Purple bacteria are poised to turn your toilet into a source of energy and useable organic material.

Dried sewage sludge.
Image credits: Hannes Grobe.

Household sewage and industrial wastewater are very rich in organic compounds, and organic compounds can be very useful. But there’s a catch: we don’t know of any efficient way to extract them from the eww goo yet. So these resource-laden liquids get treated, and the material they contain is handled as a contaminant.

New research plans to address this problem — and by using an environmentally-friendly and cost-efficient solution to boot.

The future is purple (and bacterial)

“One of the most important problems of current wastewater treatment plants is high carbon emissions,” says co-author Dr. 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.”

The study is the first effort to apply purple phototrophic bacteria — phototrophic means they absorb photons, i.e. light, as they’re feeding — together with electrical stimulation for organic waste recovery. The team showed that this approach can recover up to 100% of the carbon in any type of organic waste, supplying hydrogen gas in return — which is very nice, as hydrogen gas can be used to create power cells or energy directly.

Although green is the poster-color for photosynthesis, it’s far from the only one. Chlorophyll’s role is to absorb energy from light — we perceive this absorption as color. Green chlorophyll, for example, absorbs the wavelengths we perceive as red (which sits opposite green on the color wheel). If you’ve ever toyed around with the color-correction feature in graphical software (a la Photoshop, for example), you know that taking out the reds in a picture will make it look green. The same principle applies here.

Plants are generally green because red wavelengths carry the most energy — and plants need energy to create organic molecules. But the substance comes in all sorts of colors in a variety of different organisms. Phototrophic bacteria also capture energy from sunlight, but they use a different range of pigment — from orange, reds, and browns, to shades of purple — for the job. However, the color itself isn’t important here.

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

These bacteria use organic molecules and nitrogen gas in lieu of CO2 and water as food. This supplies all the carbon, electrons, and nitrogen they need for photosynthesis. The end result is that they tend to grow faster than other phototrophic bacteria or algae and generate hydrogen gas, proteins, and a biodegradable type of polyester as waste.

But what really sealed the deal for the team is that they can decide which of these waste products the bacteria churn out. Depending on environmental conditions such as light intensity, temperature, and the nutrients available, one of these products will predominate in the material they excrete.

The team doubled-down on this property by flooding the bacteria’s environment with electricity.

“Our group manipulates these conditions to tune the metabolism of purple bacteria to different applications, depending on the organic waste source and market requirements,” says 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.”

This concept — a “bioelectrochemical system” — works because all of the purple bacteria’s metabolic pathways use electrons as energy carriers. They use up electrons when capturing light, for example. On the other hand, turning nitrogen into ammonia releases electrons, which the bacteria need to dissipate. By applying an electrical current to the bacteria (i.e. by pumping electrons into their environment) or by taking electrons out, the team can cause the bacteria to switch from one process to the other. It also helps improve the overall efficiency of both processes (see Le Chatelier’s principle).

The team included an analysis of the optimum conditions for hydrogen production in the paper (it relies on a mixture of purple bacteria species). They also tested the effect of a negative current (electrons supplied by metal electrodes in the growth medium) on the metabolic behavior of the bacteria.

Their first key finding was that the nutrient blend that fed the highest rate of hydrogen production also minimized the production of CO2 — this would allow the bacteria to recover biofuel from wastewater with a low carbon footprint, the team explains. The negative current experiment proved that these bacteria can use cathode electrons to perform photosynthesis.

Even more striking were the results using electrodes, which demonstrated for the first time that purple bacteria are capable of using electrons from a negative electrode, or “cathode“, to capture CO2 via photosynthesis.

“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,” says Esteve-Núñez.

“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.”

The paper “Biological and Bioelectrochemical Systems for Hydrogen Production and Carbon Fixation Using Purple Phototrophic Bacteria” has been published in the journal Frontiers in Energy Research.