Hans van Leeuwen has long worked with fungi to purify food processing wastewater, starting in South Africa, then in Australia, and in recent years in his position as a professor of environmental engineering at Iowa State University in Ames. When he first considered using fungi to clean up the water used in the corn wet-milling process, he learned that although the fungi grow well, the process would be somewhat marginal in terms of increasing the profitability of a corn wet mill. When his team at Iowa State turned to the dry-mill ethanol process, they found a different story. Not only do the fungi grow prolifically, promising some interesting new coproducts, but the energy and water savings could be significant. The researchers dubbed the new process MycoMax and formed MycoInnovations Inc. to facilitate commercialization. There is one patent pending for the process and another one in development. Van Leeuwen is currently seeking funding to test the process on a pilot scale, which he estimates will cost $1 million for equipment and tests.
Their research has already garnered some attention for the Iowa State team, which includes van Leeuwen; doctoral candidate Mary Rasmussen; Samir Khanal, now with the Department of Molecular Biosciences and Bioengineering at the University of Hawaii; and Anthony Pometto, currently with the Department of Food Sciences at Clemson University in South Carolina.
The team won the grand prize for university research from the American Academy of Environmental Engineers, a project innovation award from the International Water Association, and a 2008 R&D 100 Award from R&D Magazine.
What the researchers have learned shows great promise for improving the efficiency of an ethanol plant. In the dry-mill process, after ethanol is separated from the fermented mash by distillation, centrifuges are used to remove most of the solids, which become the distillers grain coproduct and is sold as animal feed. The remaining liquid, called thin stillage, is partially recycled for use in the corn fermentation process. Only about 50 percent of the watery thin stillage can be recycled to prevent a buildup of total dissolved solids, glycerol, lactic acid, and acetic acid—fermentation byproducts that can limit the process when levels are too high, van Leeuwen says. The water from the remaining thin stillage, which contains about 6 percent solids, is evaporated in the conventional dry-mill ethanol plant creating a syrup with about 30 percent solids. It is blended with the previously removed solids and becomes the “solubles” in distillers dried grains with solubles.
Process Savings
The MycoMax process replaces the syrup formation with a system that grows the food-grade fungus Rhizopus microsporus in the nutrient-rich thin stillage while removing acetic acid, lactic acid, and glycerol. The fungus removes those substances and allows for the ability to recycle nearly all of the water in the fermentation process, van Leeuwen says. In laboratory experiments, the fungi reduced chemical oxygen demand (COD) by 80 percent, glycerol and organic acids by 100 percent, and suspended solids to nearly nondetectable levels in three to five days. Rasmussen believes the reaction time can be reduced to two days or less by using a larger volume of fungi-containing water to inoculate the process.
The fungi thrive in thin stillage. “We were surprised by how prolifically it grew,” Rasmussen says. “It grew so much on the reactors in the lab setting we moved it to a larger scale fermentor right away.” Although the fungi got hung up on the sides of the small glass vessels used for the first fermentation trials, that didn’t occur with the larger volumes and stainless steel walls of the 50-liter fermentor, she says. Providing for adequate aeration was another issue that had to be addressed in the experiments. The COD for thin stillage at 100 grams per liter is between 10 to 100 times the levels found in most wastewater treatment situations. Fungi growth would be limited by high levels of organic material, which create the high COD without adequate aeration. To boost the oxygen levels, van Leeuwen designed an air life reactor to replace the stirring and inadequate aerators that are usually used.
The process not only increases water efficiency by cleaning up the water, it also offers savings in enzyme costs. Currently, some enzymes are recycled through the portion of thin stillage that’s reintroduced to the yeast fermentation process. Researchers anticipate the recovery of more enzymes as more water is recycled; also Rhizopus microsporus is known to produce glucoamylase. Testing proved that the enzymes survived the process, but the enzyme effect needs to be analyzed and quantified in future research, van Leeuwen says, which could also involve related fungi known to produce alpha amylase.
Read the rest of the original article by Susanne Retka Schill at ethanolproducer.com