|Pilot-scale airlift reactor with external recycling for fungal processing of thin stillage. Spore inoculation occurred on Day 0 (left). Fungal pellets filled the reactor by Day 3 (right). Click to enlarge.|
Growing a fungus in the thin stillage resulting from dry-mill ethanol production can reduce energy costs by as much as one-third, recycle more water and improve the distillers dried grains byproduct, according to a team of researchers from Iowa State University and the University of Hawai’i.
The Iowa State project is focused on using fungi to clean up and improve the dry-grind ethanol production process. That process grinds corn kernels and adds water and enzymes. The enzymes break the starches into sugars, which are fermented with yeasts to produce ethanol.
The ethanol is recovered by distillation, but there are about six gallons of stillage—which contains solids and other organic materials—left over for every gallon of fuel produced. Most of the solids in the stillage are removed by centrifugation and dried into distillers dried grains that are sold as livestock feed, primarily for cattle.
The remaining liquid, known as thin stillage, still contains some solids, a variety of organic compounds from corn and fermentation as well as enzymes. Because the compounds and solids can interfere with ethanol production, only about 50% of thin stillage can be recycled back into ethanol production. The remaining thin stillage is currently concentrated by flash evaporation—an energy-intensive process—and blended with DDG, producing DDG with solubles (DDGS).
DDGS is used for livestock feed, but is low in essential amino acids, e.g., lysine, limiting its usage, particularly for hogs and chickens.
The researchers added a fungus, Rhizopus microsporus, to the thin stillage and found it would feed and grow. The fungus removes about 80% of the organic material and all of the solids in the thin stillage, allowing the water and enzymes in the thin stillage to be recycled back into production.
The fungus can also be harvested. It’s a food-grade organism that’s rich in protein, certain essential amino acids and other nutrients. It can be dried and sold as a livestock feed supplement. Or it can be blended with distillers dried grains to boost its value as a livestock feed and make it more suitable for feeding hogs and chickens.
At current production levels, eliminating the need to evaporate thin stillage would save ethanol plants up to $800 million a year in energy costs. Adding the researchers’ fungal process would improve the energy balance of ethanol production by reducing energy inputs so there is more of an energy gain.
Allowing more water recycling would reduce the industry’s water consumption by as much as 10 billion gallons per year. Recycling enzymes in the thin stillage would save about $60 million per year.
Adding value and nutrients to the livestock feed produced by ethanol plants would grow the market for that feed by about $400 million per year.
Hans van Leeuwen, an Iowa State professor of civil, construction and environmental engineering and the leader of the research project, estimated it would cost $11 million to start using the process in an ethanol plant that produces 100 million gallons of fuel per year. But, he said the cost savings at such a plant could pay off that investment in about six months.
The Iowa State research project is supported by grants of $78,806 from the Grow Iowa Values Fund, a state economic development program, and $80,000 from the US Department of Agriculture through the Iowa Biotechnology Byproducts Consortium.
The researchers have filed for a patent on the technology and are looking for investors to commercialize the invention. The process still needs to be proven at larger scales.
Van Leeuwen, and the other researchers developing the technology—Anthony L. Pometto III, a professor of food science and human nutrition; Mary Rasmussen, a graduate student in environmental engineering and biorenewable resources and technology; and Samir Khanal, a former Iowa State research assistant professor who’s now an assistant professor of molecular biosciences and bioengineering at the University of Hawai’i at Manoa—recently won the 2008 Grand Prize for University Research from the American Academy of Environmental Engineers for the project.