Biogas generally better than composting
An Austrian study compared different ways of using biowaste and found that the production of biomethane of natural gas quality is the most efficient in terms of energy balance and the greenhouse effect.
The study, which is only available in German but has a summary in English (PDF), compares the greenhouse gas offsetting potential and the amount of energy that can be gained from a ton of biowaste in various processes of composting and fermentation. Biowaste is defined as organic waste from households and commercial kitchens, non-woody plant clippings, and expired food products. Austria is looking for a way to make use of the two million tons of biowaste it produces each year.
The production of biomethane turned out to be the best way to make use of biowaste in terms of greenhouse gas emissions (measured in carbon-dioxide equivalents) and in terms of the energy that can be produced. Specifically, the net energy gains (minus the energy required for production) from biomethane come in at 599 kilowatt-hours per tons of biowaste on the average. But the amount ranged widely depending on the feedstock; for instance, a ton of organic waste from households only produced 425 kilowatt-hours compared to 856 kilowatt-hours for waste from cafeterias.
If the biogas is used to generate electricity and the waste heat is used completely, the study found that the net energy gains are still 15 percent lower than with biomethane. On the other hand, it is not quite fair to compare the net energy gains from biomethane production to local cogeneration; after all, biomethane still has to be converted into power or heat to be used as energy. On the other hand, it is quite rare indeed for all of the waste heat from biogas (or biomethane) to be used.
The effects in terms of carbon emissions are similar. A molecule of methane has an impact 25 times greater than a molecule of carbon dioxide. The study looked into energy input, greenhouse gas emissions from the production of biomass (such as methane slippage), and the reduction of carbon-equivalent emissions from the biomass production option chosen.
It turns out that biomethane production reduces carbon-equivalent emissions by 171 kilograms per tons of biowaste compared to 15-33 kilograms for the other options. In other words, in terms of climate impact it is clearly better to ferment biowaste and use it to generate energy than to compost it and put it back into the field. Indeed, composting is the least efficient way of reducing carbon-equivalent emissions. But composting does have a leg up in three cases: when waste that is low in gas content (such as household organic waste) is separated out before fermentation; in the open composting of waste after fermentation; and when the power is produced, but waste heat is not used. (Dittmar Koop / Craig Morris)
One of the huge advantages of separating the residues and using the organic part, fed through bio-digesters, is that we avoid residue dumps, which are a menace in more than one way. Read my article about Management of Organic Waste http://reibo.biz/?p=344