Anaerobic digestion demo wraps up in Wisconsin

By Anduin Kirkbride McElroy
A Wisconsin company recently completed a research and development project through the commercial demonstration of a high-solids, two-phase anaerobic digester. Afterward, Mark Heffernan, president of Bio-Products Engineering Corp., said his company's product is probably five years away from commercial operation.

In traditional anaerobic digestion, the feedstock is placed in one tank, where microbes digest it first into acid and then into methane. In the two-phase method, which was first brought to commercial scale in the 1970s, the acid and methane phases are separated. "When you separate the two microbial populations, you end up with capabilities that a conventional system doesn't have," Heffernan said. "We're taking the two-phase methodology and applying it to high solids."

The solid-loading content of conventional, two-phase systems runs between 2 percent and 6 percent, he said. High-rate digesters can operate with a solid-loading content of 8 percent. Bio-Products Engineering spent the past four years developing the acid phase that could operate at a solid-loading content of 10 percent to 15 percent at a loading rate of one ton per day per digester. "The next step is to take the digester, find money to operate it at five tons per day and build the methane phase to go with it," Heffernan said. "We want to operate that at a high-enough rate per day that shows commercial capability. The second step involves more money but is easier to do because it doesn't have the solids content."

The company would market this product to industrial food processors, brewers, wastewater treatment plants and ethanol facilities. This would benefit food processors in particular because they have a lot of waste and are also high energy consumers. "Potato plants, for example, process millions of pounds per day, but half of incoming pounds go out as waste," Heffernan said. "There's still energy left in those potatoes."

A high-solids system is necessary to process raw potatoes, which are 17 percent solids. "With our methodology, the energy potential of the raw material goes up because the efficiency of the process goes up," Heffernan said. "By going to a two-phase and higher solid-loading content, you produce a stronger acid environment. Those stronger acids are able to degrade and convert more of the raw material. The energy potential from a ton of material through our system goes up, as compared to a conventional digester. You have less solid residue because more of the original solids became liquid and then gas."