Entire industries are built around making waste seem invisible, like it was never there in the first place. But a new study from Spain suggests this might be a missed opportunity — scientists have shown that a type of purple bacteria can convert waste into hydrogen gas for energy production.
Purple bacteria are phototrophic, which means that they use photosynthesis like plants. The difference is that their source of metabolism is infrared light, as opposed to the visible light that green plants crave. "The main feature of these fascinating organisms is their versatile metabolism," says of Spain’s King Juan Carlos University, author of the study, speaking to .
"They can perform a range of metabolic reactions, making them a kind of metabolic Swiss army knife."
These bacteria adjust to various environmental factors, like light intensity, temperature, and the surrounding organics and nutrients. So when manipulated, they can deliver different results.
Puyol's team decided to add an external electric current. This shocking decision (not actually shocking) came from the bioelectrochemical system of these bacteria, how all their metabolic pathways are connected by electricity.
"All living beings have to maintain an equilibrium, which microbiologists and biotechnologists call homeostasis," Puyol says in the interview. "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."
The electric current was able to create this hydrogen gas out of human waste. In fact, it does best with human feces, a type of matter designed by nature to have as a few proteins as possible (that's because the human body has absorbed proteins and thrown away the rest).
"Waste composition plays a key role on the ability of purple bacteria to produce hydrogen," Puyol says. "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."
"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," say co-author Abraham Esteve-Núñez of the University of Alcalá, Spain, in a .
With that biofuel, Puyol sees a future where his team could "transform the wastewater treatment plant into a real biorefinery." He estimates that a medium-size wastewater treatment plant could, using the hydrogen created from this electric process, "theoretically yield energy for 43 to 107 houses."
So far, the team has been able to demonstrate its technology on a lab scale, but want to work with the waste industry to test the feasibility of their tech on a larger scale.
"Resource recovery from waste and wastewater is nothing new," Puyol says. "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."