Purple bacteria can turn human waste into clean hydrogen energy

The new method reduces carbon emissions and turns wastewater treatment plants into green generators.

A significant downside to wastewater treatment plants is their carbon footprint. Now, researchers have found a way to reduce carbon emissions from sewage and produce hydrogen energy at the same time. Their method uses purple bacteria and electrical currents to capitalize on the organic materials we flush down the toilet every day. We spoke with one of the study’s authors, Daniel Puyol of Spain’s King Juan Carlos University, to learn more.

ResearchGate: What are purple bacteria, and where do you find them in nature?

Daniel Puyol: Purple phototrophic bacteria belong to the biggest and most diverse group of bacteria. All of them are photosynthetic, but unlike plants and algae, they use infrared light as the energy source for their metabolism. This gives them a color from brown to red—including purple.

The main feature of these fascinating organisms is their versatile metabolism. They can perform a range of metabolic reactions, making them a kind of metabolic Swiss army knife. For this reason, these organisms are ubiquitous in nature. However, their preferred environment is in bodies of water, mainly lakes. They’re also frequently found in wastewater treatment plants, which gives us a clue about their applications.

RG: How did you get the idea to use them to harvest energy from our waste?

Puyol: While I was studying with Damien Batstone and Tim Hülsen, we used these organisms for wastewater treatment. We thought that we could tune their metabolism for something else. One of the possibilities is the production of hydrogen, which is a very energetic gas. We are currently exploring this option at the University Rey Juan Carlos.

RG: How do the bacteria produce hydrogen gas?

Puyol: All living beings have to maintain an equilibrium, which microbiologists and biotechnologists call homeostasis. Purple bacteria has the problem of excess electrons from their metabolism. One way of releasing this excess is through carbon dioxide fixation, like plants do. The other one is the release of electrons as hydrogen gas.

RG: Does it matter what’s in the waste?

Puyol: Yes, waste composition plays a key role on the ability of purple bacteria to produce hydrogen. The process is strongly inhibited in the presence of ammonium, which mainly comes from proteins in waste. We have to be completely sure that the ammonium is eliminated prior to the process, so a diet low in proteins would potentially help to produce more hydrogen more easily.

RG: How much energy can you harvest from poop?

Puyol: The main challenge in modern wastewater treatment plants is to go for energy neutrality, to maintain a treatment plant with no external energy. With the use of purple bacteria, we are trying to go beyond that, and transform the wastewater treatment plant into a real biorefinery. For a medium-size wastewater treatment plant, the direct transformation of organic contamination into hydrogen by purple bacteria could theoretically yield energy for 43-107 houses. Of course, most of this energy usually goes to the direct needs of the plant.

RG: What are you latest findings? Is someone putting them into practice?

Puyol: Our preliminary findings indicate that we are able to tune the metabolism of purple phototrophic bacteria to increase carbon dioxide fixation, while maintaining the same hydrogen productivity. This essentially means zero carbon footprint. We have recently obtained funding to design the process and patent the technology. With the technology demonstrated at lab-scale, we will try to convince the water sector about the feasibility of our technology. We have close contact with some water companies that would be interested.

RG: What is the ultimate goal for your research?

Puyol: We are ambitious, as we know that the possibilities around the use of purple phototrophic bacteria are wide. We are aiming to go beyond wastewater treatment, directly into bio-industry. We know that the only way of achieving success with a technology focused on resource recovery is the commercialization of these resources. So we are creating an atmosphere around purple bacteria technology, including bio-industries, water management companies, and waste management companies.

Resource recovery from waste and wastewater is nothing new. We are trying to do what nature has been being doing for millions of years. Nature, in its wisdom, has selected photosynthesis as a mechanism for these transformations. We are only accelerating them.

 

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Featured image courtesy of Kevin Millican.