Why Our Next Fuel Source May Come from Our Own Waste

By Greg Breining, Yale Environment 360. Posted July 13, 2009.

So the University of Minnesota and the Met Council began research in 2007. Through its Initiative for Renewable Energy and the Environment, the university was looking for a way to produce a renewable fuel that wouldn't compete with food crops or tie up agricultural land. The Met Council wanted cleaner wastewater.

Early on, Ruan decided against growing algae on the raw wastewater streaming into the plant: The task of managing more than 200 million gallons a day seemed daunting. Instead, research focused on the "centrate," the millions of gallons squeezed from settling-pond solids by powerful centrifuges. The foul juice is so high in nitrogen and phosphorus that it kills most organisms.

Ruan's initial task was to screen thousands of species of algae to find one, or several, that would flourish in the harsh conditions of the centrate. He dispatched his assistants to scoop green, soupy water from ponds and rivers. Most perished in the concentrated nutrients, but Ruan eventually found several species -- greenish, spherical, single-celled plankton only 5 microns across -- that survived. By acclimating these survivors, Ruan was able to produce strains that thrived in the wastewater, while reducing the levels of phosphorus by 50 to 80 percent. They yielded 30 percent of their mass as oil and grew so fast they could be harvested daily.

So far Ruan and the Met Council have shunned genetically engineered algae, though they almost certainly could boost growth and oil content. "We are not interested because eventually on a massive operation like this, some of it is going to get loose in the river," says Willet. "And I have enough regulations."

Ruan also decided against open ponds to grow his algae. Ponds are inefficient, because algae blooms block light. Commercial-scale ponds would also require large acreage, and conditions are tough to control, especially in winter. "If you're talking about an open pond system, it's almost impossible in a northern climate," says Ruan. Finally, "if the algae is dilute, it's very, very expensive to harvest it."

Instead, Ruan began building dozens of different "photo-bioreactors"-- various configurations of tubes or plates that allow good exposure to natural and artificial light, as well as easy access for harvesting and cleaning. The current generation of reactors is operating in a shed in the plant's "solids building," not only to contain the stench of the centrate, but also to keep the equipment secret until the university secures patents.

Like soybeans, algae oil can be used to make biodiesel. Or it can be "cracked" through heat and catalysts (as in an oil refinery) to produce "green diesel," identical to petroleum-derived diesel. Either biodiesel or green diesel could power the Metro Council's public bus fleet, which already uses biodiesel in blends of up to 20 percent. "We are a guaranteed market," says Willet. Remnant algae mash -- the nitrogen-rich pulp -- can be sold as fertilizer, animal feed, or raw material for ethanol.

But there's one big problem, Ruan says, and it's common to any attempt to convert algae to fuel. "We have done a lot of work to get the oil out, but we know it is expensive," says Ruan, who is lead scientist on several other promising algae biofuel projects that do not use wastewater as a feedstock.

Two methods are in common use: Drying and crushing the algae, or removing oil with a solvent. Both, says Ruan, are expensive. Researchers are exploring various ways to break down algae cell walls -- through osmotic shock or ultrasound, for example -- to make oil recovery easier.

It pays to keep trying, because with available processes, Ruan says, algae-diesel might cost $20 a gallon. But, says Willet, "that doesn't take into account the avoided costs that I will realize."

During the next year, Ruan and the Met Council hope to develop a design for a demonstration-scale plant to utilize perhaps 20,000 gallons of centrate a day. That amount is only 2 percent of the centrate the Metro plant generates, and would produce only about 160 kilograms of dry algae and 8 gallons of oil a day. But, says Ruan, an algae plant of that size could eventually be scaled up to treat the entire stream of centrate and produce near 400 gallons of oil a day. Or it could be used as is to treat the wastewater of a city of 50,000 people.

And that is the key -- wastewater treatment with the added benefit of renewable fuel. Or renewable fuel with the benefit of cleaner water. Either way, says Ruan, "we feel that this is probably a perfect combination."

See more stories tagged with: biofuel, fuel, wastewater, algae

Greg Breining is a journalist and author whose articles and essays about travel, science, and nature have appeared in The New York Times, Audubon, National Geographic Traveler, and many other publications.

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