Why Our Next Fuel Source May Come from Our Own Waste

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

In his quest for a fuel of the future, Roger Ruan has found a valuable resource in something nobody else wants -- the wastewater from Minneapolis' largest sewage treatment plant.

The University of Minnesota professor is tapping into this rather unlikely source to grow single-celled algae and produce a diesel-like biofuel. He is one of many researchers around the world working to make biofuel from algae at a price that is competitive with gasoline and diesel fuel. But Ruan's project -- along with several other sewage-to-fuel experiments -- has a distinct advantage over competing algae-to-fuel efforts: His nutrient-rich feedstock is free and available at a nearly constant rate all year long.

And perhaps most importantly, Ruan's algae can not only be used to produce fuel, but can also clean up the wastewater, potentially saving millions of dollars.

"That's what we're after," says Jason Willet, finance director for Metropolitan Council Environmental Services, which operates the wastewater plant and has helped fund Ruan's research.

A single acre of algae, even in an inefficient open pond, can produce 5,000 gallons of biodiesel per year, says Ruan--100 times as much as soybeans. And unlike many other algal biofuel experiments, Ruan's work does not rely on food-based crops, such as sugar cane, as a feedstock to produce the algae.

"This (sewage-based biofuel) potentially is a very, very good energy crop," says Ruan, a professor of biosystems and agricultural engineering. "Potentially its yield can be much, much higher than starch from corn or oil from soybeans. The main reason is that it can grow at a much, much faster rate."

Growing fuel-producing algae in waste is not Ruan's idea alone. The concept drew international attention in 2006 when a startup in New Zealand called Aquaflow successfully harvested biofuel from open-air ponds at wastewater treatment plants. The company expects to be able to produce the biofuel on a large scale, and recently attracted the attention of major players in the airline industry by announcing it had distilled a special blend that meets the technical specifications for jet fuel.

Aquaflow's advances, combined with the Pentagon's interest in biofuels as an alternative to conventional jetfuels, has sparked a flurry of academic and industrial research in the United States. A team at the University of Virginia has been working to maximize the efficiency of the algae-growing process, and the chemistry department at Old Dominion University in Norfolk, Va., has built a small-scale bioreactor at a local wastewater treatment plant that may eventually be able to produce $600,000 worth of fuel per year.

Even NASA has thrown its hat in the ring, with researchers working on the development of floating greenhouses for algae cultivation. The bags are stocked with human waste and sown with species of freshwater algae, and then deployed into the ocean. The semi-porous plastic membrane allows the exchange of CO2 and oxygen to continue uninhibited, but prevents the salty seawater from disturbing the fecund growing conditions inside. Soaking up the sun and feasting on the nutrients in the sewage, the algae produce fat-laden cells that can be harvested and refined into fuels.

U.S. entrepreneurs have also entered the market. In June, Indianapolis-based Algaewheel contracted with the city of Reynolds, Ind., to construct a module at a wastewater treatment facility that uses a wheel-like rotating contraption to filter incoming sewage through a series of algae cultures. The fuel generated from the process will be used to power the facility.

In Minneapolis, Ruan and the Metro Council are conducting research with the intention of designing a demonstration algae-to-fuel plant within about a year.

The Metropolitan Wastewater Treatment Plant sits beside the Mississippi River, just downstream from St. Paul. The 10th-largest plant of its kind in the country, it treats sewage from three-fourths of the Twin Cities metro area -- more than 200 million gallons a day. Some of its low brick buildings, adorned by graceful Art Deco lettering, date to the plant's origin in 1938. From a rooftop, the 170-acre grounds is a warren of basins, tanks, stacks, and pipes.

Incoming sewage is screened for trash and chunks, then runs into settling ponds to remove solids. In aeration ponds, carefully managed populations of microbes break down organics. After more settling to remove dead microbes, wastewater is sterilized with liquid chlorine before being discharged into the river. The effluent is often cleaner and clearer than the river itself. Amazingly, there is barely a whiff of odor.

As clean as the effluent is, Minnesota is considering new standards that will most likely require further reduction of phosphorous and nitrogen. Excess phosphorus is a real concern in the Land of 10,000 Lakes because it causes unsightly algae blooms and fish kills. Nitrogen sluicing off farmland throughout the Midwest is blamed for the hypoxic dead zone at the Mississippi River's mouth. But meeting the new standards through conventional treatment could easily cost "hundreds of million of dollars" a year, says Willet. "We need to find some options."

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.

Liked this story? Get top stories in your inbox each week from Environment! Sign up now »