Algae project removes toxins, produces biomass

Posted May 12, 2009, at 1:07 p.m. CST

A pilot project for growing algae in a wastewater treatment plant in St. Paul, Minn., will serve two functions: removing nitrogen and phosphorus from the water before it's flushed into the Mississippi River and flows into the Gulf of Mexico; and producing algal biomass for future use in the manufacturing of biofuels. In addition, the extracted nitrogen and phosphorus will be used to produce fertilizers.

A team of researchers from the University of Minnesota partnered with the Metropolitan Council for the project, using centrate-liquid waste separated from the solids-to grow several species of algae that can thrive in wastewater. The project started in 2006 on a much smaller scale, using wastewater in labs, and recently moved to Met Council's treatment plant.

"It's an opportunity to grow algae on a larger scale in colder climates," said Todd Reubold, director of communications for the Institute on the Environment. "Rather than flushing all these nutrients, we're recycling them."

"We view this [project] as an alternative for removing nutrients," said Bob Polta, lead project engineer at Met Council.

Heavy research and experimenting went into choosing the strains, and the discovery that centrate is the optimal element. "After screening all kinds of algae, we found types that can grow in concentrated wastewater without any additional nutrients," said Roger Ruan, professor of bioproducts and biosystems engineering at the university, and director of the Center for Biorefining.

Using a wastewater plant to grow the algae saves a significant amount in capital and energy, said Rod Larkins, associate director of IREE. "You have to fertilize algae, but in our case, the fertilizer is already there," he said. "You save significant money by not having to add nutrients to the algae." The necessary high volume of water and heat are available, also.

The project will eventually save Met Council the cost of removing phosphorus to meet Minnesota Pollution Control Agency mandates, which is usually done by adding salts to the water. "We're looking at capital in the hundreds of millions of dollars to do that," Polta said.

Using an enclosed photobioreactor allows algae growth to happen in a smaller area, is maintenance-free and allows growth throughout the tank, instead of just on the surface, Larkins said.

The team hopes to use gaseous waste from the Met Council's fluidized bed gasifier in the future, but currently gets its carbon dioxide supply from a pure tank. The team also is in early discussions with Xcel Energy to obtain waste carbon dioxide and maybe nitrogen oxide, according to Ruan.

Harvesting is done by centrifuge and the team uses pyrolysis to convert the biomass to bio-oil, but has not begun the process of converting that to biofuel. "We're working diligently on the pyrolysis oil," Larkins said. "The next logical step will be converting that to biodiesel." Ruan said it's uncertain when conversion will begin, as the team still is exploring processes. The biodiesel produced from that oil eventually will be used in Met Council's buses.

About 185 million gallons of wastewater come through the plant, which is Met Council's largest, on a given day, Polta said, producing about 1 million gallons of centrate. The pilot project treats about 200 gallons per day and currently is producing 2 pounds of dry solids-algal cell biomass and bacterial cell biomass-a day, Polta said. The team's goal is to eventually produce four grams of algae per liter of centrate, Larkins said, and the short-term oil production goal is 30 percent by weight. "In the long term, it'll be better than that," he said.

"If we were to do this full-scale, my estimate is we could generate 1,000 to 4,000 gallons of oil per day," Polta said.