Turning Trash into Treasure:
How can we use rotting food as fuel?
At the back of any dorm refrigerator you are apt to find all sorts of questionable products. Perhaps some black sludge that used to be a banana, a plate of white fuzz that was once take-out, or a bottle of grape juice that should now be called wine. But maybe we should not be so quick flood these refrigerators with bleach. New developments in energy science are allowing researchers to convert food waste into various types of biofuels. That wilting, mushy celery stalk you were about to throw out? Using the proper technology, it can be turned into natural gas. Of course, it takes a bit more than an old dorm fridge to turn rotting food into an alternative source of energy in a way that is both economical sensible and environmentally safe.
The Science
The idea of biofuels is not new. For decades we have been using corn and sugar to make liquid ethanol as a fuel for cars and other vehicles. But using food that we might otherwise eat causes food prices to climb dramatically. A group of scientists at the Fraunhofer Institute for Interfacial Engineering and Biotechnology in Stuttgart, Germany, has come up with a way to use food waste to create biogas, methane that has been obtained from organic sources instead of from underground. They take the food that has spoiled before it could be bought from a wholesale market and let the food ferment in large containers. Methane is released by the food and captured in another container. The natural gas can then be highly compressed and used to power cars. And it’s not just grocery stores that have excess food. Produce waste can be collected from universities, restaurants, military facilities, pretty much anywhere food has to be prepared on a large scale.
There are several different methods by which biomass (organic material, in this case excess fruits and vegetables) can be used to extract methane. Most of the differences come from how the main container is partitioned, which affects how the food waste and bacteria that help fermentation will mix and how the released gasses will be siphoned out. In general, slurry that will help the biomass decompose is mixed with the biomass at the bottom of the container (see figure 1).
The gases that are released are allowed to flow up into another chamber. The slurry/bio-waste mix is unable to enter the section holding the gases due to partitions and/or gravity. The gases can then be captured from the container by pushing them out using pressure gradients or simply allowing them to float out into another container (Polprasert, 167, 171, 174, 176).
Other factors that vary among facilities include what material is used to facilitate fermentation, the temperature of the process, and what the biomass consists of. The organic matter processed ranges from corn and sugar beets to sewer sludge. What makes all these different materials potential energy sources, though, is that as they decompose, they release gases, including methane.
Here’s a simple experiment, although it may be better to see a demonstration than try it on your own. Go to a pond with a thick layer of muck on the bottom and bring along a funnel, a lighter, and a friend. It helps if there are trees that often drop their leaves into the pond. Stick your hand in the muck (go ahead, nice and deep) and mix it around. With your other hand, hold the funnel with the wide side just under the surface of the water and the narrow end pointing into the air. The funnel should be positioned over the spot where you are mixing up the mud. As bubbles escape from the mud, they will be captured by the funnel. Have your friend, while staying as far away as possible, switch on the lighter right above the narrow end of the funnel. BOOM!
All that muck at the bottom of the pond is made of decaying organic matter, dead fish and algae, sticks and leaves that sunk to the bottom, whatever. As it decays, it releases methane, which gets trapped under the other layers of mud. When you stir up the mud with your hand, you allow these bubbles of methane to escape to the surface. The funnel focuses the methane in one place so we can ignite it with the lighter. The result: we see that methane can be burned to release energy.
Biogas plants follow the same basic principle as above, but magnified. Organic material decomposes, the resulting gases can be collected, and, after a bit more modification, they can be used for energy. Bacteria that decompose the organic material and create methane as a byproduct already exist in the pond water and dirt. But the facilities have to add their own bacteria, hence they use fertilizer, sewer sludge, or some other source of bacteria in the mixing chamber of the main holding container (Fraunhofer, 2012).
The bacteria first perform hydrolysis on the cells of the fruits and vegetables. This process breaks down the longer, more complex carbohydrates in the cells into simple sugars. Another type of bacteria then converts the sugars into various organic acids. Still another type of bacteria breaks down the organic acids further. Finally, archaea, a domain of microorganism that is distinct from bacteria or viruses, turn the acids into methane and carbon dioxide. Throughout the process, other gasses and nutrients, such as hydrogen and nitrogen, are created as by-products (Marchaim, 1992). While the researchers in Germany are interested in the methane as a fuel, they have good plans for the numerous by-products as well (see figure 2).
