Montgomery County Planning Permission/ CC BY-SA 2.0

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