Leftover fruit and vegetables from markets don’t always have to end up on the compost heap. Researchers in Stuttgart are developing a new system aims to turn that waste into biogas for cars.
In affluent countries like Germany, food doesn't always land on people's plate - quite often it ends up in the trash instead. A recent study found that Germans throw away an incredible 11 metric tons (around 12 million short tons) of food each year.
In Stuttgart, Germany's de facto automobile capital, researchers are trying to feed some of this waste from the local fruit and vegetable market directly into a biogas plant. They're even building a service station for cars to tank up with the gas directly at the plant itself.
With the rising prices of petrol, biogas made from food waste could be an attractive - and more sustainable - alternative.
From the market into the tank
Stuttgart's wholesalers market is the third biggest in Germany. Titus Steiger, head of a busy fruit and vegetable trading company, is one of hundreds of traders and farmers based at there.
Leafy vegetables have to be sold quickly, he told DW, within three days. "After that we have to give them away. With herbs we only have two days," Steiger said.
The Stuttgart market produces 2,000 kilograms (2,200 pounds) of green waste, or biowaste, every year. Currently, these food scraps are collected by the city and composted.
In many regions of Germany, biogas from organic waste is increasingly being used to run heating systems and produce electricity.
The new project in Stuttgart is being run by the Frauenhofer Institute for Interfacial Engineering and Biotechnology and is set to show that energy from food waste can also be used to run cars. The experiment is part of a project called Etamax, which received 6 million euros ($8 billion) from Germany's Federal Research Ministry.
In the pilot phase, the project will be collecting leftover fruit and vegetables from the nearby central market and several cafeterias, and then fermenting it into methane.
In a two-stage process, which lasts several days, various microorganisms digest the waste, which produces biological methane. After being pressurized, it can be used to fuel vehicles that normally run on compressed natural gas.
"Food waste has high water content and low lignin and lignocellulose content. That makes it ideal for this digestive process," Ursula Schliessman, a Fraunhofer scientist, said.
Lettuce or lemons: the right mix
Karl Kübler, who heads up Stuttgart's market, said the kind of food waste can fluctuate wildly according to season.
If it's melon season, for example, and "suddenly we have a cold spell, no one buys the melons and then we have a huge quantity of melons that are thrown out all at the same time," Kübler said.
The waste even varies from day to day. Sometimes there will be more lettuce thrown away, sometimes citrus fruits - which contain a lot of acid.
This means the scientists have to balance the pH of the material for it to be digested in the fermenter. To do this, different kinds of biowaste are stored in separate containers, where the pH and other parameters are measured.
"Then we have a specially developed system to calculate how many liters of waste have to be taken from which containers and then put in with the microorganisms," said Schliessmann.
The correct balance has to be maintained so that the microorganisms have a consistent environment in which to carry out their digestion.
Nothing is wasted
After the biogas is produced, fluid residue and any bits that cannot be fermented are put to use in other projects.
The water from the digestion process, which contains nitrogen and phosphorous, is used as a nutrient for algae, which can produce oil for use in diesel engines.
The remaining residue is turned into methane using another process - so that the organic waste is completely re-used.
The next step is to get the gas into cars.
Avoiding demand for waste
Since the system runs on food waste, it does not compete in any way with the actual production of food, as is the case when the biofuel ethanol is made from maize or other crops.
Ethanol has been the subject of a lot of criticism, especially because it uses up valuable land which could be used for growing food crops. Many also question whether it takes more energy to grow the crops than is produced.
But organic leftovers are just that. Right now in the best scenario they are composted, but for biogas proponents they represent considerable source of untapped energy.
Environment groups like Friends of the Earth Germany (BUND), say it makes sense to use food waste for biogas.
However, Berthold Friess, who heads the organization in the German state of Baden-Württemberg where the new fermenter is located, warns that technology of this kind should not be allowed to create an artificially high demand for food waste.
Studies have indicated that around half the world's food already ends up being thrown away. "The aim should really be to make sure as little food as possible is thrown away at markets," Friess said.
He also calls for the development of lighter, more economical cars and the improvement of public transport, to reduce the use of limited natural resources like oil and gas and to put less pressure on the climate.
Biogas in future energy mix
Schliessmann hopes that smaller biogas plants like this one could someday be seen in every city and play an important role in the energy mix of the future.
"The advantage of this kind of technology is that we can put it in the middle of a city where people are living because there is no smell and it is a closed system," he said.
When the plant officially starts up at the end of April, the German car company Daimler will fill up test cars with different mixes of methane gas to find out what works best.
Author: Irene Quaile and Kate Hairsine
Editor: Holly Fox