City swamps: a novel source of bioenergy?

In the old continent, bioenergy is omnipresent these days. It’s still not at the level it should be, yet progress has clearly been made. Even when the climate change debate is not on TV, solar panels are on the neighbor’s’ roof, windmills turn in the picturesque countryside, and the city bus is running on rapeseed biodiesel. The efforts toward developing sustainable energy continue as modern Homo sapiens try – better sooner than later – to move away from fossil fuel energy.

It looked first like a human-made constructed swamp. She inspected it closely enough to detect an undesirable odor, but far enough away to avoid an alligator’s sneak attack. The bubbles kept rising on the surface, allowing the foam to dance freely on the sewage’s pitch-dark rink. A sudden sadness tugged at her heart. As this place wasn’t a natural swamp at all, but the city’s waste treatment plant, no imposing macroscopic specimen would ever emerge from the thick gloomy waters. She put her camera away and continued contemplating this mysterious ecosystem. What could be the forms of life thriving in there? Flies were not buzzing overhead as she would have expected. After all, sewage is a collection of all sorts of human waste and excrement. It looked like a gross place to hang out in, she thought, but it was not actually repelling her. There was a sort of a compelling energy in the air… She could not explain it. Not quite yet.

Sewage’s value lies in its energy value, yet most of us have never heard about that. For instance, methane is a green energy produced with an excess of activated sludge. And there is another less conventional route for biofuel: It involves the microbial community that waste treatment plants harbor. However, how does the magic of recovering a pollutant carbon become a reality?

She realized she needed a microscope. Her portable camera would never be able to capture the intricacy of this invisible ecosystem. Obviously, a microscope to observe the hidden-to-the-naked-eye world! She filled her water bottle with a sewage sample and ran off to the university campus. On the way, she wondered what type of microbes were presently inhabiting her bottle. Could they be accumulating molecules of any interest?

These days, pollutant carbons such as methane can get recycled and commercialized. But first things first; in my research, we are talking about a liquid fuel, namely biodiesel. Keep in mind the fact that a rather simple chemical reaction is the pillar for its synthesis. Actually, in its most conventional industrial production, vegetable oils, animal fats or waste cooking oils get converted to biodiesel via a trans-esterification reaction.

She smeared a microscope slide with the waste treatment slurry, then adjusted the objectives and voilà, a frenetic microbial hidden world. The little guys seemed to be in a hurry to get somewhere. Perhaps to deliver a message or a cargo. What sort of molecule could it be, she inquired aloud?

Lipids! Sewage microbes – my favorite is Microthrix, a filamentous bacterium – have the unique ability to trap lipids that could be further extracted from the cells and transformed into biofuel. A source of energy that will not compete with the food production sector in terms of land use (such as rapeseed fields). Isn’t it damn clever to transform such wasted lipids into a valuable energy by-product?

[In some future time] She stepped onto the airplane. The speaker announced “We are proudly powered by waste treatment carbon. Please seat and enjoy the long ride. Next step: Amazon.” She smiled and prepared her camera while the airplane took off to the famous South American alligators’ habitat.

I am Emilie E.L. Muller and I am currently affiliated with the ‘Génétique Moléculaire, Génomique, Microbiologie’ research unit at the Strasbourg University/CNRS. The topic discussed in this article was my post-doc research at the Luxembourg Centre for Systems Biology in the team headed by Paul Wilmes (University of Luxembourg). The research was funded by the Luxembourg National Research Agency.

Text by Fernanda Haffner

Illustration by Marion Couturier