Colorful Bacteria Combines Sewage and Light to Make Clean Energy

What you just flushed down the toilet could be more useful than you ever thought. Researchers have found that a type of purple bacteria can break down the organic compounds found in sewage and potentially turn it into energy.

Organic compounds in wastewater have been the subject of studies for decades. Engineers have spent decades trying to figure out how to extract the beneficial elements from the sewage. However, an efficient extraction method doesn’t exist yet, and treatment plants discard them as contaminants.

Using just slight electric current, this purple phototrophic bacteria could be key in breaking down the waste.

"One of the most important problems of current wastewater treatment plants is high carbon emissions," said co-author Daniel Puyol of King Juan Carlos University, Spain. "Our light-based biorefinery process could provide a means to harvest green energy from wastewater, with zero carbon footprint."

Where does the purple bacteria come from?

When one thinks of photosynthesis, it’s often the green of chlorophyll that comes to mind. However, chlorophyll leaves during the autumn to leave behind yellows, orange, and red hues.

The researchers pointed out that photosynthetic pigments come in a bunch of different colors and organisms. That’s where the phototrophic bacteria come into play.

The bacteria capture energy in a variety of pigments. However, the metabolism of the organism -- not the color --  caught the attention of Puyol and the team. The bacteria used organic molecules and nitrogen gas rather than CO2 and H20 for photosynthesis.

That difference gives them an edge over other phototrophic bacteria and algae. It also means they can make anything from hydrogen gas to proteins to a type of biodegradable polyester because of that metabolism, according to the researchers.

"Purple phototrophic bacteria make an ideal tool for resource recovery from organic waste, thanks to their highly diverse metabolism," explained Puyol.

Transforming waste into biofuel with a minimal carbon footprint

The researchers explained one of the most exciting benefits to breaking down the waste sewage is just how low of an energy expenditure it takes to do it. 

"Our group manipulates these conditions to tune the metabolism of purple bacteria to different applications, depending on the organic waste source and market requirements," said co-author Professor Abraham Esteve-Núñez of University of Alcalá, Spain. "But what is unique about our approach is the use of an external electric current to optimize the productive output of purple bacteria."

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The whole system is called a bioelectrochemical system and uses the metabolic pathways in the purple bacteria to connect electrons. By improving the electron flow within the bacteria, an electric current can speed up the rate the bacteria synthesize sewage. 

"This demonstrates that purple bacteria can be used to recover valuable biofuel from organics typically found in wastewater -- malic acid and sodium glutamate -- with a low carbon footprint," reported Esteve-Núñez.

Puyol explained in a statement that there's still more room for improvement: 

"One of the original aims of the study was to increase biohydrogen production by donating electrons from the cathode to purple bacteria metabolism. However, it seems that the PPB bacteria prefer to use these electrons for fixing CO2 instead of creating H2.

"We recently obtained funding to pursue this aim with further research and will work on this for the following years. Stay tuned for more metabolic tuning."