The Environment
One of the by-products of the process is carbon dioxide. Since one of the goals of using fermenting fruits and vegetables to generate energy is to reduce the amount of greenhouse gases released into the atmosphere, it would be advisable to put that CO2 to good use. The researchers in Stuttgart have teamed up with researchers working on an algae cultivation project in Reutlingen, Germany. The algae secrete oil that can be used in diesel powered engines, but only if they have enough carbon dioxide, sunlight, and nutrients. The carbon dioxide from the biogas generation process helps the algae grow. The filtrate water that is left over from the mixing also goes to the algae, as it contains enough phosphorous and nitrogen to keep the algae well fed. Additionally, even after the bacteria cannot break down the original mixture any further, the Stuttgart researchers send the remaining sludge to the Paul Scherrer Institute in Switzerland and the Karlsruhe Institute of Technology in Germany, and converted to methane (Fraunhofer, 2012).
So the by-products of the process don't seem to be a cause for environmental concern. But is biogas as energy efficient as other fuels? And even if natural gas is the way to go, is there an advantage to getting it from fermenting produce instead of underground?
Some research suggests that biogas is far more efficient than other biofuels, such as ethanol. That is, the ratio of output energy to input energy is higher for biogas than other biofuels, given the same amount of land on which the crops were grown (see figure 3).
Another advantage food waste has over crops is that we do not need to create more growing space to produce our fuel source. The crops have to be planted, but if we use material that would have gone to a landfill, we would actually increase the amount of available land, since landfill space would be reduced. Some land would need to be set aside for building biogas plants though.
Additionally, biogas would help decrease greenhouse gas emissions. First of all, switching from gasoline or diesel powered vehicles to natural gas powered ones would reduce the amount of CO2 released by vehicles. Moreover, processing food waste means we are sending less material to landfills. Capturing methane over landfills is much more difficult and the odors are harder to control than in the biogas generation process. Finally, using manure as a means of introducing the necessary bacteria to break down the fruits and vegetables cuts some of the methane produced by livestock. Of course, burning methane will still add some carbon dioxide to the atmosphere, but less so than other fossil fuels (Borjesson and Mattiasson, 2008) (see figure 4).
While fermenting fruits and veggies to generate biogas appears to hold up environmentally, it will never get anywhere unless we have economic incentives to implement it. Natural gas is already used frequently for heat, electricity, and cooking, so residential, commercial, and industrial buildings will not have to be modified if they already use natural gas. The area that would take the most work, and money, to make using biogas feasible is in the vehicle industry. Some trucks and cars already run on compressed natural gas, but the majority does not. However, various energy companies have already gotten involved with the biogas researchers in Germany and are developing new vehicles to run on natural gas (Fraunhoffer, 2012). We will also have to tailor filling stations to biogas powered cars.
Because there is an added cost from developing the new vehicles and filling stations, biogas would have to be less expensive than gasoline and liquid biofuels. Sugar cane based ethanol from Brazil costs between $0.25 and $0.50 per liter. Biogas would have to cost $0.05 to $0.15 per equivalent liter to be feasible. In 2008, biogas derived from organic waste and manure cost between $0.45 and $0.55 per equivalent liter, so the price would have had to come down in order for using biogas in cars to be economically reasonable. Organic waste-derived biogas was still more feasible that crop-based biogas, which cost as much as $0.80 per equivalent liter. However, these calculations did not account for government subsidies. Additionally, the price of oil only has to rise above $70 per barrel for biofuels, in general, to become economical in comparison (Borjesson and Mattiasson, 2008). Currently, in March 2012, light crude oil costs $106 per barrel (CNN Money). If oil prices continue to rise, it is only a matter of time before biofuels, including biogas, become the more reasonable choice in regards to economics.
Another economic advantage to biogas is that, while natural gas from fossilized organic materials, oil, and coal, are located in a limited number of locations around the world, biogas plants could be adapted for almost everywhere (Borjesson and Mattiasson, 2008). Any sort of rotting produce works. Citrus can be used in Southern California, while old corn is used in the Midwest. The process may have to be slightly altered a bit —food waste heavy with citrus fruits prove to be a particular problem, because the acid lowers the pH level beyond the bacteria’s comfort level and counter measures must be taken to reduce acidity—but finding the solution to a slightly different mixtures of produce will only take a bit more experimenting (Fraunhoffer). As a result of this flexibility, biogas plants can be tailored to complement the local economy.
