Turning rotten tomatoes, waste fruit into biogas

London -  Rotting tomatoes, brown bananas and overripe cherries -- the waste from wholesale marts usually ends on the compost heap. But fermenting this food waste produces methane, also known as biogas, which can be compressed into high-pressure cylinders and used as fuel for vehicles, according to new research.

Natural gas is kinder on the wallet, but it is also a fossil fuel. Researchers at Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB), Germany, have now developed an alternative from fruit and vegetable waste generated by wholesale markets.

In early 2012, researchers will begin operating a pilot plant adjacent to Stuttgart's wholesale market. It will use various micro organisms to generate sought-after methane from 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," said Ing. Ursula Schließmann, head of department at IGB, according to a university statement.

But it still presents a challenge because its precise composition varies every day. Sometimes it has a high proportion of citrus fruits, while at other times there are more cherries, plums and lettuce.

On days with a higher citrus fruit content, the researchers have to adjust the pH value through substrate management because these fruits are very acidic.

"We hold the waste in several storage tanks, where a number of parameters are automatically calculated -- including the pH value," said Schließmann.

"An indicator for acidity or alkalinity is known as the pH value. A pH value of seven means a substance is neutral. The lower value indicates acidity, and a higher value is a sign of alkalinity," Schließmann said.

"The specially designed management system determines exactly how many litres of waste from which containers should be mixed together and fed to the micro organisms," explained Schließmann.

Another advantage of the new plant lies in the fact that absolutely everything it generates can be utilised - the biogas, the liquid filtrate and even the sludgy residue that cannot be broken down any further.