Ethanol process wastes could become fertilizer

Morris, Minn.-A new study sponsored by Minnesota’s Agriculture Utilization Research Institute (AURI) shows that methane and fertilizer can be produced by anaerobic digestion from the thin stillage or dissolved corn solids left over from ethanol production. Perhaps more important, AURI researchers report that turning the wastes into a usable product could boost profits and add to the “green” image of the ethanol industry. “Ethanol plants could potentially become energy independent if all the energy in thin stillage could be captured in the form of methane,” claims study author David Rein of Rein & Associates, a wastewater engineering company based in Moorhead, Minn. Rein estimates that the anaerobic digestion being used more widely to extract methane from manure could add $10 million to the bottom line of a 50-million-gallon ethanol plant, while conserving water and earning valuable carbon credits. “It’s proven technology that’s been around a long time and is widely used,” Rein says. According to Michael Sparby, AURI project director, some ethanol plants already use small digesters, called methanators, to clean up their wastewater. “But the ethanol industry is not yet using anaerobic digestion to generate power. That could change as ethanol plants seek renewable alternatives to natural gas. I’m hearing a lot of interest in stillage digestion.” AURI tested both whole stillage and thin stillage digestion, with support from the Minnesota Corn Growers Assn., Otter Tail Power Co., Otter Tail Ag Enterprises, and the city of Fergus Falls. The research was done at the Fergus Falls Wastewater Treatment Plant, which operates a municipal sludge digester. In a full-scale demonstration, whole stillage was added to the city’s digester to supplement wastewater sludge. The demonstration was a great success, Rein says. With the addition of whole stillage, which Rein calls an ideal feedstock, the digester generated enough biogas to completely satisfy the plant’s fuel needs. During the 15 to 20 days in the digester, more than 80 percent of the organic matter in the stillage was converted to biogas. The digestion process also purifies the stillage water, which can then be filtered and recycled.