Some readers might remember the Mr. Fusion unit in Back to the Future that Doc Brown fills with household garbage, including a banana peel and some beer, to power the iconic time-traveling DeLorean. While we're still some way from such direct means of running our cars on table scraps, researchers at Fraunhofer have developed a pilot plant that ferments the waste from wholesale fruit and veg markets, cafeterias and canteens to make methane, which can be used to power vehicles.

Given the rising oil prices in recent years, many drivers have been converting their cars to run on natural gas. But like oil, natural gas is a fossil fuel with limited reserves whose price has also risen in recent years and is likely to continue to do so. Fraunhofer's development provides an alternative way to obtain natural gas, not from Earth's reserves, but from fruit and vegetable waste.

The pilot plant is part of the ETAMAX project and has been constructed adjacent to Stuttgart's wholesale market. Due to begin operation in the next few months, the plant generates methane by using various microorganisms that act on the food waste in a two-stage digestion process that lasts just a few days.

"The waste contains a lot of water and has a very low lignocellulose content, so it's highly suitable for rapid fermentation," says Dr.-Ing. Ursula Schließmann, head of department at the Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB).

Because the microorganisms require constant environmental conditions to function and the waste can vary in composition every day - some days it will contain a high proportion of acidic citrus fruits, on others it might have more cherries, plums or lettuce - the researchers must adjust the pH value through substrate management.

"We hold the waste in several storage tanks, where a number of parameters are automatically calculated - including the pH value. The specially designed management system determines exactly how many liters of waste from which containers should be mixed together and fed to the microorganisms," explains Schließmann.

Enhancing the environmental benefits of the plant, everything the plant generates can be utilized, including the liquid filtrate and the sludgy residue that can't be broken down any further by the microorganisms. The filtrate water, which contains nitrogen and phosphorous, is used as a culture medium for the cultivation of algae at a second sub-project in Reutlingen. And while two thirds of the biogas produced at the Stuttgart plant is methane, around 30 percent is carbon dioxide, which is also used to cultivate the algae. Meanwhile, the remaining sludgy fermentation residue is delivered to the Paul Scherrer Institute in Switzerland and the Karlsruhe Institute of Technology, where it is also converted into methane.

In addition to Fraunhofer, the ETAMAX project also involves the participation of energy company Energie Baden-Württemberg (EnBW), which uses membranes to process the generated biogas, and Daimler, which supplies a number of experimental vehicles designed to run on natural gas. The German Federal Ministry of Education and Research (BMBF) has funded the five-year project to the tune of six million euros (approx. US$7.97 million). If the various components mesh together as hoped, similar plants could be built where large quantities of organic waste can be found.

Source: Fraunhofer