Fertile Fungus

Iowa State University researchers use fungi to treat thin stillage and create some interesting new coproducts.
By Susanne Retka Schill | January 12, 2009
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 and biological 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 ISU 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 ISU 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 they won 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 build up 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 (DDGS).

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 decreased 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 MycoMax process would add a step in a dry-grind ethanol plant to grow fungi in thin stillage. It should allow all of the water to be recycled into the fermentation process while creating a new feed coproduct.

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 are 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.

New Coproducts
Feeding trials on the dried fungal biomass left after the fungi have cleaned the water are also on the list of future research. The fungal biomass could be a high-value feed supplement containing 40 percent protein, between 2 percent and 4 percent lysine, about 1 percent methionine and 3 percent chitosan. The high protein lysine and methionine levels should make the fungal biomass a better feed supplement in distillers dried grains than the solubles it displaces, particularly in swine, poultry and fish diets. The chitosan and chitin content add a new dimension to the feed coproduct. "These substances have proven health benefits in animals, averting the use of antibiotics and improving the rate of gain," van Leeuwen says. Those benefits, however, were established with chitosan and chitin derived from other sources so research will be required to confirm the same effects with the substances derived from the MycoMax process, he adds. Chitin can also be used as a source for the popular neutraceutical, glucosamine, he says.

One advantage that could speed up the coproducts use in animal feed is that the fungus already has the "generally regarded as safe" designation from the U.S. Food and Drug Administration. The GRAS designation will simplify the process of getting the new feed supplement approved, and also opens the way for new coproducts. "You can freely eat this, although it's not used much because it's not produced in any quantity," van Leeuwen says. The primary food use is in an Asian specialty food called tempe. A related fungus is widely sold in Europe as Quorn, a meat substitute. "My big dream for the future would be to turn this into protein for human consumption," he says, improving people's diets in protein-short regions. "First we aim to get it established for animal feed."

Energy Savings Potential
Perhaps the biggest benefit of the MycoMax process will be in its energy savings. A typical dry-grind ethanol plant evaporates water to condense 2.5 gallons of thin stillage produced per gallon of ethanol from 7 percent solids into syrup that contains 32 percent solids. Using the energy requirement of a common evaporator, concentrating the syrup requires 9,563 British thermal units (Btus). Another 4,340 Btus are used to dry the syrup portion to 90 percent solids after it is blended into the distillers grains. Estimating the cost of natural gas at $13.2 per million Btu, the researchers expect a total energy savings of about 18 cents per gallon of ethanol produced.

The actual savings would vary among dry-grind ethanol plants. Some plants recover a portion of the heat used in the evaporators to provide heat for the distillation process. Researchers point out, however, that the distillation process requires substantially less heat than thin stillage evaporation. Other ethanol plants put the thin stillage and other condensates through a methanator to produce methane gas for process heat.

The energy savings realized from eliminating evaporation are not likely to be offset by the fungal cultivation process itself, which is a low energy user. Van Leeuwen says the fungi grow at 98 degrees Fahrenheit. Because the thin stillage leaves the distillation chamber at higher temperatures, the heat from that liquid could be used to maintain the temperature of the fungi growth chamber. When scaled up, the process might require heating in winter and cooling in summer, much like the fermentation process.

The economic analysis for the MycoMax process includes a cost savings from lower water requirements and estimates a 25 percent savings in enzyme costs. It also includes an estimated value for the fungal feed coproduct at $260 per ton. If all the numbers prove out when scaled up, that could total 20 cents per gallon of ethanol from the water savings and additional income.

On the expense side, the biggest operational cost for a 100 MMgy ethanol plant is about $3 million per year for the electricity to run the blowers that aerate the growing fungus.

The operation of microwave dryers, maintenance, personnel and other expenses bring the estimated operational costs to $6.9 million per year. The capital investment for a 100 MMgy ethanol plant would total an estimated $11.9 million for equipment including tanks, air blowers, separation equipment, piping and pumps.

When the savings and income are balanced against the capital and operating costs, the investment could be paid back in less than a year. Yet, in spite of such promise, van Leeuwen says difficult times in the ethanol industry have made it challenging to raise money to build a pilot facility. The process has to be proved at a larger scale to find any unknown inhibiting factors, solve any handling and separation issues, and produce enough fungal biomass for feed trials.

Susanne Retka Schill is an Ethanol Producer Magazine staff writer. Reach her at [email protected] or (701) 738-4922.