For example, the University of Illinois at Champaign-Urbana has a large agricultural program. The food waste could be collected from the dining halls, pH levels and temperatures could be varied accordingly, depending on what fruits and vegetables comprise the majority of the material, and extra manure from the cows on campus could be mixed with the produce to provide the necessary bacteria. The university can generate some of its own electricity and get plenty of positive publicity from being seen as “green.”
While there is still too much economic momentum pushing us behind gasoline to immediately switch over to biogas, we should remember that as we deplete our oil resources, gas prices will continue to increase, making alternatives look more inviting than ever. Additionally, it is nearly impossible to extrapolate with regards to technology for any extended period of time. Biogas facilities may become more efficient, as may the development of natural gas powered vehicles. And researchers may develop hybrid cars that run on natural gas and electricity. (Borjesson and Mattiasson, 2008).
In the end, turning a profit at a biogas plant may come down to whether the by-products and remaining sludge can be sold as fertilizer or something else. The researchers at Stuttgart seem to have found a solution as to what the by-products can be used for, as described earlier. However, if the by-products must be sold or used in some productive way, biogas generation may be constrained to large scale facilities. Obtaining biogas from biomass may not be feasible on the scale of an individual home if the owners lack the opportunity to put the by-products of the process to good use (Marchiam, 1992).
The Future?
If biogas plants ever become mainstream, we would have to provide proper maintenance of the equipment involved in order for biogas to continue supplying us with energy, environmental benefits, and a potential way to sell our garbage to the grid. Even in areas where the composition of the food waste and sewer sludge or fertilizer is fairly consistent, maintenance employees will need to make sure that the nutrients in the mixture are not changing over time. If the environment changes in such a way that the bacteria fermenting and breaking down the food cannot do their job, we will need to find out where the change came from and how to counteract it (Marchiam, 1992).
It seems possible that, with time, we will separate our food waste like we do our recycling and garbage in order for it to be collected and sent to the correct facilities. This would cut down on material going to landfills and provide the biogas plants with a steady source of fuel. Since many people already spend time composting, it would be a small change to go from collecting food for compost to collecting food for biogas generation.
No matter how nice biogas sounds, though, it alone won’t save us. We will still need to use other sources of fuel to cut back on our greenhouse gas emissions. In the end, because biogas will still emit some amount of carbon dioxide, we may ultimately have to move away from it. For now, however, it may ease the transition from traditional fossil fuels to renewable sources, and it gives us something productive to do with our trash in the meantime.
Works Cited
Borjesson, P. and Mattiasson, B. (2008). Biogas as a resource-efficient vehicle fuel. Trends in Biotechnology, 26, 7-13. http://dx.doi.org/10.1016/j.tibtech.2007.09.007
CNNMoney. (2012). Commodities. Retrieved from http://money.cnn.com/data/commodities/
Fraunhofer. (2012). Fuel from market waste [Press release]. Retrieved from http://www.fraunhofer.de/en/press/research-news/2012/february/fuel-from-market-waste.html
Macdonald, N. (2008, January 11). Powering up with biogas. The Dominion Post. Retrieved from http://www.lexisnexis.com/lnacui2api/results/docview/docview.do?docLinkInd=true&risb=21_T13955099375&format=GNBFI&sort=BOOLEAN&startDocNo=1&resultsUrlKey=29_T13955099379&cisb=22_T13955099378&treeMax=true&treeWidth=0&csi=256380&docNo=9http://www.lexisnexis.com/lnacui2api/results/docview/docview.do?docLinkInd=true&risb=21_T13955099375&format=GNBFI&sort=BOOLEAN&startDocNo=1&resultsUrlKey=29_T13955099379&cisb=22_T13955099378&treeMax=true&treeWidth=0&csi=256380&docNo=9
Marchiam, U. (1992). Biogas Processes for Sustainable Development. Retrieved from: http://www.fao.org/docrep/T0541E/T0541E00.HTM
Polprasert, C. (2007). Organic Waste Recycling: Technology and Management. Available from: http://books.google.com/books?id=owycqJMjoZoC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false
US Environmental Protection Agency: Office of Research and Development (November 2011). Plan to Study the Potential Impacts of Hydraulic Fracturing on Drinking Water Resources. Retrieved from http://www.epa.gov/hfstudy/HF_Study_Plan_110211_FINAL_508.pdf
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