Cow Power

There are various new carbon offset projects being undertaken that focus on harnessing agriculturally produced methane through anaerobic digestion. Three of the most promising methods are covered lagoons, plug-flow, and complete-mix digester systems, according to the National Carbon Offset Coalition. Bacteria will produce methane from manure whether or not it occurs in an open field or in an anaerobic digester, but when it is in a digester, it can accomplish two important goals. First, it provides a source of renewable energy, which can be used to run turbines or generators for energy production, or it can be burned in boilers to produce heat. The second, and perhaps even more important goal, is that it keeps methane, an extremely powerful greenhouse gas, from entering the atmosphere.

Methane is produced from manure via a chain of reactions driven by bacteria, as is illustrated below by a simple flow chart from wikipedia. First, bacteria break down the large organic molecules in manure into sugars, fatty acids and amino acids through a process called hydrolysis. Methanogens (or, methane-producing bacteria) can eat these components, or they can be further broken down by acidogenic bacteria in the process of fermentation. The products of fermentation (carbonic acids and alcohols, hydrogen, carbon dioxide and ammonia) can then be broken down into hydrogen, carbon dioxide and acetic acid by acetogens. Finally, methanogens create methane, carbon dioxide and water out of the final remaining components.


(CLICK on the flow chart to make it BIGGER)

The Iowa Department of Natural Resources gives a good overview of the three main ways harnessing methane through anaerobic digestion. The first is the covered lagoon system, where waste manure from farm animals (cows, pigs, etc.) is stored in an above-ground lagoon, which is covered by impermeable tarps and sealed at the edges. A pipe runs under the cover to collect the biogas that is created through anaerobic digestion, and send it to generators or boilers.

The second method is the plug-flow digester system. Manure and water (to maintain the proper consistency) are added to a mixing tank, or plug, which is stored below-ground. The tank is sometimes heated (with energy produced by the tank itself!) in order to speed up production of biogas, which is then driven out, either via a pump or the pressure built up by gas production, into a pipeline to be sent to a generator or boiler.

The third way of harnessing methane from manure is called the complete-mix digester system. In the complete-mix system, manure is collected in an above-ground tank (or below-ground in colder regions). The manure is heated in this tank, to begin the anaerobic process, before being pumped into a reactor tank, where it is mechanically mixed to keep the solids separated from the liquids. As biogas accumulates, it is pumped away for use, and the left-over solids can be used as fertilizer.

The Iowa DNR estimates that manure-derived methane from Iowa, if harnessed, has the potential to fulfill all of the power needs for 325, 000 homes. For this to happen, however, the price of installing and managing anaerobic methane digesters will need to decrease. According to the DNR, installation of a new system costs between about $300,000 and $500,000, which is a substantial cost to farm-owners. Perhaps in the future, as prices fall and demand increases, this renewable energy source will become a reality for more farmers.