Why Our Next Fuel Source May Come from Our Own Waste
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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."
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.