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University Fine-Tuning Anaerobic Digester

By Kevin Wilcox

Michigan State University's new anaerobic digester will create fertilizer, generate power, bolster sustainability—and pay for itself.

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Michigan State University’s Anaerobic Digestion Research and Education Center is using an advanced anaerobic digester to turn organic waste from both the campus and other, external, sources into heat and power. G.L. Kohuth

May 6, 2014—Michigan State University (MSU) is fine-tuning the design of a new anaerobic digester that will break down more than 17,000 tons of organic waste a year, releasing enough gas to generate approximately 3,000 MWh of energy—enough to power as many as 10 buildings on campus.

The university has been considering such a facility for nearly a decade. MSU is home to the Anaerobic Digestion Research and Education Center (ADREC), which provides a collaborative, multidisciplinary platform for research into capitalizing on a larger portion of the 140 billion tons of organic waste generated each year around the world.

Although the MSU leadership was keen to develop a new facility, the payback on the initial investment in the plant had to come within 10 years. Its initial designers planned to use only MSU's organic waste, but found that the payback criterion would be impossible to meet that way. In 2010, however, MSU began looking for external partners—businesses with organic waste for disposal. The tipping fees from these external partners and potential revenue from the creation of fertilizer made the project economically feasible.

Dana M. Kirk, Ph.D., P.E., an animal scientist and biosystems engineer at MSU and the manager of the ADREC, says, "The university administration wanted a project that stakeholders in Michigan could look at and say, 'this makes sustainable sense, but it also makes economic sense.' The tipping fees are really what allowed us to create a financially viable project."

The digester system comprises two tanks: a 100 ft diameter tank that stands 42 ft tall, with a capacity of 2.1 million gallons, and a smaller, 400,000 gallon tank. The smaller tank, with a hydraulic retention time of 26 days, is insulated and heated to maintain the internal temperature at approximately 101° F.

"[It is] is a complete mix digester," Kirk explains. "There are two propellers that agitate and turn the material. There is a domed gas collector on top. Both tanks are sealed airtight. So there are true anaerobic conditions inside the tanks." The larger tank serves as storage for end-product materials and also acts as a gas storage unit.

The facility required a site that is remote and yet close to the main campus, works logistically with the supply of organic waste from the campus and the manure generated from MSU's Dairy Teaching and Research Center (DTRC), and affords ready access to external partners, which include the Meijer chain of grocery stores. Meijer's Lansing distribution center generates a significant amount of fruit and vegetable waste from it fresh-cut facility, and these are augmented by external deliveries of fat, oil, and grease.

MSU found an ideal site for the digester at the DTRC—a one-acre, greenfield site. "It moves us off campus, it gets us closer to 40 percent of our feedstock, and it isolates us in case there is a mechanical or odor issue," Kirk says. "We move the cafeteria food waste by using MSU recycling logistics to the digester site; the manure from the dairy is already there; we get manure from other farms; and it allows for truck traffic for feedstock from off campus as well."

The biogas released during digestion, which is typically 62 percent methane, is fed into a combined heat-and-power unit. The system will produce about 400 kWh of electricity and 450 kWh of thermal energy. The heat generated by the unit is used to maintain the heat of the digester and to heat other research activities in the immediate area.

MSU began using the system on September 1, 2013, and was generating electricity by mid-October, though fine-tuning of the system is now under way. The system faced a harsh test this past winter—the coldest in 30 years and the snowiest on record. The harsh winter included power disruptions—one lasting for two days—and ice storms, Kirk says.

"Winter was really hard. It just never got warm," says Kirk, who found a silver lining in the situation. "It was good [because] we learned a lot. We know where the weak spots were mechanically, as far as temperature and heating. So those are being resolved."

Yet to be resolved are a few mechanical bottlenecks that do not impact the effectiveness of the digester's operation, but increase the labor required to operate the system. The design team is working to eliminate those bottlenecks now.

Although MSU has no current plans to do so, the facility was designed to accommodate expansion. The university is also considering converting the gas generated by the digester to compressed natural gas to fuel vehicles on campus. The facility recently won a Biogas Project of the Year Award from the American Biogas Council.


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