When Angenent added cobalt to the mix, he recalls, “it was unbelievable. Overnight, the process was recovered.” In lab tests, the average output yielded a quarter of a liter of methane per gram of waste fed into the digester. Angenent calculates that this number, scaled up to industrial production rates, would decrease the amount of natural gas needed to power an ethanol plant by 50 percent.
In a 2006 study, researchers at the University of Minnesota calculated the total amount of energy used in the production of ethanol, from how much it costs to build and run tractors to how much it costs to power a biofuel plant. They found that ethanol provides a scant 26 percent more energy than is used to produce it.
When Angenent plugged results of his process into the Minnesota model, he found that energy output was bumped up to 70 percent, meaning that anaerobic digestion significantly boosts the energy value of ethanol biofuel. Angenent says that percentage may change slightly in a real-life scenario if ethanol plants choose to install anaerobic digesters.
“If you put in a digester, you have a lot of liquid that needs to be recycled back into the system, and that would create changes throughout a plant,” says Angenent. “So someone will have to do a study to find out what that net energy balance really is.”
Douglas Tiffany, a research fellow at the University of Minnesota and a coauthor on the 2006 study, says that operating anaerobic digesters in ethanol plants may be a challenge, since it requires expertise to maintain a stable bacterial community at high temperatures and avoid system crashes. However, if these problems are sorted out, Tiffany says, the process may improve ethanol’s energy and environmental potential.
“We can improve these existing corn-ethanol plants dramatically and reduce greenhouse gases far more than they do today,” says Tiffany. “This process is attractive because it’s a low-energy, low-capital approach. It will take some [ethanol producers] to stick their necks out to try it, but once it’s happening in a number of plants, it should work out pretty well.”