Believe it or not, there exists a type of purple bacteria can break down the organic compounds found in sewage and turn it into energy. That means, there’s a way to take what we are all flushing down the toilet and turn it into something useful! This colorful bacteria could be the answer to a long search for a way to turn sewage into a valuable resource.
For decades now, engineers have been trying to extract beneficial elements from sewage. Yes, there’s usefulness in your poop. Scientists just haven’t figured out how to separate it. After countless studies on the organic compounds in wastewater, an efficient extraction method of “the good stuff” (its rich potential source of energy, bioplastics and even proteins for animal feed) still doesn’t exist. Treatment plants can only discard it all as contaminants.
Waste No More Waste
The purple phototrophic bacteria could be key in breaking down the waste. All that is needed is a slight electric current. It is an environmentally-friendly and cost-effective solution. The study has been published in Frontiers in Energy Research. It is the first to show that purple phototrophic bacteria, which can store energy from light, when supplied with an electric current, can recover close to 100% of carbon from any type of organic waste while generating hydrogen gas for electricity production.
Co-author Daniel Puyol of King Juan Carlos University, Spain, said:
“One of the most important problems of current wastewater treatment plants is high carbon emissions. Our light-based biorefinery process could provide a means to harvest green energy from wastewater, with zero carbon footprint.”
Impressive Photosynthetic Abilities
Photosynthetic pigments come in a bunch of different colors (not just green). They also come in a bunch of different organisms (not just leaves) like phototrophic bacteria. The bacteria capture energy in a variety of pigments. However, its ability to take in energy in any color is not what caught the attention of Puyol and the team. It was the metabolism of the organism that really got the researchers excited. Puyol said:
“Purple phototrophic bacteria make an ideal tool for resource recovery from organic waste, thanks to their highly diverse metabolism.”
The bacterium uses organic molecules and nitrogen gas rather than CO2 and H2O to provide carbon, electrons, and nitrogen for photosynthesis. That means they can make anything from hydrogen gas to proteins to a type of biodegradable polyester as byproducts of metabolism.
A Metabolism That Can Be Tuned
As a bonus, one of the benefits of breaking down the waste sewage in this process is just how low of an energy expenditure it takes to do it. The bacteria’s environmental conditions — like light intensity, temperature, and the types of organics and nutrients available – will determine which metabolic product predominates. The metabolic output can be tuned with electricity.
Co-author Professor Abraham Esteve-Núñez of University of Alcalá, Spain, said:
“Our group manipulates these conditions to tune the metabolism of purple bacteria to different applications, depending on the organic waste source and market requirements. But what is unique about our approach is the use of an external electric current to optimize the productive output of purple bacteria. 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.”
The whole system is called a bioelectrochemical system. It uses the metabolic pathways in the purple bacteria that are connected by a common currency – electrons. For example, a supply of electrons is required for capturing light energy, while turning nitrogen into ammonia releases excess electrons, which must be dissipated. By improving the electron flow within the bacteria, an electric current (provided via positive and negative electrodes, as in a battery) can speed up the rate the bacteria synthesize sewage.
The Study
- The group analyzed the optimum conditions for maximizing hydrogen production by a mixture of purple phototrophic bacteria species.
- They tested the effect of a negative current — that is, electrons supplied by metal electrodes in the growth medium — on the metabolic behavior of the bacteria.
The Results
- The nutrient blend that fed the highest rate of hydrogen production also minimized the production of CO2. Esteve-Núñez reports, “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.”
- Then, by using electrodes they discovered that purple bacteria are capable of using electrons from a negative electrode or “cathode” to capture CO2 via photosynthesis.
Esteve-Núñez reports:
“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. This indicates that the purple bacteria were using electrons from the cathode to capture more carbon from organic compounds via photosynthesis, so less is released as CO2.”
Conclusion
This first reported use of photosynthetic microbes in a battery-like ‘bioelectrochemical system’ could be useful not only for reducing carbon emissions but also for refining biogas from organic waste for use as fuel. It demonstrates how purple bacteria could turn wastewater treatment plants into zero-carbon fuel generators. However, Puyol says that this is only the beginning and the group’s true goal lies further ahead. He said:
“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.